兩個版本的PHP

Question

自減運算符在檢查PHP頁面，我發現下面的代碼：

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

一個爲什麼會把遞減運算符變過嗎？

Answer 1

--$x和$x--是不同的操作符。他們都將變量減1，但他們返回不同的事情。

• --$x：這樣可以減少$x，並返回其新值：

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

• $x--：這樣可以減少$x，並返回其原來值：

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

在for，循環它應該沒有區別。該值仍在遞減。

Answer 2

PHP支持C風格的前-和後-增量和減量運算符。

注意：遞增/遞減運算符隻影響數字和字符串。數組，對象和資源不受影響。遞減 NULL值也沒有效果，但是遞增結果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. 

爲例：

<?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

編輯：

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

OUTPUT：

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

在你的榜樣，第一個迭代將有$n = 10作爲部分--$n在for循環結束時執行。

Answer 3

他們略有不同...和它的表現的事情;嘗試類似

<?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.";