Array access

Learn to access and modify array elements by index.

Array access

You access array elements by index. Index starts at 0; for an array of length n the last index is n - 1.

int scores[5] = {85, 92, 78, 90, 88};
printf("%d\n", scores[0]);  // 85
printf("%d\n", scores[4]);  // 88
scores[2] = 80;  // change 78 to 80

If the index is out of bounds the behavior is undefined; that's why loops use i < n, not i <= n. You can both read and write an element.

Modifying elements

You can change an array element with =. The old value is lost, the new value is written to that index.

int nums[3] = {10, 20, 30};
nums[1] = 99;
printf("%d %d %d\n", nums[0], nums[1], nums[2]);
// Output: 10 99 30

Here nums[1] was 20; we set it to 99. The other elements stay unchanged. This is useful for updating elements based on conditions inside loops.

Exercise Modify an element and print

Write a C program. Declare int nums[3] = {10, 20, 30};. Change the middle element with nums[1] = 99;. Print the three values with printf("%d %d %d\n", nums[0], nums[1], nums[2]);. Put #include <stdio.h> at the top.

Show example solution

#include <stdio.h>

int main(void) {
    int nums[3] = {10, 20, 30};
    nums[1] = 99;
    printf("%d %d %d\n", nums[0], nums[1], nums[2]);
    return 0;
}

Test yourself

Access the first element

printf("%d
", scores[____]);

Show answer

printf("%d
", scores[0]);

What's the last index of scores[5]?
```
int scores[5] = {1,2,3,4,5};
```
- 0
- 5
- 4
- 6
Show answer

4 — A 5-element array's last index is 4.
Match the index concepts

scores[0]

First element
scores[4]

Last element (size 5)
nums[1] = 99

Modification
scores[5]

Out of bounds

Next lesson: Array size

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