`for` loops

Table of contents

for loops

A for loop repeats a block of code while a condition is true. We normally use a for loop when we know roughly how many times the loop needs to run, or when working through an array.

For example, this loop prints the numbers from 0 to 4:

for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}

Structure of a `for` loop

A for loop contains three expressions:

for (initialisation; condition; update) {
    // Code to repeat
}

In this example:

for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}

the three expressions are:

int i = 0    // Initialisation
i < 5        // Condition
i++          // Update

The loop then runs in this order:

int i = 0 runs once, before the loop starts.
i < 5 is checked before each iteration.
The loop body runs if the condition is true.
i++ runs after the loop body.
The condition is checked again.

Once i reaches 5, the condition i < 5 is false and the loop stops.

Counting upwards

This program prints the numbers from 1 to 5:

#include <stdio.h>

int main(void)
{
    for (int count = 1; count <= 5; count++) {
        printf("%d\n", count);
    }

    return 0;
}

Here the condition uses <= because we want the loop to include 5.

Compare this with:

for (int count = 1; count < 5; count++) {
    printf("%d\n", count);
}

This version prints only 1, 2, 3 and 4.

Counting downwards

The update does not have to increase the counter. We can use -- to count down:

#include <stdio.h>

int main(void)
{
    for (int count = 5; count > 0; count--) {
        printf("%d\n", count);
    }

    printf("Go!\n");

    return 0;
}

The counter starts at 5 and decreases after each iteration. Once it reaches 0, the condition is false and the loop stops.

Changing the step size

We can also increase or decrease the counter by more than one each time:

for (int value = 0; value <= 10; value += 2) {
    printf("%d\n", value);
}

This prints:

Here, value += 2 increases value by two after each iteration.

Processing an array

for loops are commonly used to process arrays:

#include <stdio.h>

int main(void)
{
    int values[] = { 10, 20, 30, 40, 50 };
    size_t length = sizeof(values) / sizeof(values[0]);

    for (size_t i = 0; i < length; i++) {
        printf("values[%zu] = %d\n", i, values[i]);
    }

    return 0;
}

Array indices start at zero, so the loop also begins at zero:

size_t i = 0;

The condition is:

i < length;

It is not:

i <= length;

For an array with five elements, the valid indices are 0 to 4. If the loop reaches index 5, it accesses memory beyond the end of the array.

Calculating a total

We can use a for loop to add all the values in an array:

#include <stdio.h>

int main(void)
{
    int values[] = { 1, 2, 3, 4, 5 };
    size_t length = sizeof(values) / sizeof(values[0]);
    int total = 0;

    for (size_t i = 0; i < length; i++) {
        total += values[i];
    }

    printf("Total = %d\n", total);

    return 0;
}

The variable total must be initialised to zero before the loop. Each iteration adds one array element to total.

After the loop, total contains the sum of all five elements.

Scope of the loop variable

When the counter is declared inside the for statement, it only exists within the loop:

for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}

printf("%d\n", i);    // Error: i is no longer in scope

This is normally what we want, as i is only being used to control the loop.

If the value is needed after the loop, declare it beforehand:

int i;

for (i = 0; i < 5; i++) {
    printf("%d\n", i);
}

printf("Final value: %d\n", i);

After this loop, i is 5.

`break` and `continue`

As with a while loop, break stops the loop immediately:

for (int i = 1; i <= 10; i++) {
    if (i == 8) {
        break;
    }

    printf("%d\n", i);
}

This prints the numbers from 1 to 7. When i reaches 8, the break statement stops the loop.

The continue statement skips the rest of the current iteration:

for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        continue;
    }

    printf("%d\n", i);
}

This prints the numbers from 1 to 10, except for 5.

In a for loop, continue does not skip the update. In this example, i++ still runs before the condition is checked again.

Common mistakes

Off-by-one errors

An off-by-one error occurs when a loop runs once too many or once too few.

For an array of length 5, this is correct:

for (int i = 0; i < 5; i++) {
    printf("%d\n", values[i]);
}

This is incorrect:

for (int i = 0; i <= 5; i++) {
    printf("%d\n", values[i]);
}

The second loop tries to access values[5], but the final valid element is values[4].

If you are like me, you will make this mistake more often than you would like!

Using the wrong initial value

This loop starts at index 1:

for (int i = 1; i < length; i++) {
    printf("%d\n", values[i]);
}

It therefore skips the first element, values[0].

When processing an entire array, the counter will normally start at zero.

Using the wrong update

This loop never finishes:

for (int i = 0; i < 5; i--) {
    printf("%d\n", i);
}

The condition requires i to reach 5, but i-- moves it in the opposite direction.

Check that the update is actually moving the counter towards the point where the loop will stop.

Changing the counter inside the loop

Avoid changing the counter again inside the loop body:

for (int i = 0; i < 10; i++) {
    i += 2;
    printf("%d\n", i);
}

The counter is changed by both i += 2 and the i++ in the for statement. This makes the sequence harder to follow and can cause values to be skipped.

If you want the counter to change by a different amount, put that in the for statement:

for (int i = 0; i < 10; i += 3) {
    printf("%d\n", i);
}

Choosing between `for` and `while`

Use a for loop when you can describe the starting value, stopping condition and update in one line:

for (int i = 0; i < 10; i++) {
    // ...
}

Use a while loop when the number of iterations depends on something that happens while the program is running:

while (button_pressed) {
    // ...
}

In many cases, either loop would work. Choose the one that makes it clearest when and why the loop will stop.

for loops

Structure of a for loop