Set the i-th bit to zero?

Question

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.

@JasonBaker Nope, that post is not showing how to set a bit to nothing but zero. — GivenPie, Sep 23 '14 at 20:37
@GivenPie the accepted answer has a section for each task (set, clear, toggle, check...) — Adriano Repetti, Sep 23 '14 at 20:38
@JasonBaker Right, clearly I didn't understand what clearing a bit even meant. — GivenPie, Sep 23 '14 at 20:39

score 17 · Accepted Answer · answered Sep 23 '14 at 20:35

17

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.

answered Sep 23 '14 at 20:35

rayryeng

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thanks, made alot of sense – GivenPie Sep 23 '14 at 20:50
@GivenPie - No worries!... but that duplicate question that is being pointed to is much more comprehensive. Suggest you give that a read too. – rayryeng Sep 23 '14 at 21:15

score 3 · Answer 2 · answered Sep 23 '14 at 20:39

3

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);

answered Sep 23 '14 at 20:39

Marco

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Set the i-th bit to zero?

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