涉及逗号的 perl 语法结构的含义

Question

我在一本书中遇到过这样一段代码：

#for (some_condition) {
#do something not particularly related to the question
$var = $anotherVar+1, next if #some other condition with $var
#}

我不知道 $anotherVar+1 和 next 之间的逗号 (",") 有什么作用。这种语法结构是如何调用的，它是否正确？

Answer 1

逗号是an operator, in any context. In list context where it's usually seen, it is one way to concatenate values into a list. Here it is being used in scalar context, where it runs the preceding expression, the following expression, and then returns the following expression. This is a holdover from how it works in C and similar languages, when it's not an argument separator; see https://docs.microsoft.com/en-us/cpp/cpp/comma-operator?view=vs-2019.

my @stuff = ('a', 'b'); # comma in list context, forms a list and assigns it
my $stuff = ('a', 'b'); # comma in scalar context, assigns "b"
my $stuff = 'a', 'b'; # assignment has higher precedence
                      # assignment done first then comma operator evaluated in greater context

Answer 2

perlop. You can use it to separate commands, it evaluates its left operand first, then it evaluates the second operand. In this case, the second operand is next 中描述了逗号运算符，它改变了程序的流程。

基本上，这是一种较短的写法

if ($var eq "...") {
    $var = $anotherVar + 1;
    next
}

逗号在 C 语言中的用法与此类似，您经常会在 for 循环中找到它：

for (i = 0, j = 10; i < 10; i++, j--)

Answer 3

考虑以下代码：

$x=1;
$y=1;

$x++ , $y++ if 0; # note the comma! both x and y are one statement
print "With comma: $x $y\n";

$x=1;
$y=1;

$x++ ; $y++ if 0; # note the semicolon! so two separate statements

print "With semicolon: $x $y\n";

输出结果如下：

With comma: 1 1
With semicolon: 2 1

逗号类似于分号，只是命令的两边都被视为一个语句。这意味着在只需要一个语句的情况下，逗号两边都会被评估。

涉及逗号的 perl 语法结构的含义

Meaning of the perl syntax construction involving a comma

perl

perlsyn