Bit shifting left

Question

Let's say I want to bit shift i twice to the left and store the value in f.

f = i << 2;

Is that correct? How exactly do I do this in C/C++?

Answer 1

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.

Answer 2

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.

Answer 3

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 /.

Answer 4

For the sake of completeness to help you with your bit operations you can check out this page: uow TEXTBOOK -> bitops.html