在 if() 块中分配多个变量时出现意外行为 [英] Unexpected behavior when assigning multiple variables in if() block

问题描述

本着看到 像 Python 的and"这样的短路评估的精神在存储检查结果时我决定看看如何在 PHP 中最好地解决这个问题，但我遇到了一个问题.

In the spirit of seeing Short-circuit evaluation like Python's "and" while storing results of checks I decided to see how this could be best solved in PHP but I've run into an issue.

意外

<?php
function check_a()
{
    return 'A';
}
function check_b()
{
    return 'B';
}
function check_c()
{
    return 'C';
}

if($a = check_a() && $b = check_b() && $c = check_c())
{
    var_dump($a);
    var_dump($b);
    var_dump($c);
}

结果:

bool(true)
bool(true)
string(1) "C"

<小时>

我想要发生的事情的代码

<?php
function check_a()
{
    return 'A';
}
function check_b()
{
    return 'B';
}
function check_c()
{
    return 'C';
}

// if(($a = check_a()) && ($b = check_b()) && $c = check_c()) // equivalent to line below
if(($a = check_a()) && ($b = check_b()) && ($c = check_c()))
{
    var_dump($a);
    var_dump($b);
    var_dump($c);
}

结果:

string(1) "A"
string(1) "B"
string(1) "C"

<小时>

为什么出乎意料的例子会这样?

推荐答案

这是一个运算符优先级.赋值表达式返回分配的值，因此您希望得到 A 和 B 用于前两个操作.您获得布尔值 true 的原因是 && 运算符的优先级高于赋值运算符，因此在原始表达式中

This is a question of operator precedence. An assignment expression returns the assigned value, so you would expect to get A and B for the first two operations. The reason you're getting boolean true instead is that the && operator has a higher precedence than the assignment operator, so in the original expression

$a = check_a() && $b = check_b() && $c = check_c()

$a 获取check_a() && 的值$b = check_b() &&$c = check_c(),

$b 获取check_b() && 的值$c = check_c(),

和$c得到check_c()的值.

表达式 check_a() &&$b = check_b() &&$c = check_c() 和 check_b() &&$c = check_c() return boolean true，因为使用 && 运算符会导致表达式被评估为布尔值，并且所有组件由 && 连接的表达式的计算结果为 true.

The expressions check_a() && $b = check_b() && $c = check_c(), and check_b() && $c = check_c() return boolean true, because the use of the && operator causes the expressions to be evaluated as booleans, and all components of the expressions joined by && evaluate to true.

为了得到你期望的结果，你可以像你一样添加括号，或者你可以使用and逻辑运算符而不是&&，因为它的优先级低于赋值运算符.

To get the results you expect, you can add parentheses as you did, or you can use the and logical operator instead of &&, because it has a lower precedence than the assignment operator.

if($a = check_a() and $b = check_b() and $c = check_c()) {

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