How exactly do I do this in C/C++? Let's say I want to shift int i twice to the left and store the value in f.
f = i << 2 ?
I don't need this for a program or anything, I'm just interested in how it works. Thanks.
+6
A:
Yes.
f = i << 2
Shifts are useful in a number of bit twiddling operations.
This used to be a great way to multiply a number by four. However, these days, optimizing compilers tend to take care of that for you.
Keep in mind that the two leftmost bits are discarded.
John Gietzen
2009-10-22 00:21:10
For Intel CPUs, the last bit shifted out of the operand is stored in the carry flag. This doesn't really add to the relevant discussion, I just wanted to say it.
dreamlax
2009-10-22 02:56:05
A:
For the sake of completeness to help you with your bit operations you can check out this page: uow TEXTBOOK -> bitops.html
Robert Massaioli
2009-10-22 00:24:38
+2
A:
Yes, i << 2, f = i << 2, or f <<= 2 are all things one might want to do to shift bits.
More shift things to keep in mind:
you have
>>as well. At the bit level,>>works differently for signed and unsigned types.the priority of
<<and>>is below that of+and-, which fools some people, as one might imagine them to be more like*and/.
DigitalRoss
2009-10-22 01:00:52
+3
A:
As an additional note: Even though your question is tagged C++, it is probably worth noting that C and C++ took slightly different paths with regard to shifting negative values. In C++ the result of doing << or >> on a negative value is implementation-defined. In C >> is implementation-defined, while << produces undefined behavior.
AndreyT
2009-10-22 01:31:41