Switching the bits

Question

How can I switch the certain bits of a number? For example, given bit representation (just an example, the syntax is surely wrong!):

someNumber = 00110111
changeNumber = 11100110

Then how can I change the far right bit of someNumber with the far right bit of the changeNumber without changing the rest bits of the someNumber? So the result would be:

00110111   //someNumber
11100110   //changeNumber
________
00110110

Have you looked at the bit shifting operator? http://stackoverflow.com/questions/141525/what-are-bitwise-shift-bit-shift-operators-and-how-do-they-work — Tdorno, Nov 15 '15 at 03:46

orlp · Answer 1 · 2015-11-15T04:08:01.427

3

Extract the far right bit of changeNumber:

changeNumber & 1

Remove the far right bit of someNumber:

someNumber & ~1

And OR them together:

 (changeNumber & 1) | (someNumber & ~1)

To set bit n, change 1 to 2ⁿ.

edited Nov 15 '15 at 04:08

answered Nov 15 '15 at 03:58

orlp

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R Sahu · Answer 2 · 2015-11-15T04:14:23.103

1

I suggest the following steps:

Clear the last bit of someNumber.
```
someNumber &= ~1
```
Extract the last bit of changeNumber
```
int lastBit = changeNumber & 1;
```
Set the last bit of someNumber:
```
someNumber |= lastBit;
```

edited Nov 15 '15 at 04:14

answered Nov 15 '15 at 03:59

R Sahu

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score 1 · Answer 3 · answered Nov 15 '15 at 04:09

One a similar line as Martin,

Test the last bit of someNumber, and use the result to select the operation to change some number ('bitwise and' or 'bitwise or')

#DEFINE SWITCH_MASK_OR 0b00000001
#DEFINE SWITCH_MASK_AND (~SWITCH_MASK_OR)
...
result = changeNumber & SWITCH_MASK_OR ? \
         someNumber | SWITCH_MASK_OR : someNumber & SWITCH_MASK_AND;

score 0 · Answer 4 · answered Nov 15 '15 at 03:56

AND the changenumber with a mask of 00000001, so extracting the state of the lowest bit, all others set to 0: 00000000

AND the somenumber with a mask of 11111110, so setting the lowest bit to 0 while leaving the rest unchanged: 00110110

OR the two results together, so : 00110110

Switching the bits

4 Answers4