📘 Section 3 — Arrays

Store many values under one name, then process them with loops.

🎯 What You’ll Practice

Declaring and initializing arrays
Accessing array elements using indexes
Traversing arrays with loops
Searching, summing, reversing, and filtering array elements

1. Arrays

An array is a data structure consisting of a collection of elements, each identified by an array index.

Array concept

Why Arrays?

Without arrays, storing 1000 values requires 1000 separate variables:

int number1;
int number2;
// ... now assume we need 1000 variables!

Declaring and Initializing

int array[] = {25, 50, 75, 100};     // Declare with values
int array2[4];                        // Declare with size only
array2[0] = 25;                       // Set value by index

Indexing

Arrays in C are zero-indexed — the first element is at position 0:

Index:   [0]   [1]   [2]   [3]
Value:    25    50    75   100

🧪 Exercises

Exercise 1: Array Operations

Task: Initiate an array with the following integer values: {-5, 3, 0, 1, 27} then run the following operations on the array and print out the final output:

Add 7 to the fourth element
Multiply the first element by the third element → store in the fifth element
Subtract 2 from the second element
Multiply the first element by -1
Set the value of the third element to the same value of the first element
Add 9 to the fifth element

💡 Hint

Remember: arrays are zero-indexed! The “4th element” is arr[3]
Perform operations in order — each step uses results from previous steps

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int arr[] = {-5, 3, 0, 1, 27};

    arr[3] += 7;              // 1 → 8
    arr[4] = arr[0] * arr[2]; // -5 * 0 = 0
    arr[1] -= 2;              // 3 → 1
    arr[0] *= -1;             // -5 → 5
    arr[2] = arr[0];          // 0 → 5
    arr[4] += 9;              // 0 → 9

    for (int i = 0; i < 5; i++) {
        printf("%d ", arr[i]);
    }
    return 0;
}

Output:

5 1 5 8 9

Step-by-step trace:

Step	arr[0]	arr[1]	arr[2]	arr[3]	arr[4]
Start	-5	3	0	1	27
1	-5	3	0	8	27
2	-5	3	0	8	0
3	-5	1	0	8	0
4	5	1	0	8	0
5	5	1	5	8	0
6	5	1	5	8	9

Exercise 2: Reverse an Array

Task: Write a program in C to read n number of values in an array and display them in reverse order.

💡 Hint

Read values with a forward loop (i = 0 to n-1)
Print with a backward loop (i = n-1 down to 0)

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int n;
    printf("Enter the number of elements in the array: ");
    scanf("%d", &n);
    int arr[n];

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

    for (int i = n - 1; i >= 0; i--) {
        printf("%d ", arr[i]);
    }
    return 0;
}

Exercise 3: Sum of Array Elements

Task: Write a program in C to find the sum of all elements of the array.

💡 Hint

Initialize sum = 0 before the loop
Add each element to sum as you read it (or in a separate loop)

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int n, sum = 0;
    printf("Enter the number of elements in the array: ");
    scanf("%d", &n);
    int arr[n];

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

    printf("sum=%d", sum);
    return 0;
}

💡 You can accumulate the sum while reading input — no need for a second loop.

Exercise 4: Search for a Value

Task: Check if number 1 appears in an array or not. Print "true" if found, "false" otherwise.

💡 Hint

Loop through the array
If you find 1, print and immediately return to stop searching

🟢 Click to Show Solution

#include <stdio.h>

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

    for (int i = 0; i < n; i++) {
        if (arr[i] == 1) {
            printf("true");
            return 0;
        }
    }

    printf("false");
    return 0;
}

💡 Early exit pattern: Using return 0 inside the loop lets you stop as soon as you find the answer.

Exercise 5: Find the Largest Element

Task: Find the largest element in an array of 10 elements.

💡 Hint

Assume the first element is the max
Compare every other element — update if larger

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int a[10], max;

    for (int i = 0; i < 10; i++) {
        scanf("%d", &a[i]);
    }

    max = a[0];
    for (int i = 1; i < 10; i++) {
        if (max < a[i]) {
            max = a[i];
        }
    }

    printf("Max = %d", max);
    return 0;
}

⚠️ Don’t initialize max to 0! If all values are negative, 0 would incorrectly remain the max. Always start with the first element.

Exercise 6: Palindrome Check

Task: Given a number N and an array A of N numbers. Determine if it’s palindrome or not.

💡 Hint

Compare A[0] with A[N-1], then A[1] with A[N-2], and so on
You only need to check the first half of the array
If any pair doesn’t match → not a palindrome

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int N;
    scanf("%d", &N);
    int A[N];

    for (int i = 0; i < N; i++) {
        scanf("%d", &A[i]);
    }

    for (int i = 0; i < N / 2; i++) {
        if (A[i] != A[N - i - 1]) {
            printf("NOT PALINDROME");
            return 0;
        }
    }

    printf("PALINDROME");
    return 0;
}

Example:

{1, 2, 3, 2, 1} → A[0]==A[4], A[1]==A[3] → PALINDROME
{1, 2, 3, 4, 5} → A[0]!=A[4] → NOT PALINDROME

Exercise 7: Filter by Value

Task: Given a number N and an array A of N numbers. Print all array positions that store a number less than or equal to 10 and the number stored in that position.

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int N;
    scanf("%d", &N);
    int A[N];

    for (int i = 0; i < N; i++) {
        scanf("%d", &A[i]);
    }

    printf("Positions and values less than or equal 10:\n");
    for (int i = 0; i < N; i++) {
        if (A[i] <= 10) {
            printf("Position: %d, Value: %d\n", i, A[i]);
        }
    }
    return 0;
}

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