两次优先的空合并运算符[重复]

您需要将三元条件括起来:

<?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

如你所料。

参见“三元运算符”底部的注释 PHP Ternary operator help .

正在从左到右计算表达式。所以你实际上得到了:

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

所以 $a=2 你得到:

  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');

如此 echo 'four' .

请记住,php是动态类型的,并将任何非零的非空值视为true。

论 the Comparison Operators page in the PHP Manual 他们解释说php的行为是 "non-obvious" 嵌套(堆叠)三元运算符时。

你写的代码是这样的:

$a = 2;

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

// prints 'four'

AS $a 是2而且都是 'two' 和 'three' 也是真的,你会得到 four “因此,你不再比较 'four' 是真是假。

如果你想改变这一点,你必须把括号放在不同的地方[也被注:beingsimpler和mgwynne]:

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

// prints 'two'

分组条件有问题,只需要添加括号来分隔它们。

$a = 2;
echo (
$a == 1 ? 'one' :
($a == 2 ? 'two' :
($a == 3 ? 'three' :
($a == 5 ? 'four' : 'other'))));
echo "\n";
// prints 'four'
exit;

解决了的。

这就是我想出的帮助自己理解三元运算符的左与右关联性的方法。

// PHP

$a = "T";
$vehicle =  $a == "B" ? "bus" :
            $a == "A" ? "airplane" :
            $a == "T" ? "train" :
            $a == "C" ? "car" :
            $a == "H" ? "horse" : "feet";

            // (as seen by the PHP interpreter)
            // INITIAL EXPRESSION: ((((($a == "B" ? "bus" : $a == "A") ? "airplane" : $a == "T") ? "train" : $a == "C") ? "car" : $a == "H") ? "horse" : "feet");
            // STEP 1:             (((((FALSE ? "bus" : FALSE) ? "airplane" : TRUE) ? "train" : FALSE) ? "car" : FALSE) ? "horse" : "feet")
            // STEP 2:             ((((FALSE ? "airplane" : TRUE) ? "train" : FALSE) ? "car" : FALSE) ? "horse" : "feet")
            // STEP 3:             (((TRUE ? "train" : FALSE) ? "car" : FALSE) ? "horse" : "feet")
            // STEP 4:             (("train" ? "car" : FALSE) ? "horse" : "feet")
            // STEP 5:             ("car" ? "horse" : "feet")
            // FINAL EVALUATION:   ("horse")

            // If you used the initial expression here (with the parenthesis) in a different language, it would also evaluate to "horse."

echo $vehicle; // gives us "horse"

这与:

// EVERY OTHER LANGUAGE

var a = "T";
var vehicle =   a == "B" ? "bus" :
                a == "A" ? "airplane" :
                a == "T" ? "train" :
                a == "C" ? "car" :
                a == "H" ? "horse" : "feet";

                // (as seen by the other language's interpreter)
                // INITIAL EXPRESSION: (a == "B" ? "bus" : (a == "A" ? "airplane" : (a == "T" ? "train" : (a == "C" ? "car" : (a == "H" ? "horse" : "feet")))));
                // STEP 1:             (FALSE ? "bus" : (FALSE ? "airplane" : (TRUE ? "train" : (FALSE ? "car" : (FALSE ? "horse" : "feet")))))
                // STEP 2:             (FALSE ? "bus" : (FALSE ? "airplane" : (TRUE ? "train" : (FALSE ? "car" : "feet"))))
                // STEP 3:             (FALSE ? "bus" : (FALSE ? "airplane" : (TRUE ? "train" : "feet")))
                // STEP 4:             (FALSE ? "bus" : (FALSE ? "airplane" : "train"))
                // STEP 5:             (FALSE ? "bus" : "train")
                // FINAL EVALUATION:   ("train")

                // If you used the initial expression here (with the parenthesis) in PHP, it would also evaluate to "train."

console.log(vehicle); // gives us "train"

如果您注意到,在php示例中,最里面的表达式在左侧,而在第二个示例中,最里面的表达式在右侧。每个步骤计算下一个最里面的表达式,直到有一个结果为止。如果要在php中嵌套三元操作,括号显然非常重要!