Array Traversal Patterns in DSA C - Build from Scratch

Array Traversal Patterns

📖 Scenario: You are working with a list of daily temperatures recorded over a week. You want to analyze these temperatures to find out which days were warmer than a certain threshold.

🎯 Goal: Build a program that traverses an array of temperatures, checks each temperature against a threshold, and collects the warmer temperatures into a new array.

📋 What You'll Learn

Create an array called temperatures with exactly these values: 23, 27, 21, 30, 25, 28, 22

Create an integer variable called threshold and set it to 25

Use a for loop with an index variable i to traverse the temperatures array

Inside the loop, check if the current temperature is greater than threshold

If yes, add that temperature to a new array called warmer_days

Keep track of the number of warmer days in an integer variable called count

Print the contents of warmer_days array separated by spaces

💡 Why This Matters

🌍 Real World

Analyzing temperature data to find days warmer than a certain limit is common in weather forecasting and climate studies.

💼 Career

Understanding array traversal and conditional selection is fundamental for data processing tasks in software development and data analysis roles.

1

Create the temperatures array

Create an integer array called temperatures with exactly these values: 23, 27, 21, 30, 25, 28, 22

DSA C

# Your code here

Hint

Use curly braces {} to list the values inside the array declaration.

2

Set the threshold value

Create an integer variable called threshold and set it to 25

DSA C

int temperatures[] = {23, 27, 21, 30, 25, 28, 22};
# Your code here

Hint

Use int threshold = 25; to create and set the variable.

3

Traverse the array and collect warmer days

Create an integer array called warmer_days with size 7, an integer variable count set to 0, and use a for loop with index i from 0 to 6 to check if temperatures[i] is greater than threshold. If yes, add temperatures[i] to warmer_days[count] and increment count

DSA C

int temperatures[] = {23, 27, 21, 30, 25, 28, 22};
int threshold = 25;
# Your code here

Hint

Use a for loop from 0 to 6 and an if statement inside it to check each temperature.

4

Print the warmer days

Use a for loop with index j from 0 to count - 1 to print each value in warmer_days separated by spaces

DSA C

int temperatures[] = {23, 27, 21, 30, 25, 28, 22};
int threshold = 25;
int warmer_days[7];
int count = 0;
for (int i = 0; i  threshold) {
        warmer_days[count] = temperatures[i];
        count++;
    }
}
# Your code here

Hint

Use printf("%d ", warmer_days[j]); inside the loop to print each temperature followed by a space.

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