PHP Shiftleft / <<

[jamesm87]

Hi,

I am struggling to find any sites that explain exactly what shiftleft does. I want to become Zend certified however I'm failing on some of the basic stuff (mainly because this kind of stuff I would never dream using!).

From what I gather, $i = 2 << 4; means that it shifts to the left and multiplies by 2? Well that makes no sense to me, if I was to hazard a guess I'd of thought it mean 24*2 = 48 which I am sure is the wrong answer.

Any good websites that explain the math behind it?

Thanks,

James

[laserlight]

(x << y) is mathematically equivalent to (x * pow(2, y)). Likewise, (x >> y) is mathematically equivalent to (x / pow(2, y)) with integer division.

EDIT:
One assumption is that there is no overflow. Another assumption is that the shift amount is not negative. If the shift amount is negative, then treat it as (x << (n - y)) or (x >> (n - y)) respectively, where n is the number of bits used to represent the type.

[johanafm]

And what shift left/right actually does is move the bits left/right by the specified amount, which is why it equals the above mathematical expressions.

2 = 0000 0010 b (stored as 8 bits)
2 << 4 = 0010 0000 b = (0x20 = 32 = 2 16 =) 2 2⁴

3 = 0000 0011b
3 << 4 = 0011 0000b

and so on