了解嵌套的PHP三元运算符 [英] Understanding nested PHP ternary operator

问题描述

我不明白该输出("four")是怎么来的?

I dont understand how that output ("four") comes?

$a = 2;

echo
  $a == 1 ? 'one'   :
  $a == 2 ? 'two'   :
  $a == 3 ? 'three' :
  $a == 5 ? 'four'  : 
    'other'  
  ;

// prints 'four'

我不明白为什么打印"four".

I don't understand why "four" gets printed.

推荐答案

您需要将三元条件括起来:

You need to bracket the ternary conditionals:

<?php

for ($a=0; $a < 7; $a++) {
  echo (
    $a == 1 ? 'one' :
    ($a == 2 ? 'two' :
    ($a == 3 ? 'three' :
    ($a == 5 ? 'four' : 'other'))));
    echo "\n";
    // prints 'four'
}
exit;
?>

返回:

other
one
two
three
other
four
other

如您所愿.

请参见 PHP三元运算符帮助，在三元运算符"底部的注释.

See the note at the bottom of "Ternary operators" at PHP Ternary operator help.

表达式从左到右进行求值.因此，您实际上得到了:

The expressions are being evaluated left to right. So you are actually getting:

  echo (
    ((($a == 1 ? 'one' : $a == 2)
     ? 'two' : $a == 3) ? 'three' :
    $a == 5) ? 'four' : 'other');

因此对于$a=2，您将得到:

  echo (
    ((($a==2) ? 'two' : $a == 3) ? 'three' :
    $a == 5) ? 'four' : 'other');

然后

  echo (
    ((true ? 'two' : $a == 3) ? 'three' :
    $a == 5) ? 'four' : 'other');

然后

  echo (
    ('two' ? 'three' : $a == 5) ? 'four' : 'other');

然后

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

以及echo 'four'.

请记住，PHP是动态类型的，并将任何非零，非null值都视为TRUE.

Remember that PHP is dynamically typed and treats any non-zero non-null values as TRUE.

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