C Arrays Introduction

An array is a row of boxes that hold the same kind of value, each with a number starting at 0. It is like a row of lockers you open by number.

In this page:

What is an Array?
Declaring and Initializing Arrays
Accessing Array Elements
Modifying Array Elements
Looping Through Arrays

Syntax

data_type array_name[size];

data_type array_name[size] = {value1, value2, value3};

array_name[index];  // access an element

What is an Array?

Because all elements sit in one contiguous memory block, the compiler can compute any element's address instantly from the array's starting address and the element's index, which is what makes array access O(1) rather than requiring a search.

Example: What is an Array?

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	int numbers[3] = {10, 20, 30};
	// Print formatted text to the screen
	printf("%d", numbers[0]);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Declaring and Initializing Arrays

The number of values inside the braces determines the array's size if you omit it explicitly, e.g. int nums[] = {1,2,3} creates an array of exactly 3 elements.

Leaving elements uninitialized when a size is given leaves them with unpredictable garbage values, not zero.

Example: Declaring and Initializing Arrays

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	int nums[] = {1, 2, 3};
	// Print formatted text to the screen
	printf("%d", nums[2]);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Accessing Array Elements

C performs no bounds checking, so reading or writing arr[10] on a 5-element array compiles fine but corrupts nearby memory or crashes at runtime -- always keep track of an array's declared size separately if you need to check bounds yourself.

Example: Accessing Array Elements

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	int arr[5] = {1, 2, 3, 4, 5};
	// Print formatted text to the screen
	printf("%d", arr[2]);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Modifying Array Elements

Assigning to arr[i] = value overwrites only that single memory slot; the rest of the array is untouched. This is how you build up array contents incrementally, such as filling an array with user input one entry at a time inside a loop.

Example: Modifying Array Elements

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	int arr[3] = {1, 2, 3};
	arr[1] = 99;
	// Print formatted text to the screen
	printf("%d", arr[1]);
	// Return 0 to signal that the program finished successfully
	return 0;
}

Looping Through Arrays

A typical traversal loop runs from index 0 to size-1 inclusive -- looping to i <= size instead of i < size is one of the most common off-by-one bugs in C, since it reads one element past the array's actual bounds.

Example: Looping Through Arrays

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	int arr[5] = {1, 2, 3, 4, 5};
	// Loop: repeat while the condition holds
	for (int i = 0; i < 5; i++) {
		// Print formatted text to the screen
		printf("%d ", arr[i]);
	}
	// Return 0 to signal that the program finished successfully
	return 0;
}

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