PHP中减量运算符的两个版本 - Two versions of decrement operator in PHP

Two versions of decrement operator in PHP

本文关键字：两个版本运算符 PHP | 更新日期: 2023-09-27

在检查PHP页面时，我注意到以下代码:

for ($n=10; $n>0; --$n) {
    //foo;
}

为什么要在变量前加上减量运算符 ?

--$x和$x--是不同的运算符。它们都将变量减1，但是它们返回不同的结果。

--$x:这将减少$x并返回其新值:

$y = --$x;
// Is equivalent to
// $x = $x-1;
// $y = $x;

$x--:减少$x并返回其原始值:

$y = $x--;
// Is equivalent to
// $y = $x;
// $x = $x - 1;

在for循环中，它应该没有区别。

PHP支持c风格的pre - -和post - -自增和自减操作符。

注意:自增/自减操作符只影响数字和字符串。阵列、对象和资源不受影响。递减NULL值也没有影响，但是增加它们的结果是1。

++$a    Pre-increment   Increments $a by one, then returns $a.
$a++    Post-increment  Returns $a, then increments $a by one.
--$a    Pre-decrement   Decrements $a by one, then returns $a.
$a--    Post-decrement  Returns $a, then decrements $a by one.

一个例子:

<?php
echo "<h3>Postincrement</h3>";
$a = 5;
echo "Should be 5: " . $a++ . "<br />'n";
echo "Should be 6: " . $a . "<br />'n";
echo "<h3>Preincrement</h3>";
$a = 5;
echo "Should be 6: " . ++$a . "<br />'n";
echo "Should be 6: " . $a . "<br />'n";
echo "<h3>Postdecrement</h3>";
$a = 5;
echo "Should be 5: " . $a-- . "<br />'n";
echo "Should be 4: " . $a . "<br />'n";
echo "<h3>Predecrement</h3>";
$a = 5;
echo "Should be 4: " . --$a . "<br />'n";
echo "Should be 4: " . $a . "<br />'n";
?>

PHP手册:递增/递减操作符

编辑:

for ($n=10; $n>0; --$n) {
   echo "Iterating:" . $n . "<br>";
}

输出:

Iterating:10
Iterating:9
Iterating:8
Iterating:7
Iterating:6
Iterating:5
Iterating:4
Iterating:3
Iterating:2
Iterating:1

在您的示例中，第一次迭代将把$n = 10作为部分，--$n在for循环的末尾执行。

它们略有不同…这是一种表现;试试

<?php 
for($i=0;$i<100000000;++$i){
    //heating up the cpu (if it uses some power saving feature or whatever)
}
$i=0;
$PreIncrementStart=microtime(true);
for($i=0;$i<100000000;++$i){
    //counting to 100 million, using pre-increment.
}
$PreIncrementEnd=microtime(true);
$i=0;
$PostIncrementStart=microtime(true);
for($i=0;$i<100000000;$i++){
    //counting to 100 million, using post-increment.
}
$PostIncrementEnd=microtime(true);
$PreTime=$PreIncrementEnd-$PreIncrementStart;
$PostTime=$PostIncrementEnd-$PostIncrementStart;
if($PreTime<$PostTime){
    echo "the fastest was pre-increment. (which totally makes sense, it's consistent with c/c++, and it uses fewer opcodes than the post-increment, and the `old` value is not returned, only the new value, so we don't need 2 values (old value AND new value, as the post-increment does)..)";
} else {
    echo "the fastest was post-increment... i am very surprised.";
}
echo "the difference was: ".abs($PreTime-$PostTime)." seconds.";