Set the i-th bit to zero? [duplicate]

Question

This question already has an answer here:

• How do you set, clear, and toggle a single bit? 27 answers

I would like to set the i-th bit to zero no matter what the i-th bit is.

  unsigned char pt = 0b01100001;
  pt[0] = 0; // its not how we do this... 

Setting it to one, we can use a mask pt | (1 << i) but i'm not sure how to create a mask for setting 0, if thats possible.

Answer 1

You just have to replace the logical OR with a logical AND operation. You would use the & operator for that:

pt = pt & ~(1 << i);

You have to invert your mask because logical ANDing with a 1 will maintain the bit while 0 will clear it... so you'd need to specify a 0 in the location that you want to clear. Specifically, doing 1 << i will give you a mask that is 000...010..000 where the 1 is in the bit position that you want, and inverting this will give 111...101...111. Logical ANDing with this will clear the bit that you want.

Answer 2

You could stick with this:

// Set bit at position `bitpos` in `pt` to `bitval`
unsigned char bitpos = 1;
unsigned char pt = 0b01100001;
bool bitval = 1;

// Clear the bit
pt &= ~(1u << bitpos);
// Set the bit
pt |= (bitval << bitpos);