在Perl中,当我使用@_ [0],@ _ [1]等来访问子例程的参数时,为什么会收到警告？

Question

下面的代码.

sub max 
    {
        if (@_[0] > @_[1]) 
        {
            @_[0];
        }
        else
        {
            @_[1];
        }
    }
    print "biggest is ".&max(37,25);

当我运行它时,我收到以下警告,

Scalar values @_[0] better written as $_[0] at file.pl line 3.
Scalar values @_[1] better written as $_[1] at file.pl line 3.
Scalar values @_[0] better written as $_[0] at file.pl line 5.
Scalar values @_[0] better written as $_[0] at file.pl line 9.
biggest is 37.

虽然我得到了正确的输出,但我想知道这个警告背后的原因是什么,因为我认为@_在子程序中使用是合适的$_.

Answer 1

问题是您通过使用数组切片而不是标量来引用单个数组元素.就像错误说的那样.数组切片是数组中元素的列表,例如:

my @a = (0 .. 9);
print @a[0,3,4];    # prints 034

相反,当您引用单个数组元素时,您使用标量前缀$:

print $a[3];        # prints 3

所以,当你这样做

@_[0];

Perl告诉你,引用标量值的正确方法是不使用数组切片,而是使用标量符号:

$_[0];

就这些.

Answer 2

尝试通过这个例子来理解它：

@array = (1,2,3); #array is the name of the array and @ means that it's an array
print $array[1];  
#this will print element at index 1 and you're doing it in scalar context

相似地，

@_ = (1,2,3); #_ is the name of the array
print $_[1]; 
#this will print element at index 1 of array _ and again you're doing it in scalar context