Arrays in C

Introduction

An array holds multiple values of one type in a single, contiguous block of memory. You choose the length when you declare it, then reach each slot with a zero-based index. That layout makes reads and updates fast and predictable.

Purpose of this lesson

Note: The sizeof trick stops working once an array argument decays to a pointer inside a function — pass the length explicitly in that case.

Understanding the topic

Declaring and filling arrays Creating an array means picking a type, a name, and how many slots you need — then optionally assigning starting values.

• Declaration — tell the compiler the type and capacity so it can reserve space.
• Initialization — supply starting values; any slot you skip is zero-filled.
• When values are provided at once, the compiler can infer the length for you.
• If you only initialize part of the array, the rest become zero.
• Reading with an index — Pick any slot instantly with bracket notation — the index counts from zero.
• Assigning a new value — Overwrite a slot by combining the index with the assignment operator.
• Walking through an array — Use a loop to visit each position — forward, backward, or with a step size.
• How many elements? — Divide the array's byte size by one element's byte size — but only where the array itself is in scope.

Step-by-step explanation

1. Declaring and filling arrays — Creating an array means picking a type, a name, and how many slots you need — then optionally assigning starting values.
2. Reading with an index — Pick any slot instantly with bracket notation — the index counts from zero.
3. Assigning a new value — Overwrite a slot by combining the index with the assignment operator.
4. Walking through an array — Use a loop to visit each position — forward, backward, or with a step size.
5. How many elements? — Divide the array's byte size by one element's byte size — but only where the array itself is in scope.

Syntax reference

Syntax reference:

elementType name[count];
elementType name[] = { v0, v1, ... };

Informative example

Example program:

#include <stdio.h>

int main(void) {
    int scores[] = {90, 85, 78, 92, 88, 95};
    int len = sizeof(scores) / sizeof(scores[0]);

    for (int i = 0; i < len; i++) {
        printf("%d ", scores[i]);
    }
    return 0;
}

90 85 78 92 88 95

Execution workflow

Code examples

#include <stdio.h>

int main(void) {
    int values[5] = {10, 20, 30, 40, 50};
    printf("%d ", values[2]);
    printf("%d ", values[4]);
    printf("%d ", values[0]);
    return 0;
}

30 50 10

#include <stdio.h>

int main(void) {
    int values[5] = {10, 20, 30, 40, 50};
    values[0] = 99;
    printf("%d", values[0]);
    return 0;
}

99

#include <stdio.h>

int main(void) {
    int values[5] = {10, 20, 30, 40, 50};
    printf("Forward:\n");
    for (int i = 0; i < 5; i++)
        printf("%d ", values[i]);
    printf("\nReverse:\n");
    for (int i = 4; i >= 0; i--)
        printf("%d ", values[i]);
    return 0;
}

Forward:
10 20 30 40 50 
Reverse:
50 40 30 20 10

#include <stdio.h>

int main(void) {
    int values[5] = {10, 20, 30, 40, 50};
    int count = sizeof(values) / sizeof(values[0]);
    printf("%d", count);
    return 0;
}

5

Real-world use

Related topics:

• Multidimensional arrays for tables and grids
• Pointers — closely tied to how arrays behave in expressions
• Parameter passing when sharing arrays with functions

Best practices

• Enable warnings: gcc -Wall -Wextra -std=c11 source.c -o app
• Give every variable a defined value before it is read.
• Stay inside array bounds — C will not stop you from over-running a buffer.

Common mistakes

• Reading uninitialized storage — behavior is undefined.
• Dismissing compiler warnings instead of fixing root causes.
• Ignoring NULL returns from malloc, fopen, and similar APIs.

Hands-on exercise

Practice problems:

• Locate the largest value in an integer array
• Add up every element and print the total
• Reverse elements in place
• Insert a value at a chosen index (shift right)
• Remove an item and close the gap
• Rotate elements left by k steps
• Return dynamically allocated data from a helper
• Sort numbers ascending with a simple algorithm
• Find two indices whose values sum to a target

Summary

Arrays bundle same-type data in order, support indexed access, and are usually processed with loops; measure length with sizeof while the array variable is still in scope.