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while

The simplest of the three is the while loop. It looks like this:

while (condition)
{
  do something
}

The condition (for example, (a > b)) is evaluated every time the loop is executed. If the condition is true, then statements in the curly brackets are executed. If the condition is false, then those statements are ignored, and the while loop ends. The program then executes the next statement in the program.

The condition comes at the start of the loop, so it is tested at the start of every pass, or time through the loop. If the condition is false before the loop has been executed even once, then the statements inside the curly brackets will never be executed. (See do...while, for an example of a loop construction where this is not true.)

The following example prompts the user to type in a line of text, and then counts all the spaces in the line. The loop terminates when the user hits the <RET> key and then prints out the number of spaces. (See getchar, for more information on the standard library getchar function.)

#include <stdio.h>

int main()
{
  char ch;
  int count = 0;

  printf ("Type in a line of text.\n");

  while ((ch = getchar()) != '\n')
  {
    if (ch == ' ')
    {
      count++;
    }
  }

  printf ("Number of spaces = %d.\n\n", count);
  return 0;
}