PHP 中递减运算符的两个版本

Question

在检查 PHP 页面时，我注意到以下代码：

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

为什么要将减量运算符 放在变量 之前？

Answer 1

PHP 支持 C 风格的 pre- 和 post- 递增和递减运算符。

Note: The increment/decrement operators only affect numbers and strings. Arrays, objects and resources are not affected. Decrementing NULL values has no effect too, but incrementing them results in 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 Manual: Incrementing/Decrementing Operators

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

In your example, the very first iteration will have $n = 10 as the part --$n is executed at the end of the for-loop.

Answer 2

--$x 和 $x-- 是不同的运算符。它们都将变量减 1，但它们 return 不同。

--$x：这会减少 $x 和 return 其新的值：

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

$x--：这会减少 $x 和 return 其原始值：

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

在 for 中，循环应该没有什么区别。该值仍在递减。

Answer 3

它们略有不同……这是性能问题；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.";

PHP 中递减运算符的两个版本

Two versions of decrement operator in PHP

php

decrement