“&&”之间的区别和“并且” ：运算符优先级和短路

发布于 2024-11-28 13:25:13 字数 1875 浏览 1 评论 0原文

我正在浏览 php.net 的运算符优先级部分，并遇到 this example 表示

$a = 1;
$b = null;
$c = isset($a) && isset($b);
$d = ( isset($a) and isset($b) );
$e = isset($a) and isset($b);
var_dump($a, $b, $c, $d, $e);
//Result:
int(1)
NULL
bool(false)
bool(false) <== I get this
bool(true)  <== I do not get this

我在代码中使用了大量的调试和详细的 print(_r) 语句来跟踪我在代码中的位置。因此，我使用 $debug 和 print_r($dataArray) 或 $verbose 并打印“Updating dataArray\n” 作为代码之间的单独语句，使我能够控制这些打印(_r) 声明。这来自我的 BASH 经验，我曾经写过很多 [[ $condition ]] && { #如果为真则执行 } || { #Do if false }。在 BASH 中，我知道它们是短路的，并利用这一事实编写了很多简单的单行代码。
现在，我观察到很多这种做法（编写 $verbose 和 print）正在慢慢渗透到我的 if 语句中。我知道这不是推荐的做法，而且可能会伤到我的后背。然而，我确实想掌握这项技能，因为我喜欢写这样的衬里，并想将其用作我的个人风格。

所以我的问题是：

哪个运算符（&& 或 and）短路了？
该手册说 && 优先于 and，但是有人可以通过将短路运算符特性/功能/特性与运算符优先级混合来举例说明这一点。（基本上是优先级和短路的混合和匹配）

请详细说明运算符的短路和返回值性质。

PS：1.我希望这些运算符的关联性是相同且直观的，但如果您知道任何怪癖，请赐教。
PS：2.如果您仍然想警告我这种做法的危险，请附上例子。

编辑：通过用 || 替换 or 更改我的简单 $var = mysql_(...) 或 die() 代码后，我发现使用 || 和 && 运算符而不是 and 和 or 是多么烦人的事情。 代码根本不起作用！据我了解，前一个构造将 TRUE 或 FALSE 的返回值分配给 $var，这反过来又使 的所有顺序使用>$var 生成警告/错误/意外行为。后一个构造首先将 mysql_(...) 的结果分配给 $var，然后计算 = 和 die< 的复合值。 /代码>。这对我来说是一个很好的教训，我更好 1. 开始使用 PDO/mysqli 并自己处理错误 2. 在编写我上面称为个人风格的内容之前请三思。 //自我注意：不要在用另一种脚本/解释语言编写代码时使用一种脚本/解释语言的经验，每种语言都是独一无二的，并且有自己的怪癖和陷阱，这只是悲伤*叹息*

原文

I was going through operator precedence section of php.net and came across this example which says

$a = 1;
$b = null;
$c = isset($a) && isset($b);
$d = ( isset($a) and isset($b) );
$e = isset($a) and isset($b);
var_dump($a, $b, $c, $d, $e);
//Result:
int(1)
NULL
bool(false)
bool(false) <== I get this
bool(true)  <== I do not get this

I use quite a lot of debugging and verbose print(_r) statements in my code to keep track of where I am in the code. So I use $debug and print_r($dataArray) or $verbose and print "Updating dataArray\n" as separate statements in between the code, allowing me to control these print(_r) statements. This comes from my BASH experience where I used to write lot of [[ $condition ]] && { #Do if true } || { #Do if false }. In BASH, I knew they are short circuited and used this fact to write lot of simple one liners.
Now I am observing that lot of this practice(of writing $verbose and print) is slowly creeping into my if statements. I am aware this is NOT a recommended practice and can bite me in the back. However, I do want to master this skill as I enjoy writing such one liners and want to use it as my personal style.

So my question(s) is(are) :

Which operator (&& or and) is short circuited ?
The manual says && takes precedence over and, but can someone exemplify this by mixing the short circuited operator feature/functionality/characteristic with operator precedence. (basically mix and match of precedence and short-circuiting)

Kindly elaborate on both the short-circuiting as well as return value nature of the operators.

PS: 1. I hope associativity of these operators is same and intuitive, but if you know any quirks, please enlighten me.
PS: 2. If you still feel like warning me against the perils of such practice, kindly include examples.

EDIT : After changing my simple $var = mysql_(...) or die() code by replacing or with ||, I discovered how annoying it can be to use the || and && operators instead of and and or. The code simply didn't work ! To my understanding, the former construct assigns a return value of TRUE or FALSE to $var, which in turn make all sequential use of $var to generate warning/error/unexpected behavior. The latter construct assigns result of mysql_(...) to $var first and then evaluates the compound of = and die.
This is a good lesson for me, I better 1. Start using PDO/mysqli and handle errors on my own 2. Think twice before writing something I called above as personal style.
//note to self : don't use experience of one scripting/interpretive language while writing code in another, each one is unique and has its own quirks and pitfalls, and thats just sad *sigh*