What does a bitwise shift (left or right) do and what is it used for?
I've seen the operators >> and << in various code that I've looked at (none of which I actually understood), but I'm just wondering what they actually do and what some practical uses of them are.
If the shifts are like x * 2 and x / 2, what is the real difference from actually using the * and / operators? Is there a performance difference?
Answer
Here is an applet where you can exercise some bit-operations, including shifting.
You have a collection of bits, and you move some of them beyond their bounds:
1111 1110 << 2
1111 1000
It is filled from the right with fresh zeros. :)
0001 1111 >> 3
0000 0011
Filled from the left. A special case is the leading 1. It often indicates a negative value - depending on the language and datatype. So often it is wanted, that if you shift right, the first bit stays as it is.
1100 1100 >> 1
1110 0110
And it is conserved over multiple shifts:
1100 1100 >> 2
1111 0011
If you don't want the first bit to be preserved, you use (in Java, Scala, C++, C as far as I know, and maybe more) a triple-sign-operator:
1100 1100 >>> 1
0110 0110
There isn't any equivalent in the other direction, because it doesn't make any sense - maybe in your very special context, but not in general.
Mathematically, a left-shift is a *=2, 2 left-shifts is a *=4 and so on. A right-shift is a /= 2 and so on.