CyberStars

While Loops

for loops are great when we know how many times we want to repeat. But sometimes we want to keep going as long as something is true, without knowing in advance how many iterations that means. That's the job of while

#include <stdio.h>

int main(void) {
    int i = 0;
    while (i < 10) {
        printf("%d\n", i);
        i++;
    }
    return 0;
}

Output: 0 through 9, one per line. Why doesn't 10 show up? Because 10 < 10 is false, so we exit before printing it

If we wanted 10 to be included

#include <stdio.h>

int main(void) {
    int i = 0;
    while (i <= 10) {
        printf("%d\n", i);
        i++;
    }
    return 0;
}

Now 10 <= 10 is true, so we go through one more time

while runs as long as the condition is true

Be very careful. If we forget to update i inside the loop, we get an infinite loop

#include <stdio.h>

int main(void) {
    int i = 0;
    while (i <= 100) {
        printf("%d\n", i);
        // forgot i++
    }
    return 0;
}

i stays 0, the condition is always true, and the program prints 0 forever. The platform stops it after 5 seconds. In real life, an infinite loop can freeze your entire computer

Whenever you write a while, ask yourself: "what makes this condition eventually become false?". If the answer is "nothing", you have a bug

When to choose for vs while?

  • for when the count is known ("do this 10 times", "walk every element of an array")
  • while when the stopping condition depends on something inside the loop ("keep going while there are still punch cards in the stack", "keep dividing the number until it's below 1")

Both are equally powerful: anything you can do with one, you can do with the other. Style and readability decide

C also has the do { ... } while (...) form, which runs the body at least once before checking the condition. We won't use it much in these lessons, but it's good to know it exists

Mission: The Teleprinter Countdown

At Bell Labs, before kicking off a compile run on the mainframe, the shift operator started a countdown on the teleprinter, so everyone in the computing center knew when the tape noise was about to begin.

Write a program that, inside main

  • declares an int named n
  • uses a while loop that prints the value of n, then decreases it by 1, as long as n is greater than 0
  • after the loop ends, prints Start

Example

For an n of 5, your program would print something like

5
4
3
2
1
Start