开关评价 [英] Switch evaluations

问题描述

我最近就 switch 如何处理比较产生了争论，需要帮助解决这个问题.

I recently got into an argument over how switch handles comparisons, and need help settling it.

如果我写一个 switch 如:

switch ($x){
    case ($x > 5):
        echo "foo";
        break;
    case ($x < 5):
        echo "bar";
        break;
    default:
        echo "five";
        break;
}

它等价于哪个 if 语句?A 还是 B?

Which if statement is it equivalent to? A or B?

// A

if ($x > 5) {
    echo "foo";
} elseif ($x < 5) {
    echo "bar";
} else {
    echo "five";
}

// B

if ($x == ($x > 5)) {
    echo "foo";
} elseif ($x == ($x < 5)) {
    echo "bar";
} else {
    echo "five";
}

推荐答案

给大家澄清一下:

相当于B.

它不是两者"，有时不是一个，有时是另一个，它总是B.要了解为什么有时会看到表明它可能是 A 的结果，您需要了解 类型强制在 PHP 中的工作原理.

It is not "both", it is not sometimes its one, sometimes it is the other, it is always B. To understand why you sometimes see results that indicate that it might be A, you need to understand how type coercion works in PHP.

如果您向 switch 的参数"传递一个 falsey 值，并且您在 case 中使用了 表达式 导致布尔值，它们只会在您的表达式计算结果为 FALSE 时匹配.

If you pass in a falsey value to the "argument" of switch and you use expressions in your cases that result in a boolean value, they will only match if your expression evaluates to FALSE.

switch 基本上是一个巨大的 if/elseif 树，它执行松散比较(== 而不是 ==code>===) 传递给 switch 的值(表达式的左侧)和 case 中的表达式(右侧).

switch is basically a huge if/elseif tree that performs loose comparisons (== instead of ===) between the value passed to switch (the left side of the expression) and the expression in the cases (the right side).

这个可以很好地证明对您的代码进行修改:

This can be proved quite nicely with a variation on your code:

$x = 0;

switch ($x) {
  case $x > -1: // This is TRUE, but 0 == FALSE so this won't match
    echo "case 1";
  case $x == -1: // This is FALSE, and 0 == FALSE so this will match
    echo "case 2";
}

如果我们将其转换为两个 if/elseif 树:

And if we convert that to the two if/elseif trees:

A:

$x = 0;

if ($x > -1) {
  // It matches here
  echo "case 1";
} else if ($x == -1) {
  // and not here
  echo "case 2";
}

B:

$x = 0;

if ($x == ($x > -1)) {
  // It doesn't match here
  echo "case 1";
} else if ($x == ($x == -1)) {
  // ..but here instead
  echo "case 2";
}

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