Download Programming Basics - McGill School Of Computer Science

COMP-202
Programming Basics
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
Chapter Outline
•
•
Hardware & Software
Programming Language
• Syntax and Semantics
• Identifiers
• Formatting Rules
•
•
•
•
Variables
Types
Assignment
Expressions
• Arithmetic Operators
• Increment / Decrement Operators
• Assignment Operators
•
•
Type Conversions
Development Life Cycle
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
2
What Do Other Sciences Do?
• Life Science Example:
• Study how a cell works
• A Cell is something complex that already exists
• Researcher tries to figure out how this complex cell works
• Get background knowledge
• Learn laboratory skills
• Make hypothesis
• Run many experiments to see whether hypothesis works
• Engineer Example:
• Build a bridge
• Create something new
• Must follow many rules --> must work
• Must know the math, materials, physics, …
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
3
What Do Computer Scientists Do?
• Example:
• Build the web-portal for an online bookstore
• Create something new
• Webpages
• Functionality:
• Browse with keywords
• Maintain your shopping cart
• Perform checkout
• What is more complex: a cell or a piece of
software?
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
4
What Is Programming?
• In order to understand what programming is, we
need to know what a computer is:
• A computer is a machine that executes lists of
instructions
• We feed a list of instructions to the computer and the
computer executes them
• The computer may apply the instructions on additional
information fed to the computer (the input)
• The computer may produce information as a result of
executing this list of instructions (the output)
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Recipe for Scrambled Eggs
• Ingredients: two eggs, tablespoon of oil, salt
• Instructions:
•
•
•
•
Heat pan on stove until hot
Add oil
Crack eggs into pan
Add salt
A recipe is a series of steps
What if we did not follow
the order?
• Optional: add milk / cheese
• Mix until light and flakey
• Output: scrambled eggs
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Currency Exchange
1. Input (data that is needed to solve the problem):
– Amount
– Source Currency
– Desired Currency
2. Instructions
1. Look up in table current exchange rate for the selected
currencies
2. Calculate result as amount * exchange rate
3. Output (data that is produced by executing the
instructions)
– Result
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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What is a Program?
•
•
•
•
•
A program solves a particular task
A program is a list of instructions
The list must be ordered correctly
A program has inputs and outputs
Each instruction tells the computer to do
something (an action, a calculation, a comparison)
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
8
What is Software?
• Software = Program ?
• Software typically consists of many
subcomponents / modules / subprograms
• Each subprogram solves a particular task
• Software also includes the data to be used and
manipulated
• Input / output data
• Internal data (e.g, table with exchange rates)
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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The Bank Machine: Functionality
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•
•
•
•
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•
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Withdraw money from checking account
Show balance on checking account
Show balance on saving account
Transfer money from checking account to saving account
Transfer money from saving account to checking account
Deposit money on checking account
Deposit money on saving account
…
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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The Bank Machine: Interaction
• Login (sub-program )
• Task menu (each being a sub-program)
• Withdraw, Get balance, Deposit
• Transfer
• Bill payment
• Once task is selected
• Walk-through for specific task on hand
• Requires different input data, produces different output
• Allow many tasks in one session
• Logout
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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The Bank Machine: Login
• Input: pin
• If pin is correct
• Result: go to task menu
• Else get new pin as input
• If pin is correct
• Result: go to task menu
• Else get new pin as input
• If pin is correct
• Result: go to task menu
• Else print warning, do not release card
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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The Bank Machine: Money Transfer
• Input ?
• Instructions?
• Output?
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Goal of this Course
• Translate a complex task into the software that
executes this task
• Structure the problem space into many small sub-tasks
• Write programs for each subtask
• Connect the programs to build software
• Often, output produced by one program is used as input for another
program
• At the end of the course, you will be able to write
a program that simulates a bank machine, or
implements a game, etc...
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Programming = Writing Instructions
•
But to make things challenging:
• Computers require very specific instructions, i.e., no details can be
omitted
• Computers are “stupid”, i.e, they follow instructions without knowing
what they are doing
•
If you omit the proper instructions or include the wrong
instructions, generally 4 things can happen when you run the
program
1. You are really lucky and on that particular input your program works
anyway.
2. The program gives incorrect output.
3. The program crashes.
4. The program goes on forever and ever.
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Hardware and Software
• Hardware
• The physical, tangible parts of a computer
• Keyboard, monitor, wires, chips, mouse
• Software
• Programs and data
• A program is a series of instructions
• A computer requires both hardware and software
• Each is useless without the other
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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The Personal Computer - Input Devices
CD / DVD Drive
Hard Disk
Plus Other (USB) Devices
Camera
Keyboard
Microphone
Mouse
Plus Other (USB) Devices
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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The Personal Computer - Output Devices
Monitor
CD/DVD/Disk
Speakers
Plus Other (USB) Devices
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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CPU - Central Processing Unit
• Performs Instructions
• Arithmetic operations
• Logic operations
• Decisions
• Controls (manipulates and moves) the
information /
data in a
computer
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Memory
• Memory holds the data
• Main memory (RAM):
• Data has to be in main memory so that the CPU can
access it
• Volatile: lost when program exits / computer shuts off
• Hard Disk / CD / DVD / Flash drive
• Persistent
• This is where you keep the data
for long-term storage
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Interaction
Central
Processing
Unit
Hard Disk
Monitor
Keyboard
Main
Memory
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
CD-RW
21
How Do We Store Things?
• Computer memory is electronic. It's just a bunch of wires!
• All it can recognize is “on” (current goes through) and
“off” (no current goes through)
• Using many of these on/off “switches” together, we can
encode many things
• For example, we can store whether it is morning or
afternoon using the following encoding:
• “If the 1st switch is on, then it must be PM. If the 1st switch is off,
then it must be AM.”
• By combining many (and yes, a computer has many!)
switches, we can store more complex data
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Encoding
• How could you encode the day of the week?
• If we just use 1 switch, there will not be enough
information.
• How many “switches” will we need?
• How could you encode the letters of the alphabet?
• How could you encode a 3 letter word?
• How would you encode a sentence?
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Storage is Exponential
• In general, if there are n possible values to store,
we can encode it using
• log2(n) “switches”
• Of course, there is no such thing as a fraction of a
switch, so we will always have to round up.
• Put another way, if we have n switches, we can
store 2n values
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Main Memory
Each cell has a numeric
address that uniquely identifies it
9278
9279
9280
9281
9282
9283
9284
9285
9286
..
.
Main memory is
divided into many
memory locations
(or cells)
..
.
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
Each cell can
contain a data
value, e.g. 22, or
the character ‘j’
25
Bits and Bytes
• Bit -- most basic unit of memory
• 1 or 0, on or off
• 1 Byte = 8 bits
• In general, each byte of memory has a unique address
• KB (210 bytes), MB (220 bytes), GB, TB, ...
• In a computer, data values are stored as a sequence
of bits
9278
9279
22
7
9278
9279
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
00010110
00000111
26
Binary Encoding of Numbers
• Humans use the decimal number system
• Each digit (0, 1, ..., 9) in a number represents a power of 10
• 6 = 6 x 100
31 = 3 x 101 + 1 x 100
207 = 2 x 102 + 0 x 101 + 7 x 100
• Binary number system
• Each number position (bit) represents a power of 2
• 6 = 1 x 22 + 1 x 21 + 0 x 20 = 110
31 = ?
207 = ?
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Program Execution (1)
• A program tells the CPU how to manipulate and/or
move information
• Programming is about processing information
• Take some input, manipulate it in some way, and produce a
particular output
• Fetch-Execute Cycle
• The CPU repeatedly performs the three following operations:
• Reads the next instruction in the program
• Figures out what the instruction means (add two values, load some value
from memory, store some value in memory. compare two numbers
• Performs the instruction
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Program Execution (2)
• Suppose you want to write a program that reads a
number from the keyboard, adds 1 to it, and displays the
new value to the screen
• This program might consist of the following instructions:
• READ a value from the keyboard and store it in memory
location x
• LOAD the value stored in memory location x into the CPU
• ADD 1 to the value stored in the CPU
• STORE the value currently in the CPU back into memory
location x
• DISPLAY the value stored in memory location x to the screen
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Program / CPU / Memory
• Example:
• Input (read) a number from keyboard
• Add 1 to it
• Output it on screen
Keyboard 17
Memory
Monitor
CPU
18
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Human and Computer Languages
• Consider the following English sentence:
"The lady hit the man with a baby."
• Does this mean
1. A lady hit a man who had a baby?
2. A lady used a baby to hit a man?
3. A lady and a baby ganged up on a man and hit him.
• Programming languages are designed in such a way
that individual instructions (called statements) always
have only one possible interpretation
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Programming Languages
• A programming language specifies the words and
symbols that we can use to write a program
• We know that a program is a series of instructions, and
that these instructions are performed by the computer
• We use this language to explain the instructions to the
computer
• A programming language employs a set of rules
that dictate how the words and symbols can be put
together to form valid program statements
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Syntax and Semantics
•
•
•
•
The syntax rules of a language define how we can put symbols
and statements together to make a valid program
The semantics of a program statement define what those symbols
and statements mean (their purposes or roles in a program)
A program that is syntactically correct is
not necessarily logically (semantically)
correct
A program will always do what we tell it
to do, not what we meant to tell it to do
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Programming Language Levels
• There are many programming language levels
• machine language
• assembly language
• high-level language
0011001001001
add r1,5
$a0 $3f $64 $21
mov r1,r2
load ax,3
FORTRAN, Pascal, C, Ada, C++, Java
• Each type of CPU (PowerPC processor, Intel processor,
…) has its own specific machine language. These are the
simple built-in instructions the CPU comes pre-designed
with.
• The other levels were created to make it easier for a
human to write programs
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Compilers
• A program must be translated into machine language
before it can be executed on a particular type of CPU
• A compiler is a software tool which translates source
code into a specific target language. Often, that target
language is the machine language for a particular
CPU type.
• Input: files written in a high-level programming language
• Output: executable binary file that can be processed by CPU
• A different compiler is needed for each CPU type
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Interpreter
• An interpreter is another software tool which
takes source code and translates it into a target
language
• However, the target language instructions it
produces are executed immediately
• No executable file is created
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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The Java Programming Language
• Java was created by Sun Microsystems, Inc., now part
of Oracle Corporation
• It was introduced in 1995 and has become quite
popular
• It is an object-oriented language
• It represents the program as a series of objects, each of
which belongs to one of many categories called classes
• The Java programming language combines both
compilation and interpretation
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Java Translation and Execution
• The Java compiler translates Java source code into a
special representation called bytecode
• Java bytecode is not the machine language for any
traditional CPU
• Another software tool, an interpreter called the Java
virtual machine, translates bytecode into machine
language and executes it
• Therefore the Java compiler is not tied to any particular
machine
• Java is considered to be architecture-neutral
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Java Translation and Execution
Java Source
Code
Java
Bytecode
.java file
Java
Virtual Machine
.class file
Java Compiler
.class
.class
.class
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
.class
.class
39
Java Program Structure
• In the Java programming language:
•
•
•
•
A program is made up of one or more classes
A class contains one or more methods
A method contains program statements
Statements are the actual commands you issue
• These terms will be explored in detail throughout the
course
• A Java program always contains a method called
main()
• This is where the program starts executing
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Java Program Structure: Class
// Comments about the class
public class MyProgram {
Class Header
(Name of the Class)
Class Body
(where the fields and methods go)
}
Important note: the name of the file which
contains this program MUST be the name
of the class, followed by extension .java
Here: MyProgram.java
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Java Program Structure: Class
// Comments about the class
public class MyProgram { Method Header
Name of Method
// comments about the method
public static void main(String [] args) {
Method Body
(where the statements go)
}
}
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Formatting Rules
• Spaces, blank lines, and tabs are collectively called
white space
• It is used to separate words and symbols in a program
• Extra white spaces are completely ignored
• A valid Java program can be formatted many different
ways
• Programs should be formatted for readability
• Use proper indentation!
• Use space and new lines!
• Use comments!
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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AddTwoIntegers
import java.util.Scanner;
public class AddTwoIntegers {
public static void main(String[] args) {
int input1, input2, output;
Scanner keyboard = new Scanner(System.in);
// Read the numbers
input1 = keyboard.nextInt();
input2 = keyboard.nextInt();
// Compute the result
output = input1 + input2;
// Display the result
System.out.println(output);
}
}
This is an example of a
nicely formatted program
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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AddTwoIntegersBad
import java.util.Scanner;
public class AddTwoIntegersBad {
public
static
void main(String[] args){ int input1, input2, output; Scanner
keyboard = new Scanner(System.in); input1 =
keyboard.nextInt();input2 = keyboard.nextInt();
output = input1 + input2;System.out.println(output);
}
}
Do not do this!
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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What Application should I Use to Program?
• Text Editor + Command line Compiler
• Any text editor that saves plain text files can be
used
+
• Emacs / RText / Alpha
“understand” Java
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Programming Using Text Ed + Cmd Line
•
Run text editor
• Save new text as YourClassName.java
• Give the program the same name as the class (.java)
• Type in your program
• Save the program
• Store all your files in one directory for now
•
Open command line (MacOS: Terminal, Linux: Shell)
• Compile the program (javac YourClassName.java)
• The CPU does not understand the Java language!
• Java Language is translated into language that the CPU understands
• This produces a .class file
•
•
Run the program (java NameOfClass)
Observe the result and adjust the program if necessary
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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What Application should I Use to Program?
• Integrated Development Environment (for Java)
• DrJava
• Very simple, targeted at beginners
• http://www.drjava.org/
• Eclipse
• Very powerful
• http://www.eclipse.org/
• Many others exist: NetBeans, JCreator
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Programming Using DrJava
• Type in your program
• Save the program
• Store all your files in one directory for now
• Give the program the same name as the class (.java)
• Compile the program (hit Compile button)
• The CPU does not understand the Java language!
• Java Language is translated into language that the CPU
understands
• This produces a .class file
• Run the program (hit Run button)
• Observe the result and adjust the program if necessary
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Identifiers
•
•
•
Identifiers are the words a programmer uses in a program
They are used to give names to things
Example of things that can be named with identifiers:
•
•
•
•
•
•
•
Classes
Methods
Variables
Constants
An identifier can be made up of letters, digits, the underscore
character (_), and the dollar sign ($)
Identifiers cannot begin with a digit
Java is case sensitive, therefore Result and result are
different identifiers
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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More on Identifiers
• Sometimes we choose identifiers ourselves when writing a
program (such as input1, AddTwoIntegers)
• We should choose identifiers that are meaningful so that it is more
readable for humans
• Sometimes we are using another programmer's code, so we
use the identifiers that they chose (such as println())
• Often we use special identifiers called reserved words that
already have a predefined meaning in the language
• A reserved word cannot be used in any other way
• Examples of reserved words: class, public, static, void
• There are many others
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Identifier Exercise
• Which of the following identifiers are invalid according
to the Java rules for identifiers? Why are they invalid?
myIdentifier
_my_other_identifier
yet-another-identifier
$can
2for1
twoFor1
class
my_class
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Variables
• A variable is a placeholder for values
• We can store a value in a variable
• We can use the value a variable contains to compute other values
(which may be stored in other variables) or make decisions
• Each variable has a name
• The compiler allocates memory cells to hold the value of the
variable, and assigns the name chosen for the variable to these cells
• We can then refer to the value stored in these memory cells using the
variable name
• Much easier and more practical than using the addresses of the
memory cells (and less error-prone)
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
53
Variable Declarations
•
Before we can store a value in a variable or use the value it
contains, we have to declare it
• A variable declaration is a statement that announces that we
want to create and use a new variable
• They can occur anywhere in a method, but they are typically at
the beginning
• They must indicate the name and the type of the new variable
• Once a variable has been declared, we can store a value in it or
use the value it contains
• If you try to use a variable before it is declared, the compiler will
generate an error
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Syntax for Declaring Variables
modifier
type
identifier
= value
;
(optional)
(mandatory)
(mandatory)
(optional)
(mand.)
• Example modifiers: final, static, public,
private, ...
• Example types: int, float, char, ...
• Identifier: as defined previously
• Value: an expression matching the type (the expression
is evaluated and its value is assigned to the variable)
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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More on Variable Declarations
• Note that you can declare more than one variable
with one variable declaration statement
• The variable names are separated using commas
• Example:
int var1, var2, var3;
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Variable Declaration Illustration
int weightGain = 13;
9278
9279
9280
9281
9282
9283
9284
9285
9286
..
.
00000000
00000000
00000000
00001101
weightGain
..
.
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Types
• A type is a category that a variable belongs to and determines:
• How to interpret the value it contains
• What are the possible values it can contain
• All variables have a type
• The type of a variable provides an answer to the following
question: should the value of the variable be considered as an
integer, a real number, a character, or maybe even something
else?
• For a computer, everything is stored as a series of ones and zeroes
• We need a way to tell what the ones and zeroes represent and how they
should be interpreted
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Java Types
• There are two broad kinds of types in Java
• Primitive types
• Reference types
• We will see reference types later in the course
• Primitive types represent very basic kinds of values
• Primitive types are defined by the Java compiler
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Primitive Types
•
There are exactly 8 primitive types in Java
• Four of them represent integers (positive and negative whole
numbers):
• byte, short, int, long
• Two of them represent floating point numbers (positive and
negative numbers with decimal parts):
• float, double
• One of them represents characters:
• char
• And one of them represents boolean values (true or false):
• boolean
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Numeric Types
• The difference between the various numeric
primitive types is their size, and therefore the
values they can store:
Type
Space
Literal
Minimum
Maximum
byte
8 bits
5
-128
127
short
16 bits
5
-32768
32768
int
32 bits
5
-2147483648
2147483647
long
64 bits
5
< -9 * 1018
> 9 * 1018
float
32 bits
5.2f
+/- 3.4 * 1038 (with 7 significant digits)
double
64 bits
5.2
+/- 1.7 * 10308 (with 15 signif. digits)
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Real vs. Floating Point
• Real number: number that can be given by an
infinite decimal representation
• 3.14159265...
• However, we would need infinite memory to store a
number with infinite decimal representation
• Floating point number:
• Approximation of a real number; can only hold a finite
amount of decimal places
• Needs only finite space (fits in a memory cell or a finite
set of memory cells)
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Characters
•
A char variable stores a single character from the Unicode
character set
char gender;
gender = 'M';
•
•
•
•
A character set is an ordered list of characters, and each
character corresponds to a unique number
The Unicode character set uses 16 bits (2 bytes) per character,
allowing for 65536 unique characters
It is an international character set, containing symbols and
characters from many world languages
Character literals are delimited by single quotation marks:
!'a'! 'X'! '7'! '$'! ','! '\n'
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Boolean Values
•
•
•
A boolean expression is an expression that evaluates either to
true or to false
Named after George Boole, the inventor of Boolean algebra
Similar concept in natural language
• "The traffic light is red "
• This expression is either true or false
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Boolean Type
• You can declare variables which are of type boolean
• A boolean variable represents a true or false condition
• A boolean can also be used to represent any two states, such
as a light bulb being on or off
• The literals true and false are the only valid values for a
boolean type
• Example:
boolean done = false;
// Some code here
done = true;
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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The Assignment Statement
• To store a value in a variable, we use the assignment
statement
• The assignment operator in Java is the = sign
• The syntax for the assignment statement is the following:
variable = expression;
• The expression on the right of the = sign is evaluated
• The result of evaluating the expression is stored in the variable on
the left of the = sign
• The left-hand side of the = must be a variable; it cannot be an
expression
• The = operator does not define an equation!
COMP-202 - Programming Basics, © 2013 Jörg Kienzle and others
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Reading From the Keyboard (1)
To read values from the keyboard, we use the Scanner class
• import java.util.Scanner;
• First, we must declare a variable of type Scanner, and
initialize it
•
• Scanner someName = new Scanner(System.in);
• someName can be replaced by any identifier; keyboard is a good
choice, but there is nothing special about it
• Everything else, however, must be exactly as above
•
To read a value of type int from the keyboard, we use the
following expression:
someName.nextInt();
• someName must be replaced by the identifier you used when
you declared the Scanner variable
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Reading From the Keyboard (2)
Thus, if you called your Scanner variable foo, you must write
foo.nextInt() if you want to read an int value from the keyboard
• The expression someName.nextInt() (where someName is a
variable of type Scanner) can be used as the right side of an
assignment statement:
int input;
input = someName.nextInt();
• The right side of the assignment statement is evaluated; this results in a
value of type int being read from the keyboard
• The value read from the keyboard is then stored in the variable on the
left side of the assignment statement
•
•
To read a value of type double instead of type int, replace
nextInt() by nextDouble()
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Displaying to the Monitor
• Two built-in commands to print on the screen:
• System.out.println(….stuff to print out….);
• A line-break is printed after ….stuff to print out…
• System.out.print(….stuff to print out….);
• Only ….stuff to print out…. is printed
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More on print / println
• println() and print() each take one input
(also called parameter or argument)
• a character string:
• println("Hello world!");
• the value of a variable:
• println(output);
• the combination of both:
• println("The sum is " + output);
• a combination can have more than two parts:
• println("The sum is " + sum + " and the " +
"difference is " + diff);
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Examples of Displaying Text
• System.out.println("Hello world!");
• The character string Hello world! is displayed
• int value = 5;
int result = 7 * value + 9;
System.out.println(result);
• The value 44 is displayed
• double price = 44.99;
System.out.println("This book costs "
+ price + " dollars");
• The character string This book costs 44.99 dollars is
displayed
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Countdown.java
class Countdown {
! public static void main(String args[]) {
! ! System.out.print(“Three… ”);
! ! System.out.print(“Two… ”);
! ! System.out.print(“One… ”);
! ! System.out.print(“Zero… ”);
! ! System.out.println(“Liftoff!”);
! ! System.out.println
(“Houston, we have a problem!”);
! }
}
What does this output?
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Countdown Result
• The following is displayed when the program
Countdown is executed:
Three… Two… One… Zero… Liftoff!
Houston, we have a problem!
Cursor ends up here!
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Short Introduction to Classes
• Java has many pre-defined classes
• Example: Scanner
• Look at: http://download.oracle.com/javase/6/docs/api/
• Java classes define methods that you as a
programmer can call to execute their functionality
• Examples
• int num = s.nextInt();
• System.out.println(“Hey, got any grapes?”);
• We will see later in the course how to write your
own classes
Use “.” operator to make
method calls
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The String Class
• Java defines a class String with many methods
• Example methods (there are many more):
•
•
•
•
String toLowerCase()
String toUpperCase()
char charAt(int index)
String substring(int beginIndex, int endIndex)
• Reading a String from the keyboard
• Scanner s = new Scanner(System.in);
• String wordsOfWisdom = s.nextLine();
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Arithmetic Expressions
• An expression is a combination of operators and
operands
• Arithmetic expressions compute numeric results and
make use of the arithmetic operators:
Addition
Subtraction
Multiplication
Division
Remainder
Negation
x
x
x
x
x
+
*
/
%
-x
y
y
y
y
y
• An operand can be a literal value, a variable, or the
value returned by a method call
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Division with Integers
•
•
•
If both operands to the division operator (/) are integers, the result
is an integer (the fractional part is discarded)
The remainder operator (%) returns the remainder after dividing the
second operand into the first
Example:
int numHours = 52;
int fullDays = numHours / 24;
int remainingHours = numHours % 24;
•
Division by 0
• Produces run-time error
• Program has to avoid it, or it will crash
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DivisonInt
import java.util.Scanner;
public class DivisionInt {
public static void main(String[] args) {
int numerator, denominator, output, remainder;
Scanner keyboard = new Scanner(System.in);
// Read the values
System.out.println("Enter the numerator: ");
numerator = keyboard.nextInt();
System.out.println("Enter the denominator: ");
denominator = keyboard.nextInt();
output = numerator / denominator;
remainder = numerator % denominator;
System.out.println("The result is: " + output);
System.out.println("The remainder is: " + remainder);
}
}
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Operator Precedence
• Operators can be combined into complex expressions
result
=
total + count / max - offset;
• Operators have a well-defined precedence which
determines the order in which they are evaluated
• Multiplication, division, and remainder are evaluated
prior to addition, subtraction, and string concatenation
• Arithmetic operators with the same precedence are
evaluated from left to right
• Parentheses can always be used to force the evaluation
order
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Operator Precedence Exercises
• What is the order of evaluation in the following
expressions?
a + b + c + d + e
1
2
3
a + b * c - d / e
4
3
1
4
2
a / (b + c) - d % e
2
1
4
3
a / (b * (c + (d - e)))‫‏‬
4
3
2
1
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Assignment Operator Precedence
• The assignment operator has a lower precedence
than the arithmetic operators
answer
=
sum / 4 + MAX * lowest;
1
2
3
First the expression
on the right hand
side of the =
operator is evaluated
4
Then the result is stored in the
variable on the left hand side
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TempConvert.java
import java.util.Scanner;
public class TempConvert {
! public static void main (String args[]) {
!
int base = 32;
!
double conversionFactor = 9.0 / 5.0;
!
double fahrenheitTemp;
!
int celsiusTemp;
!
Scanner scan = new Scanner(System.in);
!
celsiusTemp = scan.nextInt();
!
farenheitTemp = celsiusTemp * conversionFactor + base;
!
!
!
System.out.println(“Celsius Temp
= “ + celsiusTemp);
System.out.println(“Farenheit Temp = “ + FarenheitTemp);
}
}
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Assignment Operator Sides
• The right and left hand sides of an assignment
statement can contain the same variable
count
=
count + 1;
1
First, 1 is added to the
original value of count.
The result is stored in a
temporary memory location.
2
Then the result is stored back into count
(overwriting the original value)
The fact that the assignment operator has lower precedence
than arithmetic operators makes this possible.
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Constants
•
•
•
A constant is an identifier that is similar to a variable except that it
holds one value for its entire existence
The compiler will issue an error if you try to assign a value to a
constant more than once in the program
In Java, we use the final modifier to declare a constant
final double PI = 3.14;
•
Constants have the following advantages:
• They give names to otherwise unclear literal values
• They facilitate changes to the code
• More precision required? Change PI to 3.14159 in only one place
• No need to search the whole program for occurrences of the value
• They prevent inadvertent errors
• Cannot type the wrong value by mistake
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Circle.java
import java.util.Scanner;
public class Circle {
public static void main(String[] args) {
double radius, circumference, area;
final double PI = 3.14;
Scanner keyboard = new Scanner(System.in);
// Read the radius
System.out.println("Enter the radius: ");
radius = keyboard.nextDouble();
// Perform calculations
circumference = 2.0 * radius * PI;
area = radius * radius * PI;
System.out.println("The circumference is: " + circumference);
System.out.println("The area is: " + area);
}
}
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Boolean Expressions
• Instead of evaluating to a numeric value, boolean
expressions evaluate to either true or false
myNumber > 0 // can be either true or false
• You can assign the result of a boolean expression to a
variable of type boolean:
boolean positive;
positive = (myNumber > 0);
• Boolean expressions are often used to control the flow
of execution of a program
• We will see control flow in detail later in the course; we will
cover boolean expressions in more detail then
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Increment / Decrement Operators
• The increment and decrement operators are arithmetic
and operate on one operand
• The increment operator (++) adds one to its operand
• The decrement operator (--) subtracts one from its
operand
• The statement
count++;
is essentially equivalent to
count = count + 1;
• But there are some traps...
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Increment / Decrement Forms
•
•
•
The increment and decrement operators can be applied in prefix
form (before the variable) or postfix form (after the variable)
When used alone in a statement, the prefix and postfix forms are
basically equivalent; that is, when used alone in a statement,
++count;
is equivalent to
count++;
which is equivalent to
count = count + 1;
However, they are not equivalent when they are used in
expressions
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Increment / Decrement Details
• When used in a larger expression, the prefix and
postfix forms have a different effect
• In both cases the variable is incremented or decremented
• But the value used in the larger expression depends on
the form:
Expression
Operation
Value Used in Larger Expression
count++
add 1
add 1
subtract 1
subtract 1
old value of count
new value of count
old value of count
new value of count
++count
count---count
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Increment / Decrement Details Example
• If count currently contains 45, then
total = count++;
assigns 45 to total and 46 to count (count is
incremented only after its value has been read and used
in the expression)
• If count currently contains 45, then
total = ++count;
assigns the value 46 to both total and count (count is
incremented before its value is read and used in the
expression)
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Increment / Decrement Caveat
• When using the postfix version of an increment or
decrement operator, the value of the variable is incremented
immediately after it has been read
• Thus, if a variable is used more than once in an expression,
its value may not be the same the second time it is used
• Example:
int count = 4;
int total = 0;
total = count++ + count++ + ++count;
• After executing this code fragment, what is the value of
total?
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Increment / Decrement Advice
• Using increment and decrement operators is fine if this
operator is the only operation in a statement
i++; // OK
++j; // OK as well
• Avoid using increment and decrement operators in more
complex statements
total = count++ - --count; // Avoid!
index = ++i * --j / k++; // Avoid too!
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Assignment Operators (1)
• Often we perform an operation using a variable, then store
the result back into that variable
• Java provides additional assignment operators to simplify
that process
• They combine the assignment operator with an arithmetic operator
• total += 5;
is equivalent to
•
total = total + 5;
result = result * (count1 + count2);
is equivalent to
result *= count1 + count2;
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Assignment Operators (2)
•
•
In general, assignment operators have the form
variable op= expression;
which is equivalent to
variable = variable op (expression);
The entire expression on the right side of the assignment
operator is evaluated
• It can be a complex expression involving many levels of parentheses
•
Then, the result of evaluating that expression is used on the right
hand side of the operator in the assignment operator
• The left operand is the variable on the left of the assignment operator
•
The final result is stored back in the variable on the left-hand
side of the assignment operator
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Assignment Operators (3)
• There are many assignment operators, including
the following:
Operator
Example
Equivalent To
+=
x += y
x = x + y
-=
x -= y
x = x - y
*=
x *= y
x = x * y
/=
x /= y
x = x / y
%=
x %= y
x = x % y
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Data Conversion
• Sometimes it is convenient to convert data from one
type to another
• For example, we may want to treat an integer as a
floating point value during a computation
• Conversions must be handled carefully to avoid losing
information
• There are two types of conversions:
• Widening conversions
• Narrowing conversions
• But first, a word on precision
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Precision
• The precision of a type is the range of all possible values
you can express with a variable of that type
• Variables of type int have higher precision than variables of type
short; you can express more values with a variable of type int
than with a variable of type short
• Variables of type double have higher precision that variables of
type int; again, you can express more values with a variable of
type double than with a variable of type int
• There is a correlation between the number of bytes used to
store a value of a given type, and the precision of that type
• byte uses only 8 bits and has the lowest precision; double or
long use 64 bits and have the highest precision
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Conversion Types
• Widening conversions occur when a value whose type
has lower precision is converted to a type that has
higher precision (such as a short to an int or an int
to a double)
• They are usually safe, as there is usually no information lost
• Narrowing conversions occur when a value whose type
has higher precision is converted to a type that has
lower precision (such as an int to a short or
double to int)
• Information can be lost when conversions of this type occur
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Assignment Conversion (1)
• In Java, data conversions can occur in three ways:
• Assignment conversion
• Arithmetic promotion
• Casting
• Assignment conversion occurs when a value of one
type is assigned to a variable of another type
• Only widening conversions can occur via assignment (such as
assigning a value of type int to a variable of type double)
• If we attempt a narrowing conversion (such as assigning a
value of type double to a variable of type int), the compiler
will issue an error
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Assignment Conversion (2)
• Assignment conversions occur automatically
• For example, if we attempt to assign the value of an expression of
type int to a variable of type double, the value of the expression
will be automatically converted to have type double
• The second assignment statement below is legal:
int i = 7;!
double d = 3 + i;
• The expression on the right-hand side has type int, which has
lower precision than type double, the type of the variable on the
left-hand side
• The value of 3 + i gets converted to have type double
and gets assigned to d
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Assignment Conversion (3)
• The second assignment statement below is illegal:
double d = 7.0;
int i = 3.0 + d;
• The expression on the right-hand side has type double,
which has higher precision than type int, the type of the
variable on the left-hand side
• The compiler will therefore report an error
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Arithmetic Promotion
• Arithmetic promotion happens automatically when
operators in expressions convert their operands
double kmPerLitre;
int km = 1000;
float litres = 85.0f;
kmPerLitre = km / litres;
km ➞ float 1
2 division
result of type float
3 result ➞ double
4 assignment
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Casting (1)
• Casting (also called typecasting) is the most powerful,
general, and dangerous, technique for conversion
• It trusts that you know what you are doing
• Both widening and narrowing conversions can be
accomplished by explicitly casting a value
• It is the only way to perform a narrowing conversion
• To cast, the type you want to convert a value to is put in
parentheses in front of the value being converted. The
general syntax is thus:
(desiredType) expression
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Casting (2)
• The cast does not change the type of the value in a
variable for the rest of the program, only for the
operation in which the value is cast
• When casting from floating point to integer, the
fractional part of the floating point is discarded
double money = 25.80;
! int dollars;
! dollars = (int)money;
• dollars contains the value 25, money still contains
the value 25.8
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Casting (3)
• Casting has higher precedence than the arithmetic
operators
int a = 3;
Only the
double b = 3.5;
value of b is cast. a is not
int c, d;
cast. Result: 9
c = (int)b * a;
d = (int)(b * a);
The
product of b and a is
cast. Result: 10
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Conversion Traps (1)
• Consider the following variable declarations:
int total = 10;
int count = 4;
double result;
• What will the value of result be after the following
statement is executed?
result = total / count;
• The value of result is now 2.0 !
• The two operands of the division operator have type int;
thus, integer division is performed, truncating the
fractional part
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Conversion Traps (2)
• Casting one of the integer operands to double (or float)
will result in floating point division being performed instead
result = (float)total / count;
• The value of result is now 2.5
• First, total is cast to type float
• Arithmetic promotion occurs to promote count to have
type float; floating point division is performed and the
result of the division therefore has type float
• The quotient is converted to type double before being
assigned to result
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Conversion Exercises
•
•
•
double x = 5.9;
int y = (int) x;
What is the value of y?
int a = 5;
float b = 7.3f;
double c = 10.03;
c = b + a;!
What is the value of c?
int a = 2, b = 5;
double c = 22;
c = a / b;!
What is the value of c?
5
12.3
0.0
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Programming Errors
• Compile-time errors: the compiler finds problems with syntax
and other basic issues
• If compile-time errors exist, an executable version of the program
is not created
• Logical errors: the program runs, but produces incorrect
results
output = input1 – input2;
System.out.println("The sum is: " + output);
• Run-time errors: a problem occurs during program execution,
and causes the program to terminate abnormally (or crash)
• Division by 0
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Development Life Cycle
Debugging
•
Write Code
Compile
Syntax
Err?
•
errors
Testing 0 errors
Run Program
Yes
Log./Run Err?
No
No
Enough
Testing?
Yes
Errors might take a long
time to debug
Important Note: When
you compile for the first
time and see 150 errors,
do not despair. Only the
first 1 or 2 errors are
relevant. Fix those and
compile again. There
should be fewer errors
(like 50). Repeat until no
errors.
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Problem Solving
• The purpose of writing a program is to solve a problem
(or rather, have the computer solve a problem for us)
• The general steps in problem solving are:
•
•
•
•
•
•
•
Understand the problem
Dissect the problem into manageable pieces
Design a solution for each of the pieces
Consider alternatives to the solutions and refine them
Implement the solutions
Test the solutions and fix any problems that exist
Combine the solutions for each of the piece to obtain the
solution to the original problem
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