三元算子左联想 [英] Ternary operator left associativity

问题描述

在PHP手册中，我在操作员"下找到了以下用户提供的说明" .

In the PHP manual, I find the following 'user contributed note' under "Operators".

请注意，在php中，三元运算符?:具有左联想性，与C和C ++中具有右联想性的情况不同.

Note that in php the ternary operator ?: has a left associativity unlike in C and C++ where it has right associativity.

您不能编写这样的代码(如您在C/C ++中所习惯的那样):

You cannot write code like this (as you may have accustomed to in C/C++):

<?php 
$a = 2; 
echo ( 
    $a == 1 ? 'one' : 
    $a == 2 ? 'two' : 
    $a == 3 ? 'three' : 
    $a == 4 ? 'four' : 'other'); 
echo "\n"; 
// prints 'four' 

我实际上尝试过，它确实打印了four.但是我不明白它背后的原因，仍然觉得它应该打印two或other.

I actually try it and it really prints four. However I could not understand the reason behind it and still feel it should print two or other.

有人可以解释一下这里发生了什么以及为什么要打印四个"吗?

Can someone please explain what is happening here and why it is printing 'four'?

推荐答案

在任何理智的语言中，三元运算符都是右关联的，因此您可以期望这样的代码被解释为:

In any sane language, the ternary operator is right-associative, such that you would expect your code to be interpreted like this:

$a = 2;
echo ($a == 1 ? 'one' :
     ($a == 2 ? 'two' :
     ($a == 3 ? 'three' :
     ($a == 4 ? 'four' : 'other'))));    # prints 'two'

但是，PHP三元运算符是奇怪的左关联，因此您的代码实际上等效于此:

However, the PHP ternary operator is weirdly left-associative, such that your code is actually equivalent to this:

<?php
$a = 2;
echo (((($a == 1  ? 'one' :
         $a == 2) ? 'two' :
         $a == 3) ? 'three' :
         $a == 4) ? 'four' : 'other');   # prints 'four'

如果仍然不清楚，则评估如下:

In case it still isn't clear, the evaluation goes like this:

echo ((((FALSE    ? 'one' :
         TRUE)    ? 'two' :
         $a == 3) ? 'three' :
         $a == 4) ? 'four' : 'other');

echo ((( TRUE     ? 'two' :
         $a == 3) ? 'three' :
         $a == 4) ? 'four' : 'other');

echo ((  'two'    ? 'three' :
         $a == 4) ? 'four' : 'other');

echo (    'three' ? 'four' : 'other');

echo 'four';

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