C provides a set of shift operations for shifting bit patterns to the left and to the right.
if x having bit representation [$x_{w-1}$,
- left shift: x << k yields a value with bit representation [$x_{w-k-1}$,
$x_{w-k-2}$ , ...,$x_0$, 0,..,0] - logical right shift: A logical right shift fills the left each with k zeros, giving a result [0, ...,0,$x_{w-1}$,
$x_{w-2}$ , ...,$x_k$ ] - arithmetic right shift: An arithmetic right shift fills the left end with k repetitions of the most significant bit, giving a result [$x_{w-1}$, ...,
$x_{w-1}$ ,$x_{w-2}$ , ...,$x_k$ ]
Here a example:
| Operation | value1 | value2 |
|---|---|---|
| x | [01100011] | [10010101] |
| x << 4 | [00110000] | [01010000] |
| x >> 4 (logical) | [00000110] | [00001001] |
| x >> 4 (arithmetic) | [00000110] | [11111001] |
Pay attention: the C standards do not precisely define which type of right shift should be used with signed numbers, either arithmetic or logical shifts may be used. In practice, however, almost all compiler/machine combinations use arithmetic right shifts for signed data, and many programmers assume this to be the case.
There is a practice: 2.16
For a data ype consisting of w bits, what should be the effect of shifting by some value k >= w?
The C standards carefully avoid stating what should be done in such a case. On many machine, the shift amount is computed as k mod w.