Linear Search Algorithm in DSA C - Build from Scratch

Linear Search Algorithm

📖 Scenario: You have a list of product IDs in a store inventory. You want to find if a specific product ID is available in the list.

🎯 Goal: Build a simple linear search program in C that looks for a given product ID in an array and tells if it is found or not.

📋 What You'll Learn

Create an integer array called products with these exact values: 101, 203, 405, 607, 809

Create an integer variable called search_id and set it to 405

Write a linear search loop using for with iterator i to check each element in products

Create an integer variable called found_index and set it to -1 before the loop

If search_id is found in products, set found_index to the index i and break the loop

Print "Product found at index: X" if found, or "Product not found" if not

💡 Why This Matters

🌍 Real World

Searching for an item in a list is a common task in inventory management, shopping apps, and databases.

💼 Career

Understanding linear search helps in debugging and optimizing simple search operations in software development.

1

Create the product array

Create an integer array called products with these exact values: 101, 203, 405, 607, 809

DSA C

# Your code here

Hint

Use curly braces to list the values inside the array.

2

Set the product ID to search

Create an integer variable called search_id and set it to 405

DSA C

int products[] = {101, 203, 405, 607, 809};
# Your code here

Hint

Use int search_id = 405; to create the variable.

3

Implement the linear search loop

Create an integer variable called found_index and set it to -1. Then use a for loop with iterator i from 0 to 4 to check each element in products. If products[i] equals search_id, set found_index to i and use break to exit the loop.

DSA C

int products[] = {101, 203, 405, 607, 809};
int search_id = 405;
# Your code here

Hint

Remember to initialize found_index before the loop and use break inside the if condition.

4

Print the search result

Use printf to print "Product found at index: X" if found_index is not -1. Otherwise, print "Product not found".

DSA C

int products[] = {101, 203, 405, 607, 809};
int search_id = 405;
int found_index = -1;
for (int i = 0; i < 5; i++) {
    if (products[i] == search_id) {
        found_index = i;
        break;
    }
}
# Your code here

Hint

Use printf with %d to show the index.

Practice

1. What is the main idea behind the Linear Search Algorithm?

easy

A. Check each element one by one until the target is found or list ends

B. Divide the list into halves repeatedly to find the target

C. Sort the list first then search using binary method

D. Use a hash table to find the target instantly

Solution

1. Step 1: Understand linear search process

   Linear search checks each element in order from start to end.

2. Step 2: Compare with other search methods

   Other methods like binary search divide the list, but linear search does not.

3. Final Answer:

   Check each element one by one until the target is found or list ends -> Option A

4. Quick Check:

   Linear search = check elements one by one [OK]

Hint: Linear search checks elements one by one [OK]

Common Mistakes:

• Confusing linear search with binary search
• Thinking linear search requires sorted list
• Assuming instant search with hash tables

2. Which of the following is the correct syntax to start a linear search loop in C for an array arr of size n?

easy

A. for (int i = 0; i < n; i++)

B. for (int i = 0; i <= n; i++)

C. for (int i = 1; i < n; i++)

D. for (int i = 1; i <= n; i++)

Solution

1. Step 1: Understand array indexing in C

   Arrays in C start at index 0 and go up to n-1 for size n.

2. Step 2: Choose correct loop condition

   The loop should run from 0 to less than n, so i < n is correct.

3. Final Answer:

   for (int i = 0; i < n; i++) -> Option A

4. Quick Check:

   Loop from 0 to n-1 = for(i=0; i<n; i++) [OK]

Hint: Array index starts at 0, loop till i < n [OK]

Common Mistakes:

• Using <= n causes out-of-bounds access
• Starting loop at 1 misses first element
• Confusing array size with last index

3. What will be the output of the following C code?

int arr[] = {4, 2, 7, 1, 9};
int n = 5;
int target = 7;
int index = -1;
for (int i = 0; i < n; i++) {
  if (arr[i] == target) {
    index = i;
    break;
  }
}
printf("%d", index);

medium

A. 3

B. 2

C. -1

D. 7

Solution

1. Step 1: Trace the loop to find target 7

   Check elements: arr[0]=4 (no), arr[1]=2 (no), arr[2]=7 (yes), stop loop.

2. Step 2: Identify index where target found

   Target 7 found at index 2, so index = 2.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Target 7 at arr[2] = 2 [OK]

Hint: Stop loop when target found, print index [OK]

Common Mistakes:

• Printing value instead of index
• Not breaking loop and printing last index
• Confusing index with element value

4. Find the error in this linear search code snippet in C:

int linearSearch(int arr[], int n, int target) {
  for (int i = 1; i <= n; i++) {
    if (arr[i] == target) {
      return i;
    }
  }
  return -1;
}

medium

A. The array parameter should be a pointer, not an array

B. The function should return the element, not the index

C. The return -1 should be inside the loop

D. Loop starts at 1 and goes to <= n, causing out-of-bounds access

Solution

1. Step 1: Check loop indexing for array access

   Array indices in C start at 0 and go to n-1, but loop starts at 1 and goes to n, causing out-of-bounds at arr[n].

2. Step 2: Identify correct loop bounds

   Loop should start at 0 and run while i < n to avoid invalid access.

3. Final Answer:

   Loop starts at 1 and goes to <= n, causing out-of-bounds access -> Option D

4. Quick Check:

   Array index out-of-bounds due to wrong loop limits [OK]

Hint: Array index starts at 0, loop i < n [OK]

Common Mistakes:

• Starting loop at 1 instead of 0
• Using <= n instead of < n in loop condition
• Returning element instead of index

5. You have an unsorted array arr with 10 elements where some elements repeat. You want to find the first occurrence index of the number 5 using linear search. Which approach is best to handle duplicates and return the correct index?

hard

A. Sort the array first, then use binary search to find 5

B. Traverse entire array, count all 5s, then return last index of 5

C. Traverse from start to end, return index immediately when 5 is found

D. Use a hash map to store counts, then find index of 5

Solution

1. Step 1: Understand requirement for first occurrence

   We want the first index where 5 appears, so searching from start is needed.

2. Step 2: Choose approach that stops at first match

   Linear search that returns immediately on first 5 found gives correct first occurrence index.

3. Final Answer:

   Traverse from start to end, return index immediately when 5 is found -> Option C

4. Quick Check:

   Stop at first match to get first occurrence [OK]

Hint: Return index immediately on first match [OK]

Common Mistakes:

• Counting all occurrences then returning last index
• Sorting changes original order, losing first occurrence
• Using hash map adds unnecessary complexity