了解php中的运算符优先级 [英] Understanding operator precedence in php

问题描述

我在生产中有以下代码似乎正在引起无限循环.

I have the following code in production that appears to be causing an infinite loop.

 $z=1;
 while (!$apns = $this->getApns($streamContext) && $z < 11)
 {
    myerror_log("unable to conncect to apple. sleep for 2 seconds and try again");
    $z++;
    sleep(2);
 }

如何应用导致此行为的优先规则?

How are the precedence rules getting applied that cause this behavior?

http://php.net/manual/en/language.operators.precedence.php

我在文档中看到此注释:

I see this note in the docs:

尽管=的优先级低于大多数其他运算符，但PHP会 仍然允许类似于以下内容的表达式:if(！$ a = foo())，在 在这种情况下，foo()的返回值放入$ a中.

Although = has a lower precedence than most other operators, PHP will still allow expressions similar to the following: if (!$a = foo()), in which case the return value of foo() is put into $a.

这使我认为= =应该首先被评估.然后！然后是&& ;，这不会导致无限循环.

Which makes me think the the = should be evaluated first. then the ! then the &&, which would not cause an infinite loop.

推荐答案

您的代码正在这样评估:

Your code is evaluating like this:

while (!($apns = ($this->getApns($streamContext) && ($z < 11))))

这就是为什么您看到无限循环的原因($z >= 11，$apns为假，因此条件始终为true).此优先级的原因是，特殊规则仅适用于有效分配的左上的!(优先级低于=).它对右侧的布尔运算符没有影响，布尔运算符的行为与任何理智的语言相同.

which is why you see the infinite loop (as soon as $z >= 11, $apns is false, so the condition is always true). The reason for this precedence is that the special rules only apply to ! on the left of the assignment being valid (having lower precedence than =). It has no effect on the boolean operator on the right, which behaves as it would in any sane language.

您的风格不好.试试看，它更易读，只是最终值$z不同(如果很重要，您可以调整break语句.

Your style is bad. Try this, which is much more readable and only differs in the final value of $z (and if that's important you can tweak the break statement.

for( $z = 1; $z < 11; ++ $z ) {
    // note extra brackets to make it clear that we intend to do assignment not comparison
    if( ($apns = $this->getApns($streamContext)) ) {
        break;
    }
    myerror_log("unable to conncect to apple. sleep for 2 seconds and try again");
    sleep(2);
}

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