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lesson 27
Creating Computer
Programs
This lesson includes the following sections:
• What is a Computer Program?
• How Programs Solve Problems
• Two Approaches: Structured &
Object-Oriented Programming
What is a Computer Program?
•
Files
•
Hardware/Software Interaction
What is a Computer Program? - Files
Typically, a program is stored as a collection of files.
Some common file types used in programs are:
•
Executable (.EXE) files actually send commands to
the processor.
•
Dynamic Link Library (.DLL) files are partial .EXE
files.
•
Initialization (.INI) files contain configuration
information for a program.
•
Help (.HLP) files contain information for the user.
What is a Computer Program? Hardware/Software Interaction
• The program tells the CPU to process interrupts, or
sets of steps the CPU must follow to perform a task.
• To control hardware, a program must be written in
binary numbers (1s and 0s). This code is called
machine code or machine language.
• Programmers use programming languages to write
code in nearly human language. The resulting
description is called source code.
• Compilers and interpreters translate a program into
object code, the binary version of source code.
How Programs Solve Problems
•
Program Control Flow
• Algorithms
•
Heuristics
•
Common Flow Patterns
• Variables and Functions
How Programs Solve Problems –
Program Control Flow
•
The order in which program statements are executed
is called program control flow.
•
To determine program control flow, programmers
may use a flowchart to map the program's sequence.
•
Programmers may also create a simple text version of
a program's code – called pseudocode – to determine
how the program will flow.
This flowchart
shows that the
sequence can
vary depending
on conditions.
How Programs Solve Problems - Algorithms
•
An algorithm is a set of steps that always lead to a
solution. The steps are always the same, whether the
problem is being solved manually or with a PC.
•
A computer program may contain thousands of
algorithms, each one devoted to a single task.
•
An algorithm, for example, will find the highest point
in a mountain range by comparing all the points until
the highest one is found.
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How Programs Solve Problems - Heuristics
•
If a problem is too complex to be solved by an
algorithm, a programmer may try to solve it by using
heuristics.
•
Heuristics are like algorithms, and will always find a
solution to a problem. But unlike algorithms,
heuristics are not guaranteed to find the best possible
solution.
•
A heuristic, for example, may find the highest point in
a mountain range by comparing random points, but
this process may never find the highest one.
How Programs Solve Problems Common Flow Patterns
•
To determine when and where to pass program
control, a developer may use conditional statements or
loops.
•
A conditional statement determines whether a
condition is true. If so, control flows to the next part
of the program.
•
A loop repeats again and again until a condition is
met. Control then passes to another part of the
program.
How Programs Solve Problems Variables and Functions
•
A variable is a named placeholder for data that is
being processed. Programs contain variables to hold
inputs from users, for example.
•
A function is a set of steps that are followed to
perform a specific task. By assembling a collection of
functions together, a developer can build a complete
program.
Two Approaches: Structured &
Object-Oriented Programming
Early programmers allowed control to pass from one part
of a program to another by using goto statements.
Control would "go to" a different part of the program
when conditions allowed.
Goto statements cause programs to become very complex.
To eliminate their use, programmers developed two
approaches to development:
•
Structured programming
•
Object-oriented programming
This type of programming has fallen into disfavor.
Two Approaches: Structured & Object-Oriented
Programming - Structured Programming
Structured programming uses three types of control
structures to make program control flow more
predictable:
•
Sequence structure defines the default control flow.
•
Selection structures are built around conditional
statements.
•
Repetition (looping) structures use loops, which
execute according to the results of conditional
statements.
Two Approaches: Structured & Object-Oriented
Programming - Object-Oriented Programming
•
In object-oriented programming (OOP), programs are
built from blocks of code, called objects. Each object
has functions and characteristics (attributes), and can
contain (encapsulate) other objects.
•
Objects that share common attributes can be grouped
into classes. Classes can be divided into subclasses.
•
In OOP, objects communicate with one another by
exchanging messages.
lesson 27 Review
• Define the term computer program.
• Describe the use of flowcharts and pseudocode in
programming.
• Identify two ways in which a program can work
toward a solution.
• Differentiate the two main approaches to computer
programming.
• List and describe three elements of object-oriented
programming.