Arrays in C Explained Visually: Index, Read and Update

Start with working C code

Five integers occupy indexes 0 through 4. Before the assignment, scores[1] is 20; afterward its value is 25.

#include <stdio.h>
int main(void) {
    int scores[5] = {10, 20, 30, 40, 50};
    scores[1] = 25;
    printf("%d\n", scores[1]);  /* prints 25 */
    return 0;
}

What is an array in C?

An array is a fixed-length sequence of elements of the same type. For int scores[5], legal indexes are 0 through 4. scores[0] names the first element; scores[4] names the fifth.

Elements are stored contiguously. Each valid index identifies an element directly, so accessing it requires constant-time address calculation. The diagram uses logical boxes, not actual operating-system memory addresses.

Why array bounds matter

Unlike some languages, ordinary C array indexing does not automatically check bounds. Accessing scores[5] or scores[-1] is an out-of-bounds access and has undefined behavior. It might appear to work or it might crash, but neither result makes it safe.

When iterating over all five elements, use for (int i = 0; i < 5; ++i). Compare the index and value visually to understand which element each iteration reads.

Practice with indexing

1. Find the element at index 3 and predict its value before changing anything.
2. Change the element at index 1 to 25, then read it again.
3. Explain why scores[5] is invalid, even though the array contains five elements.