Linear Search Algorithm in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the linear search loop from the first element.

DSA C

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

Drag options to blanks, or click blank then click option'

A-1

B0

C1

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Starting the loop at 1 skips the first element.

Starting at n causes no iterations.

The linear search starts checking from the first element, which is at index 0.

2fill in blank

medium

Complete the code to return -1 when the target is not found after the loop.

DSA C

for (int i = 0; i < n; i++) {
    if (arr[i] == target) {
        return i;
    }
}
return [1];

Drag options to blanks, or click blank then click option'

An

B0

C-1

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Returning 0 can be confused with a valid index.

Returning n is out of array bounds but less common.

Returning -1 indicates the target was not found in the array.

3fill in blank

hard

Fix the error in the loop condition to prevent out-of-bounds access.

DSA C

for (int i = 0; i [1] n; i++) {
    if (arr[i] == target) {
        return i;
    }
}

Drag options to blanks, or click blank then click option'

A<

B<=

C>=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= causes out-of-bounds error.

Using > or >= makes the loop never run.

The loop must run while i is less than n to avoid accessing beyond the array.

4fill in blank

hard

Fill both blanks to complete the linear search function header and return type.

DSA C

[1] linear_search(int arr[], int n, int target) {
    for (int i = 0; i < n; i++) {
        if (arr[i] == target) {
            return i;
        }
    }
    return [2];
}

Drag options to blanks, or click blank then click option'

Aint

Bvoid

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using void as return type causes errors when returning values.

Returning 0 can be confused with a valid index.

The function returns an integer index, so return type is int and -1 indicates not found.

5fill in blank

hard

Fill all three blanks to complete the linear search call and print the result.

DSA C

int result = linear_search([1], [2], [3]);
printf("Index: %d\n", result);

Drag options to blanks, or click blank then click option'

Aarr

Btarget

Cn

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping target and n causes wrong function call.

Using undefined variables causes errors.

Call linear_search with array name, size n, and the target value.

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