Linear Search Algorithm in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the index of the target if found, or -1 if not found.

DSA Javascript

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

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

Atarget

Bi

Carr

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using the loop index i instead of the target for comparison.

Comparing arr[i] with arr instead of target.

The code compares each element arr[i] with the target value to find a match.

2fill in blank

medium

Complete the code to search through the array and return true if the target exists.

DSA Javascript

function contains(arr, target) {
  for (let i = 0; i < [1]; i++) {
    if (arr[i] === target) {
      return true;
    }
  }
  return false;
}

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

Atarget.length

Barr

Ci.length

Darr.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using target.length which is incorrect because target is a value, not an array.

Using arr instead of arr.length in the loop condition.

The loop must run from 0 to arr.length to check every element.

3fill in blank

hard

Fix the error in the code to correctly perform linear search and return the index or -1.

DSA Javascript

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

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

Aarr.length

Barr.length + 1

Carr.length - 1

Di.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using arr.length causes index out of range error.

Using arr.length + 1 causes runtime error.

The loop should run while i is less than or equal to arr.length - 1 to avoid going out of bounds.

4fill in blank

hard

Fill both blanks to create a function that returns the index of the target or -1 if not found.

DSA Javascript

function search(arr, target) {
  for (let i = 0; i [1] arr.length; i++) {
    if (arr[i] [2] target) {
      return i;
    }
  }
  return -1;
}

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

A<

B<=

C===

D!==

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= in the loop condition causes out of bounds error.

Using !== instead of === in the if condition.

The loop runs while i is less than arr.length, and inside the loop, we check if arr[i] equals the target.

5fill in blank

hard

Fill all three blanks to create a function that returns true if the target is found, false otherwise.

DSA Javascript

function exists(arr, target) {
  for (let [1] = 0; [2] < arr.length; [3]++) {
    if (arr[i] === target) {
      return true;
    }
  }
  return false;
}

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

Ai

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in loop parts causes errors.

Using 'j' in one part and 'i' in others causes reference errors.

The loop variable is 'i' used consistently in initialization, condition, and increment.

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