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Bit Manipulation Interview Questions

Brush up on your bitwise operations

Under the hood, numbers are just bits set to 0 or 1. Try some of these common and trickier questions involving bit operations.

Test Bit

Given a number, write a function that tests if its i^{th} bit is set.

def test_bit_set(number, index): ''' Returns True if number has the index'th bit set and False otherwise. '''

We'll say that the bits are numbered from the least significant bit (on the right) to the most significant bit (on the left).

So, the binary number 0000 0001 has the 0^{th} bit set and all the rest of its bits are clear (not set).

We can test if the value has a specific bit set using a left shift with an and.

First, we'll create a mask by taking 1 and shifting it left until the set bit is at the index we want to test.

1 << 0 → 0000 0001 # for the 0th bit 1 << 1 → 0000 0010 # for the 1st bit 1 << 2 → 0000 0100 # for the 2nd bit ... 1 << 7 → 1000 0000 # for the 7th bit

Then, we'll & the shifted 1 with the value we're testing. If the result is zero, then the bit isn't set; otherwise, it is.

& 0101 1101 0010 0000 ----------- 0000 0000
& 0101 1101 0100 0000 ----------- 0100 0000

Here's an implementation in code:

def test_bit_set(number, index): ''' Returns True if number has the index'th bit set and False otherwise. ''' mask = 1 << index return number & mask != 0

You could squish this into a one-liner if you wanted. We tend to prefer clarity over brevity though. :)

Set Bit

Given a number, write a function that sets its i^{th} bit to 1.

def set_bit(number, index): ''' Set the index'th bit of number to 1, and return the result. '''

We can set a specific bit using a left shift with an or.

First, we'll make a mask by taking a 1 and shifting it left until the set bit is at the index we want to set.

1 << 0 → 0000 0001 # for the 0th bit 1 << 1 → 0000 0010 # for the 1st bit 1 << 2 → 0000 0100 # for the 2nd bit ... 1 << 7 → 1000 0000 # for the 7th bit

Then, we'll | the shifted 1 with the value. This sets the bit to 1, leaving all the other bits unchanged.

| 0101 1101 0010 0000 ----------- 0111 1101

Here's an implementation in code:

def set_bit(number, index): ''' Set the index'th bit of number to 1, and return the result. ''' mask = 1 << index return number | mask

Again, this could be a one-liner if you wanted.

Clear Bit

Given a number, write a function that clears its i^{th} bit by setting it to 0.

def clear_bit(number, index): ''' Set the index'th bit of number to 0, and return the result. '''

We can clear a specific bit set using a left shift, a not, and an and.

First, we'll make our mask by taking 1, shifting it left until the set bit is at the index we want to clear, and not'ing the result. This makes a mask where every bit is set except for the one we want to clear.

~(1 << 0) → 1111 1110 # for the 0th bit ~(1 << 1) → 1111 1101 # for the 1st bit ~(1 << 2) → 1111 1011 # for the 2nd bit ... ~(1 << 7) → 0111 1111 # for the 7th bit

Then, we'll & the shifted 1 with the value we're testing. This clears the bit that we left as 0 and leaves all the other bits unchanged.

& 0101 1101 1011 1111 ----------- 0001 1101

Here's an implementation in code:

def clear_bit(number, index): ''' Set the index'th bit of number to 0, and return the result. ''' mask = ~(1 << index) return number & mask

Toggle Bit

Given a number, write a function that toggles its i^{th} bit. (If the bit is 1, set it to 0. If it's 0, set it to 1.

def toggle_bit(number, index): ''' Toggle the index'th bit of number. (If it's 0, set it to 1; if it's 1, set it to 0.) '''

We can set a specific bit using a left shift with an exclusive or.

First, we'll take 1 and shift it left until the set bit is at the index we want to set.

1 << 0 → 0000 0001 # for the 0th bit 1 << 1 → 0000 0010 # for the 1st bit 1 << 2 → 0000 0100 # for the 2nd bit ... 1 << 7 → 1000 0000 # for the 7th bit

Then, we'll ^ the shifted 1 with the value. If the bit was a 1, then the ^ with a 1 sets it to zero. If the bit was a 0, then the ^ with a 1 sets it to one. All the other bits are xor'd with zero, leaving them unchanged.

^ 0101 1101 0010 0000 ----------- 0111 1101
^ 0101 1101 0100 0000 ----------- 0001 1101

Here's an implementation in code:

def toggle_bit(number, index): ''' Toggle the index'th bit of number. (If it's 0, set it to 1; if it's 1, set it to 0.) ''' mask = 1 << index return number ^ mask

Single Bit Set

Given a number, write a function that determines if the number has exactly one bit set.

def single_bit_set(number): ''' Return True if number has exactly one bit set to 1; False if it has any other number of bits set to 1. '''

Sometimes, you'll hear this problem framed in terms of powers of two: "Write a function that determines if a number is a power of two."

All powers of two have exactly one bit set, so these questions are identical.

We can determine if a number has exactly one bit set with an and.

First, we'll take the number and subtract 1.

0100 0000 - 0000 0001 → 0011 1111 0000 1000 - 0000 0001 → 0000 0111 0101 0111 - 0000 0001 → 0101 0110 1110 1010 - 0000 0001 → 1110 1001

Notice how the subtraction clears the least-significant set bit and sets all the lower bits to 1. Everything to the left of the least-significant set bit is unchanged.

Look what happens when we & number with number - 1:

This clears the least-significant set bit and leaves the rest of the number unchanged.

If there is exactly one bit set, then the result of the & will be zero.

If there are multiple bits set, then the & will only clear the lowest bit, leaving the other bits set.

Here's how we'd write this in code:

def single_bit_set(number): ''' Return True if number has exactly one bit set to 1; False if it has any other number of bits set to 1. ''' # Special case for zero if number == 0: return False return number & (number - 1) == 0

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