Loop Design with While Loops

Chapter 5

Why Is Repetition Needed?

• Repetition allows efficient use of variables.
• We can input, add, and average multiple numbers using a limited number of variables and lines of code.
• For example, to add five numbers:
  • Declare a variable for each number, input the numbers and add the variables together.
  • Create a loop that reads a number into a variable and adds it to a variable that contains the sum of the numbers.

While Loops

Note: One example in this video is summing the numbers from 1 to 10, in which the program outputs the result of 45. However, the actual sum of the numbers from 1 to 10 is 55! See if you can find the bug in the example code.

• The syntax of the while statement is very similar to the if statement:

  while (expression)
  {
      statement;
  }

• The expression acts as a decision-maker and is evaluated as a logical expression.

• The statement can be simple or compound (i.e., multiple statements surrounded by {}).

• The statement is called the body of the loop.

• The parentheses are part of the syntax.

There are a few ways to use a while loop, which we will discuss below.

Loop Design

There are counter-, sentinel-, and flag-controlled loops.

Counter-Controlled While Loop

Here is an example where you do stuff 15 times.

int counter = 0; // set a counter to 0
while (counter < 15) // repeat the while counter is less than 15.
{
    //  Do stuff here...
    counter++; // update by one each time the loop iterates
}

In this example, counter is called the loop-control variable.

Common mistakes that cause infinite loops (loops that continue to execute endlessly):

• Make sure to include a statement in the loop body that will eventually make the test condition false.

  • Make sure to update your loop-control variable inside the loop, so the loop will exit.
  • If you have an infinite loop, press Ctrl+C to exit your program.

• Make sure you initialize the counter before the while loop or the value may be incorrect.

• Don’t put a semicolon after the test condition.

  while (counter < 15); // Infinite loop, because of the ; placement

Sentinel-Controlled While Loop

A sentinel value is used to determine if the loop should end.

Usually, this is given to you by the user (from a file).

int num = 0;
cout << "Enter numbers to add until you are finished"
    << ", them input -1" << endl;
cin >> num;

while (num != -1) // -1 is the sentinel value causing the program to exit.
{
    num += num;
    cin >> num;
}

Flag–Controlled While Loop (very similar to sentinel)

If you are looking for something and can quit once you find it, you can set a Boolean value to false and when you find what you are looking for, you set the value to true, and the loop exits.

Here is some sample code:

bool done = false;

while (!done) // done is the flag variable. (!done is preferred over done == false)
{
    //Do something...;
    if (found what you wanted to)
    {
        done = true;
    }
    //Do something else...;
}

EOF-Controlled While Loop

A use case for a sentinel-controlled loop is when reading from a file. See Reading from Files with Loops for more information.

Other Expressions

The expression in a while statement can be complex. For example,

while ((noOfGuesses < 5) && (!isGuessed))
{
    . . .
}