CODEWITHSUNDEEP
c++cbit-manipulation
Drifter
Drifter

Reputation: 61

How to shift left one specific bit?

I want to shift left only one bit in a specific place leaving its position 0, so I do not want to shift the whole variable with << operator, here is an example: say the variable has the value 1100 1010 and I want to shift the fourth bit then the result should be 1101 0010.

Upvotes: 2

Views: 3500

Answers (4)

Falk H&#252;ffner
Falk H&#252;ffner

Reputation: 5040

A simpler way is

(x & 0b11101111) + (x & 0b00001000)

that is, clear the bit that will be shifted into and add the bit to be shifted, which will overflow to the left if it is 1.

Upvotes: 1

NathanOliver
NathanOliver

Reputation: 181068

For C++, I'd just use a std::bitset. Since you set the bit of pos + 1 to the value of the bit at pos, and then set the bit at pos to 0 this translate into bitset code that is quite easy to read. That would give you a function like

unsigned char shift_bit_bitset(unsigned char val, unsigned pos)
{
    std::bitset<8> new_val(val);
    new_val[pos + 1] = new_val[pos];
    new_val[pos] = 0;
    return new_val.to_ulong();
}

Upvotes: 4

scohe001
scohe001

Reputation: 15456

Maybe not the shortest/cleanest way, but this'll do it:

unsigned shift_bit = 4;
unsigned char val = 0xCA; // 1100 1010

unsigned char bit_val = val & (1 << shift_bit - 1); // Get current bit value
val = val & ~(1 << shift_bit - 1);                  // Clear initial bit location
val = bit_val ?                                     // Update next bit to 0 or 1
        val | (1 << shift_bit) : 
        val & ~(1 << shift_bit);

See it work with the test cases specified in your question and comments here: ideone

Upvotes: 2

R Sahu
R Sahu

Reputation: 206747

Steps to get there.

  1. Pull out bit value from the original number.
  2. Left shift the bit value by one.
  3. Merge the bit-shifted value back to the original number.
// Assuming C++14 or later to be able to use the binary literal integers
int a = 0b11001010;  
int t = a & 0b00001000;  // Pull out the 4-th bit.
t <<= 1;                 // Left shift the 4-th bit.
a = a & 0b11100111;      // Clear the 4-th and the 5-th bit
a |= t;                  // Merge the left-shifted 4-th bit.

Upvotes: 7

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