What does "<<" mean in PHP?

Question

Possible Duplicate:
Reference - What does this symbol mean in PHP?

What does the << mean in this line of PHP?

$count = (1 << $count_log2) - 1;

and yet nobody made a [picture](http://stackoverflow.com/questions/2007526/what-does-the-operator-mean-in-c/2007563#2007563) :( — mario, Dec 28 '10 at 18:23

score 4 · Accepted Answer · answered Dec 28 '10 at 18:18

this is the shift left operator.

so in your example you are shifting left the value 1 , $count_log2 times to the left. so the value is 2^count_log2.

1 in 8 bit binary is 00000001 so if $count_log2 = 4, we need to get 2^4 = 16.

shifting left means moving the 1 left 4 times (since $count_log2 = 4). lets do the steps.

00000010 (2 in decimal)
00000100 (4 in decimal)
00001000 (8 in decimal)
00010000 (16 in decimal)

so we got 2^4.

a common reason for using shift operation is that it takes less time for the processor to do a shift operation than using multiplication.

score 1 · Answer 2 · answered Dec 28 '10 at 18:10

1

Left Bitshift, http://php.net/manual/en/language.operators.bitwise.php

answered Dec 28 '10 at 18:10

plundra

18,542
3
33
27

Reese Moore · Answer 3 · 2010-12-28T18:16:46.410

1

It is a left bitshift operator. See the Bitwise Operators page of the PHP manual.

To quote the manual:

$a << $b - Shift left - Shift the bits of $a $b steps to the left (each step means "multiply by two")

In this specific case, $count = (1 << $count_log2) - 1 is the same as setting $count to pow(2, $count_log2) - 1

edited Dec 28 '10 at 18:16

answered Dec 28 '10 at 18:11

Reese Moore

11,524
3
24
32

score 1 · Answer 4 · answered Dec 28 '10 at 18:12

1

<< and >> are the so-called bitshift operator.

x << n shifts the bits in the integer x n places to the left, effectively multiplying x with 2 to the power of n.

Similarly x >> n shifts to the left, dividing x by 2 to the power of n.

answered Dec 28 '10 at 18:12

sepp2k

363,768
54
674
675

Error 454 · Answer 5 · 2010-12-28T18:42:00.057

1

The << and >> are called Bitwise operators, they shift left and right respectively by a certain number of bits.

In your example: 1 << $count_log2 will shift the number 1 left by the value of $count_log2. This is easier to see in binary where the number 1 represented as an 8-bit number would be:

1 - 0000 0001

If you shift this number left by 3 (1 << 3) you would get 8:

8 - 0000 1000

edited Dec 28 '10 at 18:42

answered Dec 28 '10 at 18:18

Error 454

7,255
2
33
48

score 0 · Answer 6 · answered Dec 28 '10 at 18:11

0

Shift left or shift right. See the manual on bitwise operators.

answered Dec 28 '10 at 18:11

Quentin

914,110
126
1,211
1,335

score 0 · Answer 7 · answered Dec 28 '10 at 18:12

0

Its a bitwise operator for shifting bits to the left, this is not just php but many languages use this for if binary manipulation

http://php.net/manual/en/language.operators.bitwise.php

answered Dec 28 '10 at 18:12

RobertPitt

56,863
21
114
161

score 0 · Answer 8 · answered Dec 28 '10 at 18:14

0

<< is shift left operator in PHP

$a << $b means shift the bits of $a $b steps to the left (each step means "multiply by two")

answered Dec 28 '10 at 18:14

shankhan

6,343
2
19
22

score 0 · Answer 9 · answered Dec 28 '10 at 18:14

'>>' and '<<' are bitwise operators. '>>' shifts to the right, and '<<' shifts to the left.

Think of it like this, in binary 25 is 00011001. if you performed a shift left on 25, you'd have 00110010, which is 50.

If you performed a shift right on 50, you'd have 25.

What does "<<" mean in PHP?

9 Answers9