Linear Search Algorithm in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind the Linear Search Algorithm?

Linear Search checks each item in a list one by one until it finds the target or reaches the end.

beginner

How does Linear Search behave if the target is not in the list?

It checks every item and ends after the last one, returning a result that the target was not found.

intermediate

What is the time complexity of Linear Search in the worst case?

The time complexity is O(n), where n is the number of items in the list, because it may check all items.

beginner

Show a simple JavaScript code snippet for Linear Search.

function linearSearch(arr, target) {
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === target) {
      return i;
    }
  }
  return -1;
}

beginner

Why is Linear Search not efficient for large lists?

Because it checks each item one by one, it can take a long time if the list is very big.

What does Linear Search do when it finds the target in the list?

A Deletes the target from the list

B Continues checking all items

C Stops and returns the position of the target

D Sorts the list

Linear Search stops as soon as it finds the target and returns its position.

What is the worst-case time complexity of Linear Search?

A O(n log n)

B O(log n)

C O(1)

D O(n)

In the worst case, Linear Search checks every item, so it takes O(n) time.

If the target is not in the list, what does Linear Search return in the example code?

A 0

B -1

C null

D undefined

The example code returns -1 to indicate the target was not found.

Which of these is a key characteristic of Linear Search?

A It checks items one by one

B It requires the list to be sorted

C It uses divide and conquer

D It uses binary splitting

Linear Search checks each item one by one without needing the list to be sorted.

Which data structure is Linear Search best suited for?

A Unsorted lists or arrays

B Hash tables

C Binary trees

D Sorted arrays

Linear Search works well on unsorted lists or arrays where no order is guaranteed.

Explain how Linear Search works step-by-step on a list of numbers.

Think about looking for a name in a list by checking one by one.

Describe the advantages and disadvantages of using Linear Search.

Consider when it is easy to use and when it might be slow.

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 steps

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

   Binary search divides list, hash uses extra memory, linear is simple sequential check.

3. Final Answer:

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

4. Quick Check:

   Linear search = sequential check [OK]

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

Common Mistakes:

• Confusing linear search with binary search
• Thinking linear search requires sorted list
• Assuming linear search uses extra memory

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

easy

A. for (let i = 0; i <= arr.length; i++)

B. for (let i = 0; i < arr.length; i++)

C. for (let i = 1; i < arr.length; i++)

D. for (let i = 1; i <= arr.length; i++)

Solution

1. Step 1: Understand array indexing in JavaScript

   Arrays start at index 0 and go up to length - 1.

2. Step 2: Choose loop condition to avoid out-of-bound error

   Loop must run while i is less than arr.length, not equal or greater.

3. Final Answer:

   for (let i = 0; i < arr.length; i++) -> Option B

4. Quick Check:

   Loop runs from 0 to length-1 [OK]

Hint: Use i < arr.length to avoid index errors [OK]

Common Mistakes:

• Using <= instead of < causing out-of-bound errors
• Starting loop from 1 instead of 0
• Confusing array length with last index

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

const arr = [4, 8, 15, 16, 23, 42];
function linearSearch(array, target) {
  for (let i = 0; i < array.length; i++) {
    if (array[i] === target) {
      return i;
    }
  }
  return -1;
}
console.log(linearSearch(arr, 15));

medium

A. 2

B. 3

C. -1

D. 15

Solution

1. Step 1: Trace the loop to find target 15

   Indices: 0=4, 1=8, 2=15. At i=2, array[i] === 15 is true.

2. Step 2: Return index when target found

   Function returns 2 immediately when target matches.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Index of 15 in array = 2 [OK]

Hint: Return index when target matches during loop [OK]

Common Mistakes:

• Returning value instead of index
• Returning -1 even if target exists
• Off-by-one errors in index

4. Find the error in this linear search function and choose the correct fix:

function linearSearch(arr, target) {
  for (let i = 0; i <= arr.length; i++) {
    if (arr[i] == target) {
      return i;
    }
  }
  return -1;
}

medium

A. Initialize i to 1 instead of 0

B. Change == to === in if condition

C. Change loop condition to i < arr.length

D. Add a break statement after return

Solution

1. Step 1: Identify loop condition error

   Loop runs while i <= arr.length, which causes out-of-bound access at i = arr.length.

2. Step 2: Fix loop condition to prevent error

   Change condition to i < arr.length to stay within valid indices.

3. Final Answer:

   Change loop condition to i < arr.length -> Option C

4. Quick Check:

   Loop must stay within array bounds [OK]

Hint: Use i < arr.length to avoid index errors [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Thinking == vs === causes runtime error here
• Adding break after return (unnecessary)

5. You want to find the first occurrence of the number 7 in the array [3, 5, 7, 7, 9] using linear search. Which code snippet correctly returns the index of the first 7?

hard

A. for (let i = arr.length - 1; i >= 0; i--) { if (arr[i] === 7) return i; } return -1;

B. for (let i = 0; i < arr.length; i++) { if (arr[i] == 7) return arr[i]; } return -1;

C. for (let i = 1; i <= arr.length; i++) { if (arr[i] === 7) return i; } return -1;

D. for (let i = 0; i < arr.length; i++) { if (arr[i] === 7) return i; } return -1;

Solution

1. Step 1: Understand requirement to find first occurrence

   Start from index 0 and return index when first 7 is found.

2. Step 2: Analyze each option for correctness

   for (let i = 0; i < arr.length; i++) { if (arr[i] === 7) return i; } return -1; loops forward, returns index correctly. for (let i = arr.length - 1; i >= 0; i--) { if (arr[i] === 7) return i; } return -1; loops backward (last occurrence). for (let i = 1; i <= arr.length; i++) { if (arr[i] === 7) return i; } return -1; has wrong loop bounds. for (let i = 0; i < arr.length; i++) { if (arr[i] == 7) return arr[i]; } return -1; returns value, not index.

3. Final Answer:

   for (let i = 0; i < arr.length; i++) { if (arr[i] === 7) return i; } return -1; -> Option D

4. Quick Check:

   Forward loop returns first match index [OK]

Hint: Loop forward and return index on first match [OK]

Common Mistakes:

• Looping backward returns last occurrence, not first
• Returning value instead of index
• Using wrong loop bounds causing errors