Section 2.1. Program Development Cycle

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2.1. Program Development Cycle

We learned in the first chapter that hardware refers to the machinery in a computer system (such as the monitor, keyboard, and CPU) and software refers to a collection of instructions, called a program, that directs the hardware. Programs are written to solve problems or perform tasks on a computer. Programmers translate the solutions or tasks into a language the computer can understand. As we write programs, we must keep in mind that the computer will only do what we instruct it to do. Because of this, we must be very careful and thorough with our instructions. Note: A program is also known as a project, application, or solution.

Performing a Task on the Computer

The first step in writing instructions to carry out a task is to determine what the output should bethat is, exactly what the task should produce. The second step is to identify the data, or input, necessary to obtain the output. The last step is to determine how to process the input to obtain the desired output, that is, to determine what formulas or ways of doing things can be used to obtain the output.

This problem-solving approach is the same as that used to solve word problems in an algebra class. For example, consider the following algebra problem:

How fast is a car traveling if it goes 50 miles in 2 hours?

The first step is to determine the type of answer requested. The answer should be a number giving the speed in miles per hour (the output). (Speed is also called velocity.) The information needed to obtain the answer is the distance and time the car has traveled (the input). The formula

speed = distance/time

is used to process the distance traveled and the time elapsed in order to determine the speed. That is,

A pictorial representation of this problem-solving process is

We determine what we want as output, get the needed input, and process the input to produce the desired output.

In the following chapters we discuss how to write programs to carry out the preceding operations. But first we look at the general process of writing programs.

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Program Planning

A baking recipe provides a good example of a plan. The ingredients and the amounts are determined by what is to be baked. That is, the output determines the input and the processing. The recipe, or plan, reduces the number of mistakes you might make if you tried to bake with no plan at all. Although it's difficult to imagine an architect building a bridge or a factory without a detailed plan, many programmers (particularly students in their first programming course) try to write programs without first making a careful plan. The more complicated the problem, the more complex the plan may be. You will spend much less time working on a program if you devise a carefully thought out step-by-step plan and test it before actually writing the program.

Many programmers plan their programs using a sequence of steps, referred to as the program development cycle. The following step-by-step process will enable you to use your time efficiently and help you design error-free programs that produce the desired output.

1.	Analyze: Define the problem. Be sure you understand what the program should do, that is, what the output should be. Have a clear idea of what data (or input) are given and the relationship between the input and the desired output.
2.	Design: Plan the solution to the problem. Find a logical sequence of precise steps that solve the problem. Such a sequence of steps is called an algorithm. Every detail, including obvious steps, should appear in the algorithm. In the next section, we discuss three popular methods used to develop the logic plan: flowcharts, pseudocode, and top-down charts. These tools help the programmer break a problem into a sequence of small tasks the computer can perform to solve the problem. Planning also involves using representative data to test the logic of the algorithm by hand to ensure that it is correct.
3.	Choose the interface: Select the objects (text boxes, buttons, etc.). Determine how the input will be obtained and how the output will be displayed. Then create objects to receive the input and display the output. Also, create appropriate buttons and menus to allow the user to control the program.
4.	Code: Translate the algorithm into a programming language. Coding is the technical word for writing the program. During this stage, the program is written in Visual Basic and entered into the computer. The programmer uses the algorithm devised in Step 2 along with a knowledge of Visual Basic.
5.	Test and debug: Locate and remove any errors in the program. Testing is the process of finding errors in a program, and debugging is the process of correcting errors that are found. (An error in a program is called a bug.) As the program is typed, Visual Basic points out certain types of program errors. Other types of errors will be detected by Visual Basic when the program is executed; however, many errors due to typing mistakes, flaws in the algorithm, or incorrect use of the Visual Basic language rules can be uncovered and corrected only by careful detective work. An example of such an error would be using addition when multiplication was the proper operation.
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6.	Complete the documentation: Organize all the material that describes the program. Documentation is intended to allow another person, or the programmer at a later date, to understand the program. Internal documentation (comments) consists of statements in the program that are not executed, but point out the purposes of various parts of the program. Documentation might also consist of a detailed description of what the program does and how to use the program (for instance, what type of input is expected). For commercial programs, documentation includes an instruction manual and on-line help. Other types of documentation are the flowchart, pseudocode, and top-down chart that were used to construct the program. Although documentation is listed as the last step in the program development cycle, it should take place as the program is being coded.