Traversal Forward and Backward in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to traverse the array forward and print each element.

DSA Javascript

const arr = [1, 2, 3, 4];
for (let i = 0; i [1] arr.length; i++) {
  console.log(arr[i]);
}

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

A<

B>

C<=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' causes an index out of range error.

Using '>' or '>=' will not enter the loop.

The loop should run while i is less than the array length to avoid going out of bounds.

2fill in blank

medium

Complete the code to traverse the array backward and print each element.

DSA Javascript

const arr = [5, 6, 7, 8];
for (let i = arr.length - 1; i [1] 0; i--) {
  console.log(arr[i]);
}

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

A>

B>=

C<=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' misses the element at index 0.

Using '<' or '<=' will not run the loop correctly.

The loop should run while i is greater than or equal to 0 to include the first element.

3fill in blank

hard

Fix the error in the code to traverse forward and print elements.

DSA Javascript

const arr = ['a', 'b', 'c'];
for (let i = 0; i [1] arr.length; i++) {
  console.log(arr[i]);
}

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

A>

B>=

C<

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' causes an error by accessing arr[arr.length].

Using '>' or '>=' will not run the loop.

The loop condition must be 'i < arr.length' to avoid accessing undefined elements.

4fill in blank

hard

Fill both blanks to traverse backward and print elements.

DSA Javascript

const arr = [10, 20, 30, 40];
for (let i = arr.length - 1; i [1] 0; i[2]) {
  console.log(arr[i]);
}

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

A>=

B<=

C--

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' or '++' will cause the loop to run incorrectly or infinitely.

The loop should continue while i >= 0 and decrement i each time using '--'.

5fill in blank

hard

Fill all three blanks to create a forward traversal that prints only even numbers.

DSA Javascript

const arr = [1, 2, 3, 4, 5, 6];
for (let i = 0; i [1] arr.length; i[2]) {
  if (arr[i] [3] 2 === 0) {
    console.log(arr[i]);
  }
}

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

A<

B++

C%

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using '--' will cause an infinite loop.

Using '<=' will cause out of bounds error.

Using other operators instead of '%' will not correctly check even numbers.

The loop runs while i < arr.length, increments i with '++', and checks if arr[i] % 2 === 0 for even numbers.

Practice

1. What does it mean to traverse a linked list forward?

easy

A. Visiting nodes randomly without order

B. Visiting nodes from the first node to the last node in order

C. Skipping every other node while visiting

D. Visiting nodes from the last node to the first node in reverse order

Solution

1. Step 1: Understand forward traversal

   Forward traversal means starting at the first node (head) and moving to the next node repeatedly until the end.

2. Step 2: Compare with other options

   Backward traversal is the opposite, from last to first. Skipping or random visiting is not traversal.

3. Final Answer:

   Visiting nodes from the first node to the last node in order -> Option B

4. Quick Check:

   Forward traversal = first to last [OK]

Hint: Forward means start to end in order [OK]

Common Mistakes:

• Confusing forward with backward traversal
• Thinking traversal skips nodes
• Assuming traversal is random

2. Which of the following is the correct syntax to traverse an array arr forward in JavaScript?

easy

A. for (let i = 0; i < arr.length; i++) { console.log(arr[i]); }

B. for (let i = arr.length; i >= 0; i--) { console.log(arr[i]); }

C. for (let i = 1; i <= arr.length; i++) { console.log(arr[i]); }

D. for (let i = arr.length - 1; i > 0; i--) { console.log(arr[i]); }

Solution

1. Step 1: Identify forward traversal syntax

   Forward traversal starts at index 0 and goes up to index arr.length - 1, so i from 0 to less than arr.length.

2. Step 2: Check each option

   for (let i = 0; i < arr.length; i++) { console.log(arr[i]); } uses i = 0 to i < arr.length, correct. for (let i = arr.length; i >= 0; i--) { console.log(arr[i]); } and D count backward. for (let i = 1; i <= arr.length; i++) { console.log(arr[i]); } starts at 1 and accesses arr[arr.length], which is undefined.

3. Final Answer:

   for (let i = 0; i < arr.length; i++) { console.log(arr[i]); } -> Option A

4. Quick Check:

   Forward array loop = i from 0 to length-1 [OK]

Hint: Start index 0, end before length for forward loops [OK]

Common Mistakes:

• Starting loop at 1 instead of 0
• Using <= instead of < in loop condition
• Looping backward for forward traversal

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

const arr = [10, 20, 30];
for (let i = arr.length - 1; i >= 0; i--) {
  console.log(arr[i]);
}

medium

A. 10 20 30

B. undefined undefined undefined

C. 30 20 10

D. SyntaxError

Solution

1. Step 1: Analyze loop indices

   The loop starts at i = arr.length - 1 = 2, then decrements i until 0, so indices 2, 1, 0.

2. Step 2: Print elements in reverse order

   arr[2] = 30, arr[1] = 20, arr[0] = 10, so output is 30 20 10.

3. Final Answer:

   30 20 10 -> Option C

4. Quick Check:

   Backward loop prints elements in reverse [OK]

Hint: Loop backward from length-1 to 0 for reverse output [OK]

Common Mistakes:

• Confusing forward and backward indices
• Expecting output in original order
• Off-by-one errors in loop condition

4. Identify the error in the following backward traversal code for an array nums:

for (let i = nums.length; i >= 0; i--) {
  console.log(nums[i]);
}

medium

A. No error, code is correct

B. Loop should increment i instead of decrement

C. Loop condition should be i <= nums.length

D. Loop should start at nums.length - 1 and decrement i

Solution

1. Step 1: Check loop start and direction

   Array indices go from 0 to nums.length - 1. Starting at nums.length is out of bounds.

2. Step 2: Correct loop start and decrement

   Loop should start at nums.length - 1 and decrement i to 0 to traverse backward correctly.

3. Final Answer:

   Loop should start at nums.length - 1 and decrement i -> Option D

4. Quick Check:

   Backward loop start = length-1, decrement i [OK]

Hint: Start backward loop at length-1, not length [OK]

Common Mistakes:

• Starting loop at length causing undefined access
• Incrementing index in backward loop
• Wrong loop condition allowing infinite loop

5. Given a doubly linked list, which traversal method allows visiting nodes both forward and backward efficiently?

hard

A. Using next pointers for forward and prev pointers for backward traversal

B. Using only next pointers for both directions

C. Using only prev pointers for both directions

D. Traversing randomly without pointers

Solution

1. Step 1: Understand doubly linked list structure

   Doubly linked lists have two pointers per node: next (forward) and prev (backward).

2. Step 2: Use pointers for traversal

   Forward traversal uses next pointers; backward traversal uses prev pointers efficiently.

3. Final Answer:

   Using next pointers for forward and prev pointers for backward traversal -> Option A

4. Quick Check:

   Doubly linked list uses next and prev pointers [OK]

Hint: Doubly linked list has next and prev pointers [OK]

Common Mistakes:

• Assuming single pointer can traverse both ways
• Ignoring prev pointers in backward traversal
• Thinking traversal can be random