Is this a numeric context thing?

mephit has asked for the wisdom of the Perl Monks concerning the following question:

Greetings, Monks.

I was skimming my older nodes and came across this reply, which says:

qw(foo bar))[$_[0]] 
would still yield 'foo' when @_ is empty (therefore, $_[0] is undef).
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What I'm wondering, is if that's a numeric constant thing? If @_ is empty, does the array usage of [] force the undefined $_[0] into numeric context, which would make is a 0? Is that how that works? I did some tests, such as:

Enter h or `h h' for help, or `man perldebug' for more help.

h
main::(-:1):    h
  DB<1> x $ARGV[0]
0  undef
  DB<2> x 3 + $ARGV[0]
0  3
  DB<3> x 3 * $ARGV[0]
0  0
[download]

which is what I was expecting, thinking that an undef would be forced into a numeric 0 because of arithmetic operands. So, I'm pretty sure that's what's happening, but that's just an educated guess. Am I right?

Thanks.

Mephit (See my home node for my rant about Opera and PerlMonks, and my earliest nodes.

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Re: Is this a numeric context thing? by davorg (Chancellor) on May 17, 2002 at 07:01 UTC
From perldoc perlsyn: A variable holds the undefined value (undef) until it has been assigned a defined value, which is anything other than undef. When used as a number, undef is treated as 0; when used as a string, it is treated the empty string, ""; and when used as a reference that isn't being assigned to, it is treated as an error. If you enable warnings, you'll be notified of an uninitialized value whenever you treat undef as a string or a number. Well, usually. Boolean ("don't-care") contexts and operators such as ++, --, +=, -=, and .= are always exempt from such warnings.	[reply]

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Re: Is this a numeric context thing?
by davorg (Chancellor) on May 17, 2002 at 07:01 UTC

From perldoc perlsyn:

A variable holds the undefined value (undef) until it has been assigned a defined value, which is anything other than undef. When used as a number, undef is treated as 0; when used as a string, it is treated the empty string, ""; and when used as a reference that isn't being assigned to, it is treated as an error. If you enable warnings, you'll be notified of an uninitialized value whenever you treat undef as a string or a number. Well, usually. Boolean ("don't-care") contexts and operators such as ++, --, +=, -=, and .= are always exempt from such warnings.

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