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

manpreet Best Answer 2 years ago

_x000D_ Setting a bit Use the bitwise OR operator (|) to set a bit. number |= 1UL << n; That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth bit. Use 1ULL if number is wider than unsigned long; promotion of 1UL << n doesn't happen until after evaluating 1UL << n where it's undefined behaviour to shift by more than the width of a long. The same applies to all the rest of the examples. Clearing a bit Use the bitwise AND operator (&) to clear a bit. number &= ~(1UL << n); That will clear the nth bit of number. You must invert the bit string with the bitwise NOT operator (~), then AND it. Toggling a bit The XOR operator (^) can be used to toggle a bit. number ^= 1UL << n; That will toggle the nth bit of number. Checking a bit You didn't ask for this, but I might as well add it. To check a bit, shift the number n to the right, then bitwise AND it: bit = (number >> n) & 1U; That will put the value of the nth bit of number into the variable bit. Changing the nth bit to x Setting the nth bit to either 1 or 0 can be achieved with the following on a 2's complement C++ implementation: number ^= (-x ^ number) & (1UL << n); Bit n will be set if x is 1, and cleared if x is 0. If x has some other value, you get garbage. x = !!x will booleanize it to 0 or 1. To make this independent of 2's complement negation behaviour (where -1 has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation. number ^= (-(unsigned long)x ^ number) & (1UL << n); or unsigned long newbit = !!x; // Also booleanize to force 0 or 1 number ^= (-newbit ^ number) & (1UL << n); It's generally a good idea to use unsigned types for portable bit manipulation. or number = (number & ~(1UL << n)) | (x << n); (number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth bit to x. It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.