Array access

Learn to access and modify array elements by index.

Array access

To access array elements you write the index inside square brackets. nums[0] gives the first element, nums[2] the third. Because indices start at zero, nums[i] actually represents the (i+1)th element.

The same syntax lets you change an element: nums[1] = 99; sets the second element to 99. Using an index beyond the array size (such as nums[10] on a five-element array) causes undefined behavior, so keep the index under control.

Modifying arrays

You can change array contents by assigning to its elements. Writing nums[1] = 99; changes only that element; others stay the same. This lets you update the array from a single point.

You can rewrite all elements by assigning in a loop. For example for (int i = 0; i < 5; i++) nums[i] = 0; zeroes every element. Because the array size is fixed, memory management is automatic; you do not need new or delete.

Exercise Change the second element and print it

The nums array is set to {10, 20, 30, 40, 50}. Change the second element with nums[1] = 99; and print it with std::cout << nums[1];. The output must be 99.

Show example solution

#include <iostream>

int main() {
    int nums[5] = {10, 20, 30, 40, 50};
    nums[1] = 99;
    std::cout << nums[1] << std::endl;
    return 0;
}

Test yourself

Write the third element

int nums[5] = {10, 20, 30, 40, 50};
std::cout << ____;

Show answer

int nums[5] = {10, 20, 30, 40, 50};
std::cout << nums[2];

Match each access pattern to its meaning

nums[0]

First element
nums[1]

Second element
nums[4]

Last element (5 elements)
nums[i]

i-th index
What does nums[3] = 99; do?
```
int nums[5] = {10,20,30,40,50};
nums[3] = 99;
```
- Reads the third element
- Divides the array by 3
- Sets the fourth element to 99
- Sets the total to 99
Show answer

Sets the fourth element to 99 — nums[3] = 99; sets the fourth element to 99 (index 3).

Next lesson: std::array

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