Find the Only Non Repeating Element Using XOR in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the variable that will hold the XOR result.

DSA Javascript

let result = [1];

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

A0

B1

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing with 1 or null causes incorrect results.

We start with 0 because XOR with 0 returns the other number unchanged.

2fill in blank

medium

Complete the for loop to iterate over the array elements.

DSA Javascript

for (let i = 0; i [1] arr.length; i++) {

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

A<

B>

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= causes index out of range error.

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

3fill in blank

hard

Fix the error in the XOR operation inside the loop.

DSA Javascript

result = result [1] arr[i];

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

A&

B+

C-

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using + or & instead of ^ causes wrong results.

The XOR operator in JavaScript is ^, which is used to find the unique element.

4fill in blank

hard

Fill both blanks to complete the function that returns the unique element.

DSA Javascript

function findUnique(arr) {
  let result = [1];
  for (let i = 0; i [2] arr.length; i++) {
    result = result ^ arr[i];
  }
  return result;
}

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

A0

B<

C<=

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 for initialization or <= in loop condition.

Initialize result with 0 and loop while i is less than arr.length.

5fill in blank

hard

Fill all three blanks to create a concise one-liner function that finds the unique element.

DSA Javascript

const findUnique = arr => arr.reduce(([1], [2]) => [1] ^ [2], 0);

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

Aacc

Bcurr

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names or missing initial value 0.

Use 'acc' and 'curr' as parameter names and XOR them inside reduce, starting with 0.

Practice

1. What does the XOR operation do when applied between two identical numbers?

easy

A. It returns 0

B. It returns the number itself

C. It returns 1

D. It returns the sum of the numbers

Solution

1. Step 1: Understand XOR property for identical numbers

   XOR of a number with itself always results in 0 because bits cancel out.

2. Step 2: Apply XOR to example

   For example, 5 ^ 5 = 0 as each bit is the same and cancels.

3. Final Answer:

   It returns 0 -> Option A

4. Quick Check:

   X ^ X = 0 [OK]

Hint: XOR same numbers always zeroes out [OK]

Common Mistakes:

• Thinking XOR returns the number itself
• Confusing XOR with OR operation
• Assuming XOR sums numbers

2. Which of the following is the correct syntax to XOR two variables a and b in JavaScript?

easy

A. a && b

B. a ^ b

C. a xor b

D. a | b

Solution

1. Step 1: Recall JavaScript bitwise XOR operator

   In JavaScript, the XOR operator is represented by the caret symbol ^.

2. Step 2: Check each option

   a ^ b uses ^ correctly. a xor b uses 'xor' which is not valid syntax. Options A and D are logical AND and bitwise OR respectively.

3. Final Answer:

   a ^ b -> Option B

4. Quick Check:

   Use ^ for XOR in JavaScript [OK]

Hint: Use ^ symbol for XOR in JavaScript [OK]

Common Mistakes:

• Using 'xor' keyword
• Confusing XOR with && or ||
• Using | instead of ^

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

const arr = [2, 3, 2, 4, 3];
let result = 0;
for (const num of arr) {
  result ^= num;
}
console.log(result);

medium

A. 2

B. 0

C. 4

D. 3

Solution

1. Step 1: XOR all elements in the array

   Start with result = 0, then XOR with each element: 0 ^ 2 = 2, 2 ^ 3 = 1, 1 ^ 2 = 3, 3 ^ 4 = 7, 7 ^ 3 = 4.

2. Step 2: Final XOR result is the non-repeating element

   The only element that does not repeat is 4, which is the final XOR result.

3. Final Answer:

   4 -> Option C

4. Quick Check:

   Final XOR = 4 [OK]

Hint: XOR all numbers; result is unique element [OK]

Common Mistakes:

• Not starting XOR from 0
• XORing incorrectly in loop
• Assuming sum instead of XOR

4. Identify the error in the following code that tries to find the non-repeating element using XOR:

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

medium

A. Variable 'result' is not initialized to 0

B. Loop condition should be i <= arr.length

C. XOR operator is incorrect, should be &&

D. Array should be sorted before XOR

Solution

1. Step 1: Check initialization of 'result'

   The variable 'result' is declared but not initialized, so it is undefined initially.

2. Step 2: Effect of uninitialized 'result' on XOR

   XOR with undefined results in NaN, so the final output will be incorrect. Initializing 'result' to 0 fixes this.

3. Final Answer:

   Variable 'result' is not initialized to 0 -> Option A

4. Quick Check:

   Initialize result = 0 before XOR loop [OK]

Hint: Always initialize XOR accumulator to 0 [OK]

Common Mistakes:

• Not initializing XOR variable
• Using wrong loop condition
• Misusing XOR operator

5. Given an array where every element appears three times except one element which appears once, can XOR be used directly to find the unique element? Why or why not?

hard

A. Yes, but only if the array length is odd

B. Yes, XOR works for any number of repetitions

C. No, because XOR requires sorted arrays

D. No, because XOR only cancels pairs of identical numbers

Solution

1. Step 1: Understand XOR behavior with repeated elements

   XOR cancels out pairs of identical numbers because x ^ x = 0.

2. Step 2: Analyze effect of triply repeated elements

   Elements repeated thrice do not cancel out with XOR because XOR of three identical numbers is the number itself, not zero.

3. Step 3: Conclusion on XOR usage

   Therefore, XOR alone cannot find the unique element if others repeat thrice; other methods are needed.

4. Final Answer:

   No, because XOR only cancels pairs of identical numbers -> Option D

5. Quick Check:

   XOR cancels pairs, not triples [OK]

Hint: XOR cancels pairs only, not triples [OK]

Common Mistakes:

• Assuming XOR works for any repetition count
• Thinking sorting helps XOR
• Confusing XOR with sum or count