Creating Computer Programs

lesson 27

This lesson includes the following sections:

• What is a Computer Program? • How Programs Solve Problems

• Two Approaches: Structured & Object-Oriented Programming

• Files

• Hardware/Software Interaction

What is a Computer Program?

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.

• 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.

What is a Computer Program? - Hardware/Software Interaction

• Program Control Flow

• Algorithms

• Heuristics

• Common Flow Patterns

• Variables and Functions

How Programs Solve Problems

• 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.

How Programs Solve Problems – Program Control Flow

This flowchartshows that thesequence canvary dependingon conditions.

• 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.

How Programs Solve Problems - Algorithms

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• 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 - Heuristics

• 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 - Common Flow Patterns

• 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.

How Programs Solve Problems - Variables and Functions

• Structured programming

• 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:

Two Approaches: Structured & Object-Oriented Programming

This type of programming has fallen into disfavor.

• 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.

Structured programming uses three types of control structures to make program control flow more predictable:

Two Approaches: Structured & Object-Oriented Programming - Structured 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.

Two Approaches: Structured & Object-Oriented Programming - Object-Oriented Programming

• 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.

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