CS 1408 Intro to Computer Science with VB.NET
[pic] |Department of Computer
and
Mathematical Sciences |CS 1410
Intro to Computer Science with
C++ |8 | |
Lab 8: While Loops
Objectives:
The objective of this lab is to help understand the various categories of WHILE-LOOPS available in C++.
One aspect of programming that quickly becomes apparent to the beginner is the need to repeatedly execute the same set of instructions in a program, for a fixed number of times or even an indefinite number of times. For example, we may wish to allow the user of a program to repeat the same task as many times as he or she desires. Recall the operating system program FORMAT that we used in the first lab to format a disk. After formatting a disk, each time the program asks the user if he or she wants to format another disk. If the user answers "yes", then the program executes the same set of instructions to format the next disk. Or suppose we are faced with the problem of writing a program to process large quantities of information. Consider, for instance, a program that must generate the payroll checks for all employees in a large company. This program must repeatedly process the same type of information in the same way, namely, the pay rate, hours worked, etc. for each employee. To solve this problem efficiently, the same set of instructions must be executed for each employee RECORD (a record is a small collection of related data values). A mechanism in a programming language that allows the same set of instructions to execute multiple times is called a LOOP. C++ has many types of statements that implement loops, but the most basic and powerful of these statements is the WHILE-LOOP. The syntax for a WHILE-LOOP is as follows:
while (boolean expression)
one statement;
The reserved word in this statement is "while". When a WHILE-LOOP executes, the Boolean expression within the parentheses is evaluated. This expression is called the LOOP CONDITION. If it is TRUE, the statement within the loop executes. Otherwise, execution proceeds to the next statement in the program. Note that this is quite similar to an IF-ELSE statement. The big difference is this: After the statement within the loop executes, then the Boolean expression is evaluated again. If it is TRUE, the statement within the loop executes another time, then the Boolean expression is evaluated again, and so forth. Therefore this looping process continues until the Boolean expression finally evaluates as FALSE. Usually, it is necessary to include more than one statement in the body of a loop. This can be done by using a compound statement, called the LOOP BODY, as shown below:
while ( loop condition )
{
list of statements
}
Therefore, the entire list of statements within the body of this loop will execute over and over again, until the loop condition becomes FALSE.
Task 1: WHILE-LOOP statements
The purpose of this task is to illustrate SENTINEL-CONTROLLED, FLAG-CONTROLLED, and COUNT-CONTROLLED while-loops.
In practice, when loops are used in programs, the following general pattern of statements often appears:
Statements to initialize loop process
Statements to initialize loop condition
while ( loop condition )
{
Statements to perform loop process
Statements to update loop condition
}
The LOOP PROCESS is the key set of steps or instructions that the WHILE-LOOP is designed to repeat. Of course, the statements necessary to carry out this process will be contained within the body of the loop. Many times, it is necessary to place certain statements before the loop to prepare or initialize this process. Likewise, statements are usually needed before the loop to initialize the loop condition to TRUE. If the loop condition is not initialized to TRUE before the WHILE-LOOP is encountered, the statements within the body of the loop may never execute even one time. At the same time, statements must be placed within the body of the loop to update the loop condition. If the loop condition is initially TRUE and is never updated, then it will remain TRUE and the loop will continue executing indefinitely - it will never terminate. This is called an ENDLESS or INFINITE LOOP.
The style of the loop condition can be used to categorize WHILE-LOOPs. There are several categories of loops, including FLAG-CONTROLLED, SENTINEL-CONTROLLED, and COUNT-CONTROLLED loops. A SENTINEL-CONTROLLED loop continues executing until a special data value is input that signals all information has been processed and it's time to terminate (this special data value is called a SENTINEL). On the other hand, a FLAG-CONTROLLED loop is controlled by a special Boolean variable called a FLAG, which is initialized to TRUE before the loop begins executing. This type of loop continues executing until something happens within the loop body that causes the flag variable to become FALSE. A very common type of loop is a COUNT-COUNTROLLED loop. For this type of loop, the loop body executes a fixed number of times. Usually in a count-controlled loop, an integer variable called a COUNTER is used to keep track of how many times the loop body has executed. When the value stored in the counter reaches the fixed number of times the loop is supposed to execute, then the loop terminates.
Statement of the Problem: Create a C++ program called HiLo that will create a guessing game. The program will generate a random number between 1 and 100 for the user to guess. Then ask the user for a number as a guessing number. The program will determine the user’s guessing number as follows:
• If the guessing number is equal to the random number, output the message that the guessing number is correct and quit the game.
• If the guessing number is less than the random number, output a message “Too low.”, and ask if the user want to continue. If yes, ask the user for another guess.
• If the guessing number is greater than the random number, output a message “Too high.”, and ask if the user want to continue. If yes, ask the user for another guess.
Output a goodbye message.
After analyzing the above given problem, the steps of the algorithm can be designed as follows:
1. Generate a random number.
2. Prompt the user for an input as a guessing number.
3. Determine if the guessing number is
a. Equal to the random number: If it is, output a message that says the guessing is correct and quit the game.
b. Less than the random number: If it is,
i. Output a message “Too low,”
ii. Ask if the user wants to continue the game. If yes, ask for another guessing number; otherwise quit the game.
c. Greater than the random number: If it is,
i. Output a message “Too high,”
ii. Ask if the user wants to continue the game. If yes, ask for another guessing number; otherwise quit the game.
4. Output a goodbye message.
Activity 1.1: Use Visual Logic software to convert the above algorithm to a programming flowchart. Then verify your programming flowchart for correct results. Note that in Visual Logic, the fuction Random(100) generates a random number between 1 and 100.
[pic]
Activity 1.2: Each step of the above algorithm and the programming flowchart in Activity 1.1 can be converted to C++ code as follows.
1. Generate a random number.
/*****These statements generate a random integer from 0 to 100*****/
srand ( time(NULL) );
Number = rand()%100+1;
/******************************************************************/
2. Prompt the user for an input as a guessing number.
cout ................
................
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