Common array operations - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of array sum calculation

What is the output of this C program that sums elements of an integer array?

C

#include <stdio.h>

int main() {
    int arr[] = {1, 2, 3, 4, 5};
    int sum = 0;
    for (int i = 0; i < 5; i++) {
        sum += arr[i];
    }
    printf("%d\n", sum);
    return 0;
}

A 10

B 15

C 5

D 0

💡 Hint

Add all numbers in the array: 1 + 2 + 3 + 4 + 5.

The loop adds each element of the array to sum. The total is 1+2+3+4+5 = 15.

❓ Predict Output

intermediate

Output of array element modification

What is the output of this C program that modifies elements of an array?

C

#include <stdio.h>

int main() {
    int arr[3] = {10, 20, 30};
    for (int i = 0; i < 3; i++) {
        arr[i] = arr[i] * 2;
    }
    for (int i = 0; i < 3; i++) {
        printf("%d ", arr[i]);
    }
    return 0;
}

A 20 40 60

B 10 20 30

C 0 0 0

D 30 40 50

💡 Hint

Each element is multiplied by 2.

The loop doubles each element, so 10->20, 20->40, 30->60.

❓ Predict Output

advanced

Output of searching for an element in array

What is the output of this C program that searches for a value in an array?

C

#include <stdio.h>

int main() {
    int arr[] = {5, 10, 15, 20, 25};
    int target = 15;
    int found = 0;
    for (int i = 0; i < 5; i++) {
        if (arr[i] == target) {
            found = 1;
            break;
        }
    }
    if (found) {
        printf("Found\n");
    } else {
        printf("Not Found\n");
    }
    return 0;
}

A 0

B Not Found

C 15

D Found

💡 Hint

Check if 15 is in the array.

The program finds 15 in the array and prints "Found".

❓ Predict Output

advanced

Output of array reversal

What is the output of this C program that reverses an array and prints it?

C

#include <stdio.h>

int main() {
    int arr[] = {1, 2, 3, 4};
    int n = 4;
    for (int i = 0; i < n / 2; i++) {
        int temp = arr[i];
        arr[i] = arr[n - 1 - i];
        arr[n - 1 - i] = temp;
    }
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    return 0;
}

A 4 3 2 1

B 1 2 3 4

C 2 1 4 3

D 1 4 3 2

💡 Hint

The program swaps elements from start and end.

The array is reversed, so the output is "4 3 2 1 ".

🧠 Conceptual

expert

Number of elements in a statically declared array

Given the declaration int arr[10];, what is the value of sizeof(arr) / sizeof(arr[0]) in C?

A Depends on compiler

B 4

C 10

D 40

💡 Hint

sizeof(arr) gives total bytes, sizeof(arr[0]) gives bytes per element.

The expression calculates the number of elements by dividing total size by element size, which is 10.

Practice

1. What does the following C code do?
int arr[5] = {1, 2, 3, 4, 5};

easy

A. Creates an array of 5 integers with values 1 to 5

B. Creates a pointer to an integer

C. Declares a single integer variable

D. Creates a 2D array of integers

Solution

1. Step 1: Understand the declaration

   The code declares an integer array named arr with size 5.

2. Step 2: Check the initialization

   The array is initialized with values 1, 2, 3, 4, and 5 in order.

3. Final Answer:

   Creates an array of 5 integers with values 1 to 5 -> Option A

4. Quick Check:

   Array declaration with initialization = Creates an array of 5 integers with values 1 to 5 [OK]

Hint: Array size and values match initialization count [OK]

Common Mistakes:

• Confusing array with pointer declaration
• Thinking it's a single variable, not an array
• Assuming it's a 2D array

2. Which of the following is the correct syntax to access the third element of an array named arr?

easy

A. arr(3)

B. arr[2]

C. arr{3}

D. arr[3]

Solution

1. Step 1: Recall array indexing in C

   Arrays in C use zero-based indexing, so the first element is arr[0].

2. Step 2: Identify the third element index

   The third element is at index 2, so arr[2] accesses it.

3. Final Answer:

   arr[2] -> Option B

4. Quick Check:

   Third element index = 2 [OK]

Hint: Remember C arrays start at index 0 [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based indexing (arr[3])
• Using curly braces for access

3. What is the output of this C code?

int arr[4] = {10, 20, 30, 40};
int sum = 0;
for(int i = 0; i < 4; i++) {
    sum += arr[i];
}
printf("%d", sum);

medium

A. 100

B. 90

C. Syntax error

D. 40

Solution

1. Step 1: Sum all elements in the array

   The loop adds arr[0]=10, arr[1]=20, arr[2]=30, arr[3]=40 to sum.

2. Step 2: Calculate the total sum

   10 + 20 + 30 + 40 = 100.

3. Final Answer:

   100 -> Option A

4. Quick Check:

   Sum of array elements = 100 [OK]

Hint: Add all elements using loop index 0 to size-1 [OK]

Common Mistakes:

• Off-by-one error in loop causing wrong sum
• Confusing sum with last element value
• Syntax errors from missing semicolons

4. Identify the error in this code snippet:

int arr[3] = {1, 2, 3};
for(int i = 0; i <= 3; i++) {
    printf("%d ", arr[i]);
}

medium

A. Array size should be 4

B. printf format specifier is wrong

C. Loop condition should be i < 3, not i <= 3

D. Variable i should be declared outside the loop

Solution

1. Step 1: Analyze loop condition

   The loop runs from i=0 to i=3 inclusive, so it accesses arr[3].

2. Step 2: Check array bounds

   Array arr has size 3, valid indices are 0,1,2. Accessing arr[3] causes out-of-bounds error.

3. Final Answer:

   Loop condition should be i < 3, not i <= 3 -> Option C

4. Quick Check:

   Array index must be less than size [OK]

Hint: Loop index must be less than array size [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Assuming array size auto-expands
• Misunderstanding printf format specifier

5. You want to find the largest number in an integer array arr of size n. Which code snippet correctly does this?

hard

A. int max = arr[n]; for(int i = 0; i < n; i++) { if(arr[i] > max) max = arr[i]; }

B. int max = 0; for(int i = 0; i <= n; i++) { if(arr[i] > max) max = arr[i]; }

C. int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] < max) max = arr[i]; }

D. int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] > max) max = arr[i]; }

Solution

1. Step 1: Initialize max with first element

   Set max = arr[0] to start comparison from first element.

2. Step 2: Loop from second element to end

   Loop i from 1 to n-1, compare arr[i] with max, update max if arr[i] is larger.

3. Step 3: Check other options for errors

   int max = 0; for(int i = 0; i <= n; i++) { if(arr[i] > max) max = arr[i]; } uses max=0 which fails if all numbers are negative; loop condition i <= n causes out-of-bounds; int max = arr[n]; for(int i = 0; i < n; i++) { if(arr[i] > max) max = arr[i]; } initializes max with arr[n] which is invalid index; int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] < max) max = arr[i]; } finds minimum instead of maximum.

4. Final Answer:

   int max = arr[0]; for(int i = 1; i < n; i++) { if(arr[i] > max) max = arr[i]; } -> Option D

5. Quick Check:

   Initialize max with first element and compare correctly [OK]

Hint: Start max with arr[0], loop i from 1 to n-1 [OK]

Common Mistakes:

• Using wrong loop bounds causing out-of-bounds access
• Initializing max with 0 ignoring negative values
• Confusing max with min logic