Array Traversal Patterns in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Nested Loop Array Traversal

What is the printed output of the following C code snippet that traverses a 2D array?

DSA C

int arr[2][3] = {{1, 2, 3}, {4, 5, 6}};
for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 3; j++) {
        printf("%d ", arr[i][j]);
    }
    printf("\n");
}

A 1 2 3 4 5 6 \n

B 4 5 6 \n1 2 3 \n

C 1 4 2 \n3 5 6 \n

D 1 2 3 \n4 5 6 \n

💡 Hint

Think about how the outer loop controls rows and the inner loop controls columns.

The outer loop runs twice for each row, and the inner loop prints all columns in that row before moving to the next line.

❓ Predict Output

intermediate

Output of Single Loop Traversal with Step

What is the output of this C code that traverses an array with a step of 2?

DSA C

int arr[] = {10, 20, 30, 40, 50, 60};
int n = 6;
for (int i = 0; i < n; i += 2) {
    printf("%d ", arr[i]);
}

A 10 30 50

B 20 40 60

C 10 20 30 40 50 60

D 60 50 40 30 20 10

💡 Hint

The loop increments by 2, so it skips every other element.

Starting at index 0, the loop prints elements at indices 0, 2, and 4.

🔧 Debug

advanced

Identify the Error in Reverse Array Traversal

What error does this C code produce when trying to print an array in reverse?

DSA C

int arr[] = {1, 2, 3, 4, 5};
int n = 5;
for (int i = n - 1; i >= 0; i--) {
    printf("%d ", arr[i]);
}

A Prints: 5 4 3 2 1

B Compilation error: invalid loop condition

C Prints: 5 4 3 2 1 0 (out of bounds access)

D Runtime error: Array index out of bounds

💡 Hint

Check the starting index and valid array indices.

Array indices go from 0 to n-1. Starting at n accesses arr[5], which is out of bounds causing runtime error.

🧠 Conceptual

advanced

Understanding Traversal Order in Zigzag Pattern

Given a 2D array, which traversal pattern prints elements in a zigzag order (left to right on even rows, right to left on odd rows)?

DSA C

int arr[3][3] = {{1,2,3},{4,5,6},{7,8,9}};
for (int i = 0; i < 3; i++) {
    if (i % 2 == 0) {
        for (int j = 0; j < 3; j++) {
            printf("%d ", arr[i][j]);
        }
    } else {
        for (int j = 2; j >= 0; j--) {
            printf("%d ", arr[i][j]);
        }
    }
    printf("\n");
}

A 3 2 1 \n4 5 6 \n9 8 7 \n

B 1 2 3 \n4 5 6 \n7 8 9 \n

C 1 2 3 \n6 5 4 \n7 8 9 \n

D 1 3 2 \n4 6 5 \n7 9 8 \n

💡 Hint

Even rows go left to right, odd rows go right to left.

The code prints row 0 left to right, row 1 right to left, row 2 left to right, creating a zigzag pattern.

🚀 Application

expert

Count Unique Elements Using Array Traversal

Given an unsorted integer array, which code snippet correctly counts the number of unique elements using only array traversal and no extra data structures?

DSA C

int arr[] = {2, 3, 2, 4, 3, 5};
int n = 6;
int count = 0;
for (int i = 0; i < n; i++) {
    int j;
    for (j = 0; j < i; j++) {
        if (arr[i] == arr[j]) {
            break;
        }
    }
    if (j == i) {
        count++;
    }
}
printf("%d", count);

A 5

B 4

C 3

D 6

💡 Hint

Count increments only when current element is not found before in the array.

The code checks if the current element appeared before; if not, it counts it as unique. The unique elements are 2, 3, 4, 5 totaling 4.

Practice

1. What does the following code snippet do?
for (int i = 0; i < n; i++) { printf("%d ", arr[i]); }

easy

A. Prints only the last element of the array

B. Prints all elements of the array from end to start

C. Prints only the first element of the array

D. Prints all elements of the array from start to end

Solution

1. Step 1: Understand the loop range

   The loop starts at 0 and runs while i is less than n, incrementing i by 1 each time.

2. Step 2: Analyze the print statement inside the loop

   Each iteration prints the element at index i, so it prints elements from arr[0] to arr[n-1].

3. Final Answer:

   Prints all elements of the array from start to end -> Option D

4. Quick Check:

   Loop from 0 to n-1 prints all elements [OK]

Hint: Loop from 0 to n-1 prints array forward [OK]

Common Mistakes:

• Confusing loop direction and thinking it prints backward
• Assuming it prints only one element
• Misreading the loop condition

2. Which of the following is the correct syntax to traverse an array arr of size n backwards in C?

easy

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

B. for (int i = n; i > 0; i++) { printf("%d", arr[i]); }

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

D. for (int i = 1; i <= n; i++) { printf("%d", arr[i]); }

Solution

1. Step 1: Identify correct loop start and end for backward traversal

   Backward traversal starts from index n-1 down to 0.

2. Step 2: Check loop syntax correctness

   for (int i = n-1; i >= 0; i--) { printf("%d", arr[i]); } uses i = n-1; i >= 0; i-- which correctly traverses backward.

3. Final Answer:

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

4. Quick Check:

   Backward loop uses i = n-1 to 0 with decrement [OK]

Hint: Backward loop starts at n-1 and decrements to 0 [OK]

Common Mistakes:

• Starting loop at n instead of n-1
• Using i++ instead of i-- for backward traversal
• Accessing arr[i] with out-of-range indices

3. What is the output of the following code?

int arr[] = {5, 10, 15, 20};
int sum = 0;
for (int i = 1; i < 4; i++) {
    sum += arr[i];
}
printf("%d", sum);

medium

A. 50

B. 45

C. 30

D. Syntax Error

Solution

1. Step 1: Identify loop indices and elements summed

   The loop runs i from 1 to 3, summing arr[1], arr[2], arr[3] which are 10, 15, and 20.

2. Step 2: Calculate sum of selected elements

   sum = 10 + 15 + 20 = 45

3. Final Answer:

   45 -> Option B

4. Quick Check:

   Sum of arr[1..3] = 45 [OK]

Hint: Sum elements from index 1 to 3 inclusive [OK]

Common Mistakes:

• Starting sum from arr[0] mistakenly
• Adding wrong indices or missing last element
• Confusing loop range boundaries

4. Find the error in the following code that attempts to print all elements of an array arr of size n:

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

medium

A. Loop condition should be i < n, not i <= n

B. Array indexing should start from 1, not 0

C. printf statement syntax is incorrect

D. No error, code is correct

Solution

1. Step 1: Analyze loop condition and array indexing

   Loop runs from i = 0 to i = n inclusive, accessing arr[n] which is out of bounds.

2. Step 2: Correct loop condition to avoid out-of-bounds access

   Change condition to i < n so indices go from 0 to n-1, valid for array of size n.

3. Final Answer:

   Loop condition should be i < n, not i <= n -> Option A

4. Quick Check:

   Array indices valid from 0 to n-1 [OK]

Hint: Use i < n to avoid out-of-bounds errors [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Starting index at 1 instead of 0
• Ignoring array size limits

5. Given an integer array arr of size n, which code snippet correctly prints every second element starting from the first element?

hard

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

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

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

D. for (int i = n-1; i >= 0; i -= 2) { printf("%d ", arr[i]); }

Solution

1. Step 1: Understand requirement to print every second element starting at first

   Start at index 0 and increment index by 2 each time to skip every other element.

2. Step 2: Check loop correctness and bounds

   for (int i = 0; i < n; i += 2) { printf("%d ", arr[i]); } uses i = 0; i < n; i += 2 which correctly prints arr[0], arr[2], arr[4], etc.

3. Final Answer:

   for (int i = 0; i < n; i += 2) { printf("%d ", arr[i]); } -> Option A

4. Quick Check:

   Increment by 2 starting at 0 prints every second element [OK]

Hint: Increment index by 2 starting at 0 for every second element [OK]

Common Mistakes:

• Starting at index 1 instead of 0
• Using <= n causing out-of-bounds
• Looping backward when forward is needed