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Basic syntax tutorials I followed do not make this clear:

Is there any practical/philosophical/context-dependent/tricky difference between accessing an array using the former or latter subscript notation?

$ perl -le 'my @a = qw(io tu egli); print $a[1], @a[1]'

The output seems to be the same in both cases.

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3 回答 3

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$a[...]   # array element

returns the one element identified by the index expression, and

@a[...]   # array slice

returns all the elements identified by the index expression.

As such,

  • You should use $a[EXPR] when you mean to access a single element in order to convey this information to the reader. In fact, you can get a warning if you don't.
  • You should use @a[LIST] when you mean to access many elements or a variable number of elements.

But that's not the end of the story. You asked for practical and tricky (subtle?) differences, and there's one noone mentioned yet: The index expression for an array element is evaluated in scalar context, while the index expression for an array slice is evaluated in list context.

sub f { return @_; }

$a[ f(4,5,6) ]     # Same as $a[3]
@a[ f(4,5,6) ]     # Same as $a[4],$a[5],$a[6]
于 2013-03-31T04:07:53.253 回答
8

If you turn on warnings (which you always should) you would see this:

Scalar value @a[0] better written as $a[0]

when you use @a[1].

The @ sigil means "give me a list of something." When used with an array subscript, it retrieves a slice of the array. For example, @foo[0..3] retrieves the first four items in the array @foo.

When you write @a[1], you're asking for a one-element slice from @a. That's perfectly OK, but it's much clearer to ask for a single value, $a[1], instead. So much so that Perl will warn you if you do it the first way.

于 2013-03-31T03:39:19.850 回答
4

The first yields a scalar variable while the second gives you an array slice .... Very different animals!!

于 2013-03-31T03:35:57.433 回答