Comparison of the differences between ++i and i++ in php_PHP tutorial

++i and i++ are used in many programming to add +1 to a variable, but there is a problem of sequence. Let me introduce some differences between them in operation.

1. Usage of ++i (take a=++i, i=2 as an example)

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

Then the final a value is equal to 3 and the i value is equal to 3.

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

2. Usage of i++ (take a=i++, i=2 as an example)

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

Then the final a value is equal to 2 and the i value is equal to 3.

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

3. ++i and i++

a=++i is equivalent to i++ , a=i

a=i++ is equivalent to a=i , i++

4. When ++i and i++ are used alone, they are equivalent to i=i+1

If assigned to a new variable, ++i first adds 1 to the i value, and i++ first assigns i to the new variable.

Performance optimization

while(++$j < 10000) ;

The code is as follows

代码如下

复制代码

方式一:   $begin = time(); $i = 0; while(++$i < 10000) {   $j = 0;   while(++$j < 10000)     ;   ; } $end = time();   时间 : 16s   方式二:   $begin = time(); $i = 0; while($i < 10000) {   $j = 0;   while($j < 10000)     ++$j;   ++$i; } $end = time();   时间:13s   方式三:   $begin = time(); $i = 0; while($i < 10000) {   $j = 0;   while($j < 10000)     $j++;   $i++; } $end = time();   时间:15s   方式四:   $begin = time(); $i = 0; while($i++ < 10000) {   $j = 0;   while($j++ < 10000)     ;   ; } $end = time(); 时间:13s

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Method 1:

$begin = time();

$i = 0;

while(++$i < 10000)

{

$j = 0;

; }

$end = time();

Time: 16s Method 2: $begin = time(); $i = 0; while($i < 10000) { $j = 0; while($j < 10000) ++$j; ++$i;

} $end = time();

Time:13s

Method 3: $begin = time(); $i = 0; while($i < 10000) { $j = 0; while($j < 10000) $j++; $i++; } $end = time(); Time:15s Method 4: $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, what is ultimately executed is OPCODE, each line of opline has two operands. For the operands, there are generally three types of access methods, temporary variables, variables, and compile-time variables. These three types of variables Among them, the third type with the fastest access is compiler variables. During the execution of OpCode, a variable plus a first-level reference will be stored in a hash structure, used to speed up access. http://www.bkjia.com/PHPjc/629024.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/629024.htmlTechArticle++i and i++ are used in many programming, and the +1 operation is added to the variable, but there is a sequencing problem , let me introduce some differences between them in operation. 1. Usage of ++i (with a=++...