Reverse Bits of a Number in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to shift bits to the right by one position.

DSA Rust

let mut n = num;
let mut rev = 0;
for _ in 0..32 {
    rev = rev << 1;
    rev |= n & 1;
    n = n [1] 1;
}
rev

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

A<<

B+

C>>

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using left shift '<<' instead of right shift '>>'.

Using addition '+' or subtraction '-' instead of bit shifts.

We shift the bits of n to the right by 1 to process the next bit.

2fill in blank

medium

Complete the code to shift the reversed bits to the left by one position.

DSA Rust

let mut rev = 0;
for _ in 0..32 {
    rev = rev [1] 1;
    rev |= n & 1;
    n = n >> 1;
}
rev

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

A<<

B>>

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using right shift '>>' instead of left shift '<<'.

Using addition '+' or subtraction '-' instead of bit shifts.

We shift rev to the left by 1 to make space for the next bit.

3fill in blank

hard

Fix the error in the code to correctly reverse bits of a 32-bit number.

DSA Rust

fn reverse_bits(num: u32) -> u32 {
    let mut n = num;
    let mut rev = 0;
    for _ in 0..32 {
        rev = rev << 1;
        rev |= n & 1;
        n = n [1] 1;
    }
    rev
}

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

A>>

B<<

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using left shift '<<' instead of right shift '>>'.

Using addition '+' or subtraction '-' instead of bit shifts.

We must shift n to the right by 1 to process bits from least significant to most significant.

4fill in blank

hard

Fill both blanks to complete the code that reverses bits and returns the result.

DSA Rust

fn reverse_bits(num: u32) -> u32 {
    let mut n = num;
    let mut rev = 0;
    for _ in 0..32 {
        rev = rev [1] 1;
        rev |= n & 1;
        n = n [2] 1;
    }
    rev
}

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

A<<

B>>

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the shift directions for rev and n.

Using addition or subtraction instead of bit shifts.

Shift rev left to make space, and shift n right to process next bit.

5fill in blank

hard

Fill all three blanks to create a function that reverses bits and prints the result.

DSA Rust

fn reverse_bits(num: u32) -> u32 {
    let mut n = num;
    let mut rev = 0;
    for _ in 0..32 {
        rev = rev [1] 1;
        rev |= n & [2];
        n = n [3] 1;
    }
    rev
}

fn main() {
    let num = 43261596;
    let reversed = reverse_bits(num);
    println!("Reversed bits: {}", reversed);
}

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

A<<

B1

C>>

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Masking with 0 instead of 1.

Using wrong shift directions.

Forgetting to mask n before OR operation.

Shift rev left, mask n with 1 to get last bit, shift n right to move to next bit.

Practice

1. What does the operation of reversing bits of a number mean in simple terms?

easy

A. Adding 1 to the binary number

B. Changing all 0s to 1s and 1s to 0s

C. Flipping the order of bits from left to right

D. Shifting bits to the left by one position

Solution

1. Step 1: Understand bit representation

   Bits are the 0s and 1s that represent a number in binary form.

2. Step 2: Define reversing bits

   Reversing bits means reading the bits from right to left instead of left to right, effectively flipping their order.

3. Final Answer:

   Flipping the order of bits from left to right -> Option C

4. Quick Check:

   Reverse bits = flip order [OK]

Hint: Reverse bits means flip their order, not invert values [OK]

Common Mistakes:

• Confusing reversing bits with bitwise NOT (inverting bits)
• Thinking reversing means shifting bits
• Assuming reversing adds or subtracts from the number

2. Which Rust code snippet correctly reverses the bits of a 4-bit unsigned integer n?

easy

A. let mut rev = 0; for _ in 0..4 { rev <<= 1; rev |= n & 1; n >>= 1; }

B. let mut rev = 0; for _ in 0..4 { rev |= n & 1; rev <<= 1; n >>= 1; }

C. let mut rev = 0; for _ in 0..4 { rev += n & 1; n <<= 1; }

D. let mut rev = 0; for _ in 0..4 { rev <<= 1; rev |= n << 1; n >>= 1; }

Solution

1. Step 1: Analyze bit reversal logic

   We shift rev left by 1, add the least significant bit of n, then shift n right by 1.

2. Step 2: Check each option

   let mut rev = 0; for _ in 0..4 { rev <<= 1; rev |= n & 1; n >>= 1; } correctly does rev <<= 1; rev |= n & 1; n >>= 1; in loop. Others have wrong order or operations.

3. Final Answer:

   let mut rev = 0; for _ in 0..4 { rev <<= 1; rev |= n & 1; n >>= 1; } -> Option A

4. Quick Check:

   Shift rev left, add bit, shift n right [OK]

Hint: Shift rev left before adding bit, shift n right after [OK]

Common Mistakes:

• Adding bit before shifting rev left
• Shifting n left instead of right
• Using addition instead of bitwise OR

3. What is the output of the following Rust code snippet?

fn reverse_bits(mut n: u8) -> u8 {
    let mut rev = 0;
    for _ in 0..8 {
        rev <<= 1;
        rev |= n & 1;
        n >>= 1;
    }
    rev
}

fn main() {
    let num = 13u8; // binary 00001101
    println!("{}", reverse_bits(num));
}

medium

A. 176

B. 11

C. 101

D. 208

Solution

1. Step 1: Convert 13 to binary and reverse bits

   13 in 8 bits is 00001101. Reversing bits gives 10110000.

2. Step 2: Convert reversed bits back to decimal

   10110000 binary equals 128+32+16=176 decimal.

3. Final Answer:

   176 -> Option A

4. Quick Check:

   00001101 reversed = 10110000 = 176 [OK]

Hint: Reverse bits visually, then convert binary to decimal [OK]

Common Mistakes:

• Not using 8 bits, missing leading zeros
• Confusing bit order or reversing incorrectly
• Calculating decimal value wrong

4. Identify the bug in this Rust function that attempts to reverse bits of a 4-bit number:

fn reverse_bits(mut n: u8) -> u8 {
    let mut rev = 0;
    for _ in 0..4 {
        rev |= n & 1;
        rev <<= 1;
        n >>= 1;
    }
    rev
}

medium

A. Using bitwise OR instead of addition

B. Shifting rev left after adding bit causes incorrect result

C. Not shifting n left instead of right

D. Loop should run 8 times for 4-bit number

Solution

1. Step 1: Analyze order of operations in loop

   rev |= n & 1 adds bit, then rev <<= 1 shifts left, causing last bit to be zeroed out.

2. Step 2: Correct order is shift rev left first, then add bit

   Shifting rev left after adding bit loses the last bit added, so order must be reversed.

3. Final Answer:

   Shifting rev left after adding bit causes incorrect result -> Option B

4. Quick Check:

   Shift rev left before OR bit [OK]

Hint: Shift rev left before OR bit to avoid losing bits [OK]

Common Mistakes:

• Adding bit before shifting rev left
• Loop count mismatch for bit size
• Shifting n left instead of right

5. Given a 16-bit unsigned integer, how would you efficiently reverse its bits using Rust's built-in methods combined with bitwise operations?

hard

A. Manually loop 16 times shifting bits and reconstruct reversed number

B. Use n.swap_bytes() to reverse bits

C. Convert number to string, reverse characters, then parse back

D. Use n.reverse_bits() method directly on the number

Solution

1. Step 1: Know Rust's built-in bit reversal

   Rust's primitive integer types have a reverse_bits() method that reverses bits efficiently.

2. Step 2: Compare options

   Use n.reverse_bits() method directly on the number uses the built-in method directly, which is efficient and simple. Others are manual or incorrect methods.

3. Final Answer:

   Use n.reverse_bits() method directly on the number -> Option D

4. Quick Check:

   Use built-in reverse_bits() for efficiency [OK]

Hint: Use built-in reverse_bits() method for fast bit reversal [OK]

Common Mistakes:

• Trying manual loops instead of built-in method
• Confusing byte swap with bit reversal
• Using string conversion which is inefficient