Merging bitshifts

Question

I'm trying to exact a 3 bit value from a byte for example:

unsigned char r = ((253 << 5) >> 5);

Result value should be 5 but why does it returns 253?

Answer 1

Two possibilities:

• Your expression is evaluated as an integer and cast as a sngle-byte value in the end.
• your compiler optimizes away the shift operations.

Note that your result should be 5, not 3. What you want is:

const unsigned char c = 253 & 0x07 ; // (00000111)

Your bit mask is:

253 | 1 1 1 1 1 1 0 1 | F D
  7 | 0 0 0 0 0 1 1 1 | 0 7
+++++++++++++++++++++++++++
  5 | 0 0 0 0 0 1 0 1 | 0 5

Answer 2

Your expression is evaluated using ints. Change it to:

unsigned char r = ((unsigned char)(253 << 5) >> 5);

and you should get the desired result.