Loops

Loops

Loops allow us to repeat parts of code several times.

They are very powerful when used with variables, as they can do different things depending on the variable.

The While Loop

while (COND) { ... }

The while loop will repeat its code block for as long as given condition is true.

int number = 3;
while (number > 0) {
    printf("%d\n", number);
    number = number - 1;
}
printf("Blast Off!");


3
2
1
Blast Off!

Every time the loop begins, the COND is evaluated.

If it is true - the loop will run.
Otherwise the program will go to the next code block.

int limit = 5;
int number = 6;

printf("Starting at %d\n", number);

while (number < limit) {                // this
    printf("Counted to %d\n", number);  // code
    number = number + 1;                // won't
}                                       // run

printf("Finished at %d\n", number);

The For Loop

for (INIT;COND;STEP) {...}

When using the while loop, we had to separately define a variable, a condition and step instruction.

int number = 3;             // INIT variable
while (number > 0) {        // COND check
    printf("%d\n", number);
    number = number - 1;    // STEP instruction
}
printf("Blast Off!");

With a for loop, we can combine everything together!

for (int number = 3; number > 0; number = number - 1) {
//   ^ INIT          ^ COND      ^ STEP
    printf("%d\n", number);
}
printf("Blast Off!");

3
2
1
Blast Off!

Write a program to print out the numbers 1-26, and their corresponding letters in the alphabet.

#include <stdio.h>

int main(void) {
    
    // WRITE YOUR CODE HERE

    return 0;
}

#include <stdio.h>

int main(void) {
    int i = 1;
    char c = 'A';

    while (i <= 26) {              // i = 1..26
        printf("%d - %c\n", i, c); // e.g "1 - A"
        i++;                       // Shorthand for i = i + 1
        c++;                       // Shorthand for c = c + 1
    }

    return 0;
}

When using for loops, we can also initialise multiple values, and perform several step instructions.

Note: All the variables must be the same data type!

#include <stdio.h>

int main(void) {
    for (int a = 1, b = 2; a < 5; a++, b += 2) {
        //                             ^ short for b = b + 2
        printf("%d", b);
    }

    return 0;
}

How many times does this loop run? A: Four times

What does the program output? 2 4 6 8

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