C Programming — Arrays Complete Guide

1

What is an Array — and Why Use One?

0 – 8 min

Without arrays, storing 100 student marks would need 100 separate variables: int m1, m2, m3, ... m100; — and you could not loop through them. An array stores multiple values of the same type under one name, in a contiguous block of memory.

Same type — all elements must be int, or all float, or all char, etc.
Contiguous memory — all elements sit next to each other in RAM — no gaps
Fixed size — the size is set at declaration and cannot change
Index starts at 0 — first element is arr[0], last is arr[n-1]

Without array vs with array — storing 5 marks

❌ 5 variables

85

m1

92

m2

78

m3

96

m4

88

m5

Cannot loop through!

✅ 1 array

85

[0]

92

[1]

78

[2]

96

[3]

88

[4]

Loop with for(i=0;i<5;i++)

2

Declaring, Initialising & Indexing

8 – 22 min

Method	Syntax	What happens
Declare only	`int scores[5];`	5 ints reserved — contain garbage
Declare + initialise	`int scores[5] = {85,92,78,96,88};`	5 ints set to given values
Auto-size	`int scores[] = {85,92,78};`	Compiler counts — size = 3
Partial init	`int scores[5] = {85,92};`	First 2 set, remaining filled with 0
Zero all	`int scores[5] = {0};`	All 5 elements set to 0

int scores[5] = {85, 92, 78, 96, 88} — memory layout

scores[ ]

85

[0]

92

[1]

78

[2]

96

[3]

88

[4]

0x10000x10040x10080x100C0x1010

Each int takes 4 bytes → addresses advance by 4

Declaring, initialising, and reading arrays

array_basics.c

C

#include <stdio.h>

int main() {

    // Method 1: declare then assign one by one
    int ages[4];
    ages[0] = 20;
    ages[1] = 25;
    ages[2] = 19;
    ages[3] = 30;

    // Method 2: declare and initialise together
    int   scores[] = {85, 92, 78, 96, 88};
    float prices[] = {9.99, 24.50, 4.75};
    char  grade[]  = {'A', 'B', 'C', 'F'};

    // Access individual elements by index
    printf("First score:  %d\n", scores[0]);  // 85
    printf("Third score:  %d\n", scores[2]);  // 78
    printf("Last score:   %d\n", scores[4]);  // 88

    // Modify an element
    scores[2] = 95;
    printf("Updated [2]:  %d\n", scores[2]);  // 95

    // sizeof trick to get number of elements
    int n = sizeof(scores) / sizeof(scores[0]);
    printf("Number of elements: %d\n", n);   // 5

    return 0;
}

terminal

output

First score:  85
Third score:  78
Last score:   88
Updated [2]:  95
Number of elements: 5

⚠️ C does NOT check array bounds — ever!
scores[10] on a 5-element array gives no error — it silently reads or writes random memory. This is one of the most dangerous bugs in C. Always keep your index between 0 and n-1. Use sizeof(arr)/sizeof(arr[0]) to always get the correct count.

3

Array Operations — Loop, Sum, Min, Max

22 – 36 min

The real power of arrays comes when paired with loops. A for loop lets you process every element with the same code — whether there are 5 elements or 5000, the loop looks identical.

Sum, average, min, max — all array fundamentals

array_operations.c

C

#include <stdio.h>

int main() {
    int arr[] = {64, 25, 12, 92, 43, 37};
    int n     = sizeof(arr) / sizeof(arr[0]);

    // ── Print all elements ──
    printf("Array: ");
    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
    printf("\n");

    // ── Sum and average ──
    int sum = 0;
    for (int i = 0; i < n; i++)
        sum += arr[i];
    printf("Sum:     %d\n", sum);
    printf("Average: %.1f\n", (float)sum / n);

    // ── Find maximum ──
    int max = arr[0];   // assume first is max
    for (int i = 1; i < n; i++)
        if (arr[i] > max) max = arr[i];
    printf("Maximum: %d\n", max);

    // ── Find minimum ──
    int min = arr[0];   // assume first is min
    for (int i = 1; i < n; i++)
        if (arr[i] < min) min = arr[i];
    printf("Minimum: %d\n", min);

    // ── Reverse print ──
    printf("Reversed: ");
    for (int i = n - 1; i >= 0; i--)
        printf("%d ", arr[i]);
    printf("\n");

    return 0;
}

terminal

output

Array:    64 25 12 92 43 37
Sum:      273
Average:  45.5
Maximum:  92
Minimum:  12
Reversed: 37 43 92 12 25 64

User input into array

array_input.c

C

#include <stdio.h>

int main() {
    int n;
    printf("How many numbers? ");
    scanf("%d", &n);

    int arr[n];   // variable-length array (VLA)

    printf("Enter %d numbers:\n", n);
    for (int i = 0; i < n; i++) {
        printf("  [%d]: ", i);
        scanf("%d", &arr[i]);
    }

    printf("You entered: ");
    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
    printf("\n");

    return 0;
}

💡 Always start max/min from arr[0], not 0!
Starting max = 0 fails if all values are negative — max = arr[0] is always correct because it starts with an actual value from the array.

4

Sorting & Searching Arrays

36 – 52 min

Two of the most fundamental array algorithms. Linear search checks every element one by one. Bubble sort repeatedly compares adjacent elements and swaps them until sorted. These are the building blocks of everything more complex.

Linear search — find an element by value

linear_search.c

C

#include <stdio.h>

int main() {
    int arr[]  = {10, 35, 7, 42, 19, 88, 3};
    int n      = sizeof(arr) / sizeof(arr[0]);
    int target = 42;
    int found  = -1;   // -1 means not found yet

    for (int i = 0; i < n; i++) {
        if (arr[i] == target) {
            found = i;
            break;   // no need to keep searching
        }
    }

    if (found != -1)
        printf("Found %d at index %d\n", target, found);
    else
        printf("%d not found in array\n", target);

    return 0;
}

terminal

output

Found 42 at index 3

Bubble sort — sort array in ascending order

bubble_sort.c

C

#include <stdio.h>

void printArr(int arr[], int n) {
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    printf("\n");
}

int main() {
    int arr[] = {64, 25, 12, 92, 43};
    int n     = sizeof(arr) / sizeof(arr[0]);

    printf("Before: "); printArr(arr, n);

    // Bubble sort — n-1 passes
    for (int i = 0; i < n - 1; i++) {
        for (int j = 0; j < n - i - 1; j++) {
            if (arr[j] > arr[j + 1]) {
                int temp  = arr[j];
                arr[j]     = arr[j + 1];
                arr[j + 1] = temp;
            }
        }
    }

    printf("After:  "); printArr(arr, n);
    return 0;
}

terminal

output

Before: 64 25 12 92 43
After:  12 25 43 64 92

💡 How bubble sort works: Each pass through the array, the largest unsorted element "bubbles up" to its correct position. After pass 1 — 92 is in place. After pass 2 — 64 is in place. After n-1 passes, everything is sorted. Each inner loop runs one step shorter because the end is already sorted.

5

2D Arrays — Rows & Columns

52 – 68 min

A 2D array is an array of arrays — a table with rows and columns. Declare with int mat[rows][cols]. Access each element with two indices: mat[row][col]. Traverse with nested loops — outer loop for rows, inner loop for columns.

First index → row (0 = first row)
Second index → column (0 = first column)
Memory is stored row by row — row 0 all elements, then row 1, etc.

int mat[3][4] — 3 rows × 4 columns. mat[1][2] = 7

col 0

col 1

col 2

col 3

row 0

1

2

3

4

row 1

5

6

7

8

row 2

9

10

11

12

mat[1][2] = 7 | mat[0][3] = 4 | mat[2][0] = 9

2D array — declare, initialise, print, sum

array_2d.c

C

#include <stdio.h>

int main() {
    // 3 rows, 4 columns
    int mat[3][4] = {
        {1,  2,  3,  4},   // row 0
        {5,  6,  7,  8},   // row 1
        {9, 10, 11, 12}    // row 2
    };

    // Print matrix using nested loops
    printf("Matrix:\n");
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++)
            printf("%4d", mat[i][j]);
        printf("\n");
    }

    // Sum of all elements
    int total = 0;
    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 4; j++)
            total += mat[i][j];
    printf("Sum of all: %d\n", total);

    // Sum of each row
    for (int i = 0; i < 3; i++) {
        int rowSum = 0;
        for (int j = 0; j < 4; j++)
            rowSum += mat[i][j];
        printf("Row %d sum: %d\n", i, rowSum);
    }

    return 0;
}

terminal

output

Matrix:
   1   2   3   4
   5   6   7   8
   9  10  11  12
Sum of all: 78
Row 0 sum: 10
Row 1 sum: 26
Row 2 sum: 42

Matrix addition — add two 2×2 matrices

matrix_add.c

C

#include <stdio.h>

int main() {
    int a[2][2] = {{1,2},{3,4}};
    int b[2][2] = {{5,6},{7,8}};
    int c[2][2];

    for (int i = 0; i < 2; i++)
        for (int j = 0; j < 2; j++)
            c[i][j] = a[i][j] + b[i][j];

    printf("A + B =\n");
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 2; j++)
            printf("%4d", c[i][j]);
        printf("\n");
    }
    return 0;
}

terminal

output

A + B =
   6   8
  10  12

6

Passing Arrays to Functions

68 – 80 min

When you pass an array to a function, C passes the address of the first element automatically — not a copy. This means:

The function can modify the original array directly
No & needed — array name is already an address
The function does NOT know the array size — always pass it as a second parameter
Use const int arr[] if the function should not modify the array

Functions that receive and modify arrays

array_functions.c

C

#include <stdio.h>

// const = read-only — promise not to modify
void printArray(const int arr[], int n) {
    for (int i = 0; i < n; i++) printf("%d ", arr[i]);
    printf("\n");
}

int sumArray(const int arr[], int n) {
    int total = 0;
    for (int i = 0; i < n; i++) total += arr[i];
    return total;
}

// No const — this function MODIFIES the original
void doubleAll(int arr[], int n) {
    for (int i = 0; i < n; i++)
        arr[i] *= 2;   // modifies original array!
}

void reverseArray(int arr[], int n) {
    for (int i = 0; i < n / 2; i++) {
        int temp    = arr[i];
        arr[i]       = arr[n - 1 - i];
        arr[n-1-i] = temp;
    }
}

int main() {
    int nums[] = {1, 2, 3, 4, 5};
    int n = 5;

    printf("Original: "); printArray(nums, n);
    printf("Sum: %d\n",   sumArray(nums, n));

    doubleAll(nums, n);
    printf("Doubled:  "); printArray(nums, n);

    reverseArray(nums, n);
    printf("Reversed: "); printArray(nums, n);

    return 0;
}

terminal

output

Original: 1 2 3 4 5
Sum: 15
Doubled:  2 4 6 8 10
Reversed: 10 8 6 4 2

💡 Use const when a function should not change the array.
void print(const int arr[], int n) — the const keyword prevents the function from modifying the array. If it tries, the compiler throws an error. Always use const for read-only functions — it documents intent and prevents bugs.

7

Putting It All Together — Student Records

80 – 90 min

A complete student marks program using everything — arrays, loops, functions, sorting, and user input. Each function does one job. The program reads marks, finds statistics, sorts and displays results.

Complete student marks program

student_marks.c

C

#include <stdio.h>

void bubbleSort(int a[], int n) {
    for (int i = 0; i < n-1; i++)
        for (int j = 0; j < n-i-1; j++)
            if (a[j] > a[j+1]) {
                int t=a[j]; a[j]=a[j+1]; a[j+1]=t;
            }
}

char grade(int m) {
    if(m>=90)return'A'; if(m>=75)return'B';
    if(m>=50)return'C'; return 'F';
}

int main() {
    int n;
    printf("Enter number of students: ");
    scanf("%d", &n);

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

    // Stats before sorting
    int sum=0, max=marks[0], min=marks[0];
    for (int i=0; i<n; i++) {
        sum += marks[i];
        if(marks[i]>max) max=marks[i];
        if(marks[i]<min) min=marks[i];
    }

    printf("\n=== RESULTS ===\n");
    printf("Average : %.1f\n", (float)sum/n);
    printf("Highest : %d (%c)\n", max, grade(max));
    printf("Lowest  : %d (%c)\n", min, grade(min));

    bubbleSort(marks, n);
    printf("Sorted  : ");
    for(int i=0;i<n;i++) printf("%d ",marks[i]);
    printf("\n");

    return 0;
}

terminal — 5 students

output

Enter number of students: 5
  Student 1 marks: 78
  Student 2 marks: 92
  Student 3 marks: 65
  Student 4 marks: 88
  Student 5 marks: 45

=== RESULTS ===
Average : 73.6
Highest : 92 (B)
Lowest  : 45 (F)
Sorted  : 45 65 78 88 92

Q

Quick Quiz

Question 1 of 5

Given int arr[5] = {10,20,30,40,50}; — what is arr[3]?

Question 2 of 5

What happens when you access arr[10] on an array of size 5?

Question 3 of 5

How do you get the number of elements in int arr[] = {1,2,3,4,5}; safely?

Question 4 of 5

When you pass an array to a function, the function receives:

Question 5 of 5

Given int m[3][3] — how do you access row 2, column 1?

✓

Lesson Checklist

I know an array stores same-type values in contiguous memory
I know array index starts at 0 — first is [0], last is [n-1]
I can declare and initialise arrays both ways
I use sizeof(arr)/sizeof(arr[0]) to get element count
I know C does not check bounds — accessing out of range is undefined
I can loop through an array to find sum, min, max, average
I start max/min from arr[0] — not from 0
I can implement linear search — find element by value
I can implement bubble sort — sort array in ascending order
I can declare and traverse a 2D array with nested loops
I know arrays are passed by reference to functions — always
I always pass the array size as a second parameter
I use const when a function should not modify the array
I completed the quiz

→

What to Learn Next

Continue — arrays go deeper

🧵 Strings — char arrays with special rules and string.h functions Next
📌 Pointers & arrays — arr[i] is *(arr+i) — same thing Next
📦 Dynamic arrays — malloc to create arrays at runtime Advanced
🏗️ Array of structs — store records of mixed types Advanced

C Arrays — Complete Guide

What is an Array — and Why Use One?

Without array vs with array — storing 5 marks

Declaring, Initialising & Indexing

int scores[5] = {85, 92, 78, 96, 88} — memory layout

Array Operations — Loop, Sum, Min, Max

Sorting & Searching Arrays

2D Arrays — Rows & Columns

int mat[3][4] — 3 rows × 4 columns. mat[1][2] = 7

Passing Arrays to Functions

Putting It All Together — Student Records

Quick Quiz

Lesson Checklist

What to Learn Next