Array Traversal Patterns in DSA C

Mental Model

Go through each item in the list one by one to see or use it.

Analogy: Like reading a book from the first page to the last page, looking at every word in order.

[0] -> [1] -> [2] -> [3] -> [4] -> null
 ↑

Dry Run Walkthrough

Input: array: [10, 20, 30, 40, 50]

Goal: Visit each element in the array from start to end and print it

Step 1: Start at index 0, read value 10

[0:10↑] -> [1:20] -> [2:30] -> [3:40] -> [4:50] -> null

Why: We begin at the first element to start traversal

Step 2: Move to index 1, read value 20

[0:10] -> [1:20↑] -> [2:30] -> [3:40] -> [4:50] -> null

Why: Move to next element to continue traversal

Step 3: Move to index 2, read value 30

[0:10] -> [1:20] -> [2:30↑] -> [3:40] -> [4:50] -> null

Why: Keep moving forward to visit all elements

Step 4: Move to index 3, read value 40

[0:10] -> [1:20] -> [2:30] -> [3:40↑] -> [4:50] -> null

Why: Continue until last element

Step 5: Move to index 4, read value 50

[0:10] -> [1:20] -> [2:30] -> [3:40] -> [4:50↑] -> null

Why: Last element visited to complete traversal

Result:

[0:10] -> [1:20] -> [2:30] -> [3:40] -> [4:50] -> null
Printed values: 10 20 30 40 50

Annotated Code

DSA C

#include <stdio.h>

void traverseArray(int arr[], int size) {
    for (int i = 0; i < size; i++) {
        // print current element
        printf("%d ", arr[i]);
    }
    printf("\n");
}

int main() {
    int arr[] = {10, 20, 30, 40, 50};
    int size = sizeof(arr) / sizeof(arr[0]);
    traverseArray(arr, size);
    return 0;
}

for (int i = 0; i < size; i++) {

loop through each index from start to end

printf("%d ", arr[i]);

print the current element at index i

Output

10 20 30 40 50 

Complexity Analysis

Time: O(n) because we visit each of the n elements once

Space: O(1) because we use only a fixed amount of extra space

vs Alternative: Compared to random access, traversal is simple and direct with no extra cost

Edge Cases

empty array

loop does not run, nothing is printed

DSA C

for (int i = 0; i < size; i++) {

array with one element

prints the single element correctly

DSA C

for (int i = 0; i < size; i++) {

When to Use This Pattern

When you need to look at or use every item in a list one by one, use array traversal to visit each element in order.

Common Mistakes

Mistake: Using <= instead of < in the loop condition causing out-of-bounds access
Fix: Change loop condition to i < size to stay within array limits

Mistake: Starting loop index at 1 instead of 0, skipping first element
Fix: Start loop index at 0 to include the first element

Summary

It visits each element in an array one by one.

Use it when you want to process or read all items in order.

Remember to start at index 0 and stop before the array size.

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