I'm trying to use masks and manipulating specific bits in a byte. For example:
I want to write a program in C that flips two bits at particular positions e.g. the bit at position 0 and the one at the third position.
So, 11100011, would become 01110011.
How can I swap these bits?
14 Answers
Flipping a bit is done by XOR-ing with a mask: set bits at the positions that you want to flip, and then execute a XOR, like this:
int mask = 0x90; // 10010000
int num = 0xE3; // 11100011
num ^= mask; // 01110011Here are a few notes:
- bits are commonly counted from the least significant position, so your example flips bits in positions 4 and 7, not at positions 0 and 4
- To construct a bit mask for a single position, use expression
1 << n, wherenis the position number counting from the least significant bit. - To combine multiple bits in a single mask, use
|operator. For example,(1 << 4) | (1 << 7)constructs the mask for flipping bits 4 and 7.
If your byte is x, and you want to switch the bits at the i-th and j-th position:
x = x ^ ((1<<i) | (1<<j));So, in your case, it would just be (1<<4) | (1<<7). :)
1First of all, good luck!
One remark - it is more useful to count the bits from the right and not left, since there are various byte/word sizes (8-bit,16-bit,etc.) and that count preserves compatibility better. So in your case you are referring to bits #7 and #4 (zero-count).
Did you mean 'flip' (change 0<->1 bits) or 'switch' them between one and the other?
For the first option, the answer above (XOR with "int mask = 0x90; // 10010000") is very good. For the second one, it's a bit more tricky (but not much).
2To flip bits, you can use the exclusive OR bitwise operator. This takes two operands (typically, the value you want to operate on and the mask defining what bits will be flipped). The eXclusive OR (XOR) operator will only flip a bit if, and only if, one of the two is set to 1, but NOT both. See the (simple) example below:
#include <stdio.h>
int main(int argc, char** argv)
{ int num = 7; //00000111 int mask = 3; //00000011 int result = num ^ mask; //00000100 printf("result = %d\n", result); //should be 4 return 0;
}
