AN EXAMPLE OF A PROBLEM INVOLVING
THE SELECTION CONTROL STRUCTURE

The Situation

You want a program that can identify positive and negative numbers and then display the words "positive" or "negative" accordingly. Your program must also deal with zero separately, displaying the word "zero" if it is entered. After displaying the appropriate word, the program should skip a line and display the words "To try another number, run this program again.".

Analysis:

The analysis and documentation for this problem appears below followed by the C++ source code.

Constraints:

• This example will produce screen output that matches the format defined in the sample softcopy screens on the following page. More than one sample is provided because programs that make decisions often have more than one possible outcome. A sample of each type of outcome should be provided.
• This example was not developed using multiple modules. It was written as one main function.
• To simplify this example, the program's displayed introduction consists of only the single line "Sign Notification Program" followed by a blank line as shown in the sample softcopy.
• The algorithm has been documented graphically as a flowchart.
• Multiple desk checks have been performed on the algorithm for at least the values shown in the sample output screens to ensure that all branches (a.k.a. legs) of the algorithm have been tested. Notice that the tracing charts include extra columns to track the results of the tests performed by the program.

SOLUTION

The following documentation is intended to serve as an example of what would be expected from an analyst when designing a program related to the situation described above. All emphasized text below serves only to clarify the documentation for students, but would not actually appear in the real documentation.

PROBLEM STATEMENT:

Request and store a number and then display a word indicating its sign. If the number is positive, display the word "positive". If it is negative, display the word "negative". If it is zero, display the word "zero". After displaying the appropriate word, the program should skip a line and display the words "To try another number, run this program again.".

SAMPLE SOFTCOPY

Programs that involve selection of alternative actions will always have more than one (and often many) possible results. The output produced by a program can differ drastically depending on the decisions it makes. Because of this, samples of the results expected from each decision must be demonstrated separately by providing multiple sample outputs; one for every possible path through the flowchart. These sets of possible program reactions are sometimes referred to as "scenarios". Program designers are required to determine multiple sets of sample input data that will result in each path being demonstrated, and then document each of these in separate samples for each scenario as shown below.

Note: The first two lines are the "Introduction". Bracketed items indicate user input.

Scenario 1 - Entry of a value of zero:

Scenario 2 - Entry of a positive number:

Scenario 3 - Entry of a negative number:

SYMBOLIC CONSTANT LIST: (NONE)

VARIABLE LIST:

FLOWCHART (Graphic Algorithm) for the program "SIGN":

A flowchart is a graphic algorithm that helps to illustrate the flow of control of a program as the processor would advance from one step to another, often in simple sequential order, but sometimes following more complex structures. In this case the processor must use its ability to compare and recognize relationships between data such as greater, less than, or equal to, and make a decision about which step(s) to process next. As long as the order of execution of steps continues to proceed toward the end of the program without branching backwards to a previous step, then the structure being followed is known as selection. For more information about what each shape below represents, view the web page about Flowcharting Symbols & Guidelines.

ALGORITHM (AS AN OUTLINE)

(The outline below would be redundant if a flowchart is provided, but is added in this example to show an optional algorithm format)

a. Start
b. Display the Introduction (see Sample Softcopy).
    b1. Display "Sign Notification Program" message and return.
    b2. Display a blank line.
c. Request and store N
    c1. Prompt on screen with "Enter a number to analyze: ".
    c2. Store keyed-in response in N.
d. If N is zero:
    then: display "That number is zero." message and return
    otherwise:
        If N is greater than zero:
            then: display "That is a positive number." message and return
            otherwise: display "That is a negative number." message and return.
e. Display a blank line.
f. Display the words "To try another number, run this program again."
g. End

DESK CHECK:

Tracing Chart for Scenario 1 - an entry of zero:

Test Softcopy for Scenario 1 - an entry of zero:

Separate pairs of documents (Tracing Chart and Test Output) would be provided to prove that the flowchart worked for each scenario.

Blended Desk Check Document for Scenario 2 - An entry greater than zero:

Blended Desk Check Document for Scenario 3 - An entry less than zero:

C++ SOURCE CODE:

/* sign.cpp - an example of the selection structure */
/* Written by: Randolph Gibson - Date: Sept. 11, 2011 */

#include <iostream>
using namespace std;

int N; /* A number to be evaluated */

int main ()

{
  cout << "Sign Notification Program\n\n";
  cout << "Enter a number to analyze: ";
  cin >> N;

  if (N == 0) cout << "That number is zero.\n";
  else if (N > 0) cout << "That is a positive number.\n";
       else cout << "That is a negative number.\n";

  cout << "\nTo try another number, run this program again.\n";
  return 0;   /* Return zero error code to parent process */
}