Question

在检查PHP页面时，我注意到以下代码：

{{1}}

为什么要将变量运算符放在变量之前？

Answer 1

PHP支持C样式的前-和后-递增和递减运算符。

注意：递增/递减运算符仅影响数字和字符串。数组，对象和资源不受影响。递减 NULL值也没有效果，但递增它们会产生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

修改

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

<强>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，第一次迭代将具有$n = 10，因为在for循环结束时执行了部分--$n。

Answer 2

--$x和$x--是不同的运营商。它们都将变量减1，但它们返回不同的东西。

--$x：这会减少$x并返回其新值：

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

$x--：这会减少$x并返回其原始值：

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

在for循环中，它不应该有所作为。该值仍在递减。

Answer 3

它们略有不同......它是一种表现的东西;尝试像

这样的东西

<?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中的两个版本的递减运算符

3 个答案: