# PHP shift operations, shift Operation Learning notes _php Tutorial

Source: Internet
Author: User
Here are some common about PHP shift operations, shift operation Learning notes, I hope the article to the students bring value.

The application of bitwise arithmetic to Formulas

Clear 0 take a bit to use with, a position one is available or

To reverse and swap, easily use XOR or

Shift Operations

Point 1 They are both binocular operators, two operation components are shaped, and the result is shaping.

2 "<<" Shift left: fill 0 on the right-hand side, and the left bit will be squeezed out of the head, and its value is equal to 2.

3 ">>" right Shift: the right bit is squeezed out. For the left-hand exit, if it is a positive number, the vacancy is 0, if it is negative, it may be 0 or 1, depending on the computer system used.

4 ">>>" operator, the right bit is squeezed out, for the left side to move out of the vacancy is 0.

The application of the bitwise operator (source operand s mask mask)

(1) Bitwise AND--&

1 Clear 0 Special positioning (mask in the specific position 0, the other bit is 1,s=s&mask)

2 Take a number of the middle finger positioning (mask in a particular position 1, the other bit is 0,s=s&mask)

(2) Bitwise OR--|

It is commonly used to set the source operand to some location 1, other bits unchanged. (Mask in a specific position 1, the other bit is 0 s=s|mask)

(3) Bit XOR--^

1 reverse the value of a particular bit (at a specific position in mask 1, the other bit is 0 s=s^mask)

2 without introducing a third variable, swap the value of two variables (set A=A1,B=B1)

Target Operation Post-operation status

A=A1^B1 a=a^b A=A1^B1,B=B1

B=A1^B1^B1 b=a^b A=A1^B1,B=A1

A=B1^A1^A1 a=a^b A=B1,B=A1

Binary complement Operation formula:

-X = ~x + 1 = ~ (x-1)

~x =-x-1

-(~x) = X+1

~ (-X) = X-1

X+y = x-~y-1 = (x|y) + (x&y)

XY = x + ~y + 1 = (x|~y)-(~x&y)

X^y = (x|y)-(X&y)

X|y = (x&~y) +y

X&y = (~x|y)-~x

X==y: ~ (x-y|y-x)

X!=y:x-y|y-x

x< y: (XY) ^ ((x^y) & ((x-y) ^x))

X<=y: (X|~y) & ((x^y) |~ (y-x))

x< y: (~x&y) | ((~x|y) & (x-y))//unsigned x, y comparison

X<=y: (~x|y) & ((x^y) |~ (y-x))//unsigned x, y comparison

Application examples

(1) determine whether the int variable A is an odd or an even number

a&1 = 0 Even

a&1 = 1 Odd

(2) Take the K-bit (k=0,1,2......sizeof (int)) of the type int variable A, i.e. a>>k&1

(3) The K-position of the int variable A is cleared 0, i.e. a=a&~ (1<<>< p=""><>

(4) Place the k position of the int variable a 1, i.e. a=a| (1<<>< p=""><>

(5) int type variable loop left shift k times, namely a=a< >16-k (set sizeof (int) =16)

(6) int type variable a loop right shift k times, namely A=a>>k|a<<16-k (set sizeof (int) =16)

(7) Average of integers

For two integers x, y, if averaging is used (x+y)/2, an overflow is generated because the x+y may be greater than Int_max, but we know that their average is definitely not overflow, we use the following algorithm:

int average (int x, int y)//Returns the mean of X, y

{

Return (X&y) + ((x^y) >>1);

}

(8) Determine whether an integer is a power of 2, for a number x >= 0, to determine whether he is a power of 2

Boolean power2 (int x)

{

Return ((x& (x-1)) ==0) && (x!=0);

}

(9) Swap two integers without temp

void swap (int x, int y)

{

x ^= y;

Y ^= x;

x ^= y;

}

(10) Calculate absolute value

int abs (int x)

{

int y;

y = x >> 31;

Return (x^y)-y; Or: (x+y) ^y

}

(11) The modulo operation is converted into a bitwise operation (in the case of no overflow)

A% (2^n) equivalent to A & (2^n-1)

(12) The multiplication operation is converted into a bitwise operation (in the case of no overflow)

A * (2^n) equivalent to a<< n

(13) The division operation translates into a bitwise operation (in the case of no overflow)

A/(2^n) equivalent to a>> n

Example: 12/8 = = 12>>3

(+) A% 2 equivalent to A & 1

if (x = = a) x= b;

else x= A;

Equivalent to x= a ^ b ^ x;

(x) The opposite number is expressed as (~x+1)

Finally add some about bits move operation

PHP is mainly designed for text operations, in fact, PHP is not suitable for mathematical operations, efficiency is not high, but because this project has a thing must use the binary displacement operation, in PHP encountered some trouble.

Because PHP has only 32-bit signed integers, there are no 64-bit long integers, and there is no unsigned integer. The range of its integral type is -231-1~231, beyond which it will be interpreted as a floating-point number. Therefore, 0xFFFFFFFF, direct printing, shows the 4294967295, and 232:

>> 0xFFFFFFFFF
4294967295
>> GetType (0xFFFFFFFF)
' Double '

In a 32-bit signed integer, 0xFFFFFFFF should represent-1:

>> (int) 0xFFFFFFFFF
-1

While PHP does not support floating-point bits, if it is to be done, it will first be converted to an integer, and the final result will be returned as an integer type:

>> 1 << 31
-2147483648
>> 1 << 30
1073741824
>> 1 << 32
1
>> 0xFFFFFFFF >> 1
-1

At the same time, the right shift operation of PHP, the high will fill the sign bit, and PHP does not provide Java-like >>> to force the fill 0:

>> 1 << 32
1
>> 0xFFFFFFFF >> 1
-1
>> 0xFFFFFFFF >> 2
-1
>> 0xFFFFFFFF >> 3
-1
>> 0xFFFFFFFF >> 31
-1

How to solve this problem, I have considered using the Bcmath mathematical function library, directly processing the string representation of the integer, or the gmp/bigint extension and so on. But I think since the use of strings, then I can complete the string, convert the number to 32 binary string, and then manually fill 0, and finally converted back.

Don't know who has a better way, please tell me.

The code is as follows:
(in addition, the code can be extended to any bit 2 of the displacement operation, here I did not do)

Php

 The code is as follows Copy Code /*** Unsigned 32-bit right shift* @param mixed \$x The number to be manipulated, if it is a string, it must be in decimal form* @param string \$bits right Shift number* @return Mixed result, floating-point numbers will be returned if the integer range is exceeded*/function Shr32 (\$x, \$bits) {Two cases where displacement is out of rangeif (\$bits <= 0) {return \$x;}if (\$bits >= 32) {return 0;}Convert to a string representing a binary number\$bin = Decbin (\$x);\$l = strlen (\$bin);The length of the string exceeds the bottom 32 bits, the length is not enough, then the fill high is 0 to 32 bitsif (\$l > 32) {\$bin = substr (\$bin, \$l-32, 32);}elseif (\$l < 32) {\$bin = Str_pad (\$bin, +, ' 0 ', str_pad_left);}Remove the number of digits to move and fill the left 0Return Bindec (Str_pad (substr (\$bin, 0, 32-\$bits), +, ' 0 ', str_pad_left));}/*** Unsigned 32-bit left shift* @param mixed \$x The number to be manipulated, if it is a string, it must be in decimal form* @param string \$bits left shift number* @return Mixed result, floating-point numbers will be returned if the integer range is exceeded*/function Shl32 (\$x, \$bits) {Two cases where displacement is out of rangeif (\$bits <= 0) {return \$x;}if (\$bits >= 32) {return 0;}Convert to a string representing a binary number\$bin = Decbin (\$x);\$l = strlen (\$bin);The length of the string exceeds the bottom 32 bits, the length is not enough, then the fill high is 0 to 32 bitsif (\$l > 32) {\$bin = substr (\$bin, \$l-32, 32);}elseif (\$l < 32) {\$bin = Str_pad (\$bin, +, ' 0 ', str_pad_left);}Remove the number of digits you want to move and fill the right 0Return Bindec (Str_pad (substr (\$bin, \$bits), +, ' 0 ', str_pad_right));}

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