Mastering C Arrays: Declaration, Initialization, and Common Operations

Mastering C Arrays

Discover how to declare, initialize, and manipulate C arrays with practical examples, best‑practice tips, and safety guidelines.

In C, an array is a contiguous block of memory that stores multiple values of the same type. For example, to hold 100 integers you would write:

int data[100];

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Declaring an Array

Use the syntax:

dataType arrayName[arraySize];

Example:

float mark[5];

Here, mark can store five float values. Note that the size and type are fixed after declaration.

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Accessing Elements

Array indices start at 0. For an array of size n, the last element is array[n-1]. Example:

mark[0]  // first element
mark[4]  // last element for size 5

Key Points:

• First index is 0, not 1.
• For float (4 bytes), each subsequent index increments the address by 4.
• Never access beyond array[n-1] to avoid undefined behavior.

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Initializing an Array

Initialize at declaration:

int mark[5] = {19, 10, 8, 17, 9};

Or let the compiler deduce the size:

int mark[] = {19, 10, 8, 17, 9};

Resulting values:

mark[0] = 19
mark[1] = 10
mark[2] = 8
mark[3] = 17
mark[4] = 9

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Modifying Elements

int mark[5] = {19, 10, 8, 17, 9};
mark[2] = -1;  // third element
mark[4] = 0;   // fifth element

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Input & Output

Reading into an array element:

scanf("%d", &mark[2]);          // third element
scanf("%d", &mark[i-1]);        // i-th element (1‑based)

Printing an element:

printf("%d", mark[0]);        // first
printf("%d", mark[2]);        // third
printf("%d", mark[i-1]);      // i‑th

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Example 1: Array Input/Output

#include <stdio.h>

int main() {
    int values[5];

    printf("Enter 5 integers: ");
    for (int i = 0; i < 5; ++i) {
        scanf("%d", &values[i]);
    }

    printf("Displaying integers:\n");
    for (int i = 0; i < 5; ++i) {
        printf("%d\n", values[i]);
    }
    return 0;
}

Output

Enter 5 integers: 1
-3
34
0
3
Displaying integers:
1
-3
34
0
3

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Example 2: Calculate Average

#include <stdio.h>

int main() {
    int marks[10], n, sum = 0;

    printf("Enter number of elements: ");
    scanf("%d", &n);

    for (int i = 0; i < n; ++i) {
        printf("Enter number%d: ", i + 1);
        scanf("%d", &marks[i]);
        sum += marks[i];
    }

    int average = sum / n;
    printf("Average = %d", average);
    return 0;
}

Output

Enter n: 5
Enter number1: 45
Enter number2: 35
Enter number3: 38
Enter number4: 31
Enter number5: 49
Average = 39

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Beware of Out‑of‑Bounds Access

Accessing testArray[12] when the array has only 10 elements leads to undefined behavior—crashes, corrupted data, or silent errors. Always validate indices before accessing.

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Multidimensional Arrays

So far we’ve covered one‑dimensional arrays. In the next tutorial, we’ll explore multidimensional arrays (arrays of arrays) and how to use them effectively.