Download Session 2 - Computer Science

Survey of Computer Science
CSCI 110, Spring 2011
Lecture 2
Simple Python Programs
Variables, Numbers and Strings
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Using the Python Interpreter
It's traditional to start with a program that prints out 'Hello World'.
In Python, we can do this with one command to the interpreter.
Python prompt
Python statement
>>> print "Hello World!"
Hello World!
Python output
The read-evaluate-print cycle:
•The Python interpreter waits for a command.
•When the command is entered, Python evaluates the command
•Python then prints the result.
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Writing multiple statements
We can continue in the interpreter mode:
>>> print "Hello world!"
Hello world!
>>> print "How are you?"
How are you?
This can get tedious. We can use an editor to write multiple
statements that Python can execute all at once:
print "Hello world!"
print "How are you?"
print "CSCI 110 rocks!"
We will demonstrate the result of running this in class.
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Adding Comments
•Comments are not evaluated by the interpreter
•They are added to the source code to make it easier for
programmers to understand the code or to add information.
•Comments in Python begin with a #
Example:
# Program: Hello
# Purpose: To write out 'Hello World'
print "Hello world!"
print "How are you?"
print "CSCI 110 rocks!"
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Program Prologues
All of your programs for this class must have a prologue:
# Program: Hello
# Author: Angela Student
# Class: CSCI 110
# Date: 9/5/08
# Assignment: Lab 1
# Purpose: To write out "Hello World!"
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Variables (identifiers)
If a user types something on a keyboard, the program must read
it in and store it.
Information is stored in the computer's memory.
A variable names a piece of data. It references a location in
memory where the information is stored.
yourName
'Frank'
aNumber
12
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Assigning values to variables
We can assign a value to a variable using the assignment operator.
The assignment operator is an equals sign (=).
Example:
>>> yourName = "Frank"
>>> yourName
'Frank'
The variable, yourName, now identifies a location in memory where
the text string, 'Frank', is stored.
We can also assign numeric values to variables:
>>> aNumber = 12
>>> aNumber
12
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Reading Text from the
Keyboard
We use variables to read information in from the keyboard:
The raw_input( ) command is used to read in text:
>>> name = raw_input("Please enter your name: ")
Please enter your name: Angela
>>> name
Angela
1) raw_input( ) will output the text within the brackets and
then wait for the user response.
2) The user response is assigned to the name variable.
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Reading in a Number
Use the command, input( ), to enter a number from the keyboard.
Example:
>>> aNumber = input("Please enter a number: ")
Please enter a number: 12
>>> aNumber
12
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An example
Write a program that asks for a user's name, and then says hello
using the user's name.
# Program: Hello
# Purpose: To say hello to the user
# We will write this in class
yourName = raw_input("Hello! What is your name? ")
print "Hello " + yourName + "!"
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Naming Variables
Rules for naming variables:
1) Do not use spaces
2) Variable names must start with a letter
3) Do not use special characters (%, *, /, etc)
4) Case (upper vs. lower case letters) matters.
Good variable names:
age_of_dog
TaxRate98
Not allowed:
age% 98TaxRate
printHeading
age-of-cat
my dog
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Classes and Objects
Python is an object oriented programming language.
All items of data are objects.
Objects have properties: Things that describe the objects
Objects have methods: Things an object can do.
A class describes a group of objects that have the same methods
and set of properties.
Example: A car class
Properties: color, number of doors, body type
Methods: accelerate, brake, steer
My car is an object that is a member of the car class.
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Primitive Numeric Types
Type
integer
Kind of info
whole number
Uses
Arithmetic expressions
long integer
Large whole number
floating point decimal number
Examples:
integer: 12
-37
"
"
"
"
5672
long integer: 29387651298L
floating point: 12.6
0.2
-7.615
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Arithmetic Expressions
with Floating Point Numbers
Arithmetic operators for real numbers:
+
*
/
**
These work the way you would expect:
4.0 + 3.3
7.3
5.0 * 7.0
35.0
3.6 / 3
1.2
2.0**3.0
8.0
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Arithmetic Expressions
with Integers
Arithmetic operators for integers:
+
*
/
%
**
All work the way you would expect, except / and %
DIV: Integer division truncates the decimal portion.
12 / 3
4
8/3
2
1/2
0
MOD: Gives the remainder of division of two integers:
7%2
1
11 % 4
3
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Example Using Numbers
# Program: dogYears.py
# Purpose: To calculate a dogs age in dogyears.
# We will write this in class.
ageOfDog = input("How old is the dog? ")
dogYears = 7*ageOfDog
print "The dog is", dogYears, "years old in dogyears."
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Strings
A line of text is called a string.
In Python, the string class is an ordered collection of characters.
An ordered collection may also be called a list or an array.
A string is indicated using quotes.
Example:
>>>"dog"
'dog'
Triple quotes allow newlines in the middle of a string.
Example:
>>> ' ' 'This string has a break
in the middle. ' ' '
'This string has a break\nin the middle.'
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String Methods
Python provides a variety of operations with strings:
Concatenation: +
Repetition: *
Length: len(stringName)
"dog" + "house" -> "doghouse"
"cool"*3
-> "coolcoolcool"
len("cool")
-> 4
Accessing a character by position: stringName[position]
>>>name = "Angelina"
>>>name[0]
'A'
>>>name[5]
'i'
Accessing a substring: stringName[first: last]
>>>name[1:6]
'ngeli'
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