Comparison between ++i and i++ in PHP _php tutorial

++i and i++ are available in many programs, adding +1 to the variable, but there are problems, let me introduce some of the differences they are running.

1, the use of ++i (take A=++i, i=2 as an example)

The I value is first added to 1 (that is, i=i+1) and then assigned to the variable a (that is, a=i),

Then the final a value is equal to 3, and the I value equals 3.

So a=++i equivalent to i=i+1, a=i

2, the use of i++ (take a=i++, i=2 as an example)

First assign the I value to the variable a (that is, a=i), then I value 1 (that is, i=i+1),

Then the final a value is equal to 2, and the I value equals 3.

So a=i++ equivalent to A=i, i=i+1

3, ++i and i++

A=++i equivalent to i++, a=i

a=i++ equivalent to A=i, i++

4, ++i and i++ when used alone, equivalent to I=i+1

If you assign a new variable, the ++i first assigns the I value to 1 and the i++ first to the new variable.

Performance optimization

The code is as follows

Copy Code

Way One: $begin = time (); $i = 0; while (+ + $i < 10000) { $j = 0; while (+ + $j < 10000) ; ; } $end = time (); Time: 16s Way two: $begin = time (); $i = 0; while ($i < 10000) { $j = 0; while ($j < 10000) + + $j; + + $i; } $end = time (); Time: 13s Way three: $begin = time (); $i = 0; while ($i < 10000) { $j = 0; while ($j < 10000) $j + +; $i + +; } $end = time (); Time: 15s Mode four: $begin = time (); $i = 0; while ($i + < 10000) { $j = 0; while ($j + < 10000) ; ; } $end = time (); Time: 13s

Compare the first method and the second method, because in PHP, the final execution is opcode, each line opline

There are two operands, and for operands there are generally 3 types of access, temporary variables, variables, and compile-time variables, three variables

Among them, the fastest access is the third, the compiler variable, in the opcode execution process, will say a variable plus a reference stored in a hash

Structure, which is used to speed up access.

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