Please explain 'list' assignment.

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

Hello. Why is it that

my ($var1, $var2, $var3) = "Hello";
[download]

..not the same as


my $var1 = "Hello";
my $var2 = "Hello";
my $var2 = "Hello";
[download]

??? Thanks.

Entities should not be multiplied unnecessarily.

2006-11-03 Retitled by planetscape, as per Monastery guidelines

( keep:1 edit:12 reap:0 )

Original title: 'Whis is this not?'

Comment on Please explain 'list' assignment. Select or Download Code

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Re: Please explain 'list' assignment. by Errto (Vicar) on Nov 03, 2006 at 19:11 UTC
The way to do what you're looking for is `my $var1 = my $var2 = my $var3 = "Hello";` [download] The reason your first piece of code doesn't do that is because it's a list assignment. That is, it takes a list of variables on the left side and a list of values on the right side. But the list on the left side has three items and the list on the right only has one, so the other two variables will get a value of `undef`.	[reply] [d/l] [select]
Re: Please explain 'list' assignment. by friedo (Prior) on Nov 03, 2006 at 19:10 UTC
Because list assignments are parallel. A list of three items on the left expects three items on the right. Since there's only one, `$var2` and `$var3` remain undefined. You can do what you want with: ~~`my ($var1, $var2, $var3) = "Hello" x 3;`~~ ~~[download]~~ Update: Oops - I read your question slightly wrong. To assign the same value three times, use `my ($var1, $var2, $var3) = map "Hello", 1..3;` [download] or `my ($var1, $var2, $var3) = ("Hello") x 3;` [download] (thanks Sidhekin)	[reply] [d/l] [select]
Re^2: Please explain 'list' assignment. by shenme (Priest) on Nov 03, 2006 at 19:19 UTC
The 'x' operator can act upon strings or upon lists. The use above, `"Hello" x 3"` created three concatenated copies of the string "Hello", thus "HelloHelloHello". What you wanted was "three copies of an item in list context", like so: `my ( $var1, $var2, $var3 ) = ("Hello") x 3;` [download] The parentheses force the value into list context, the 'x' then makes three copies of that single value list, and you get ("Hello", "Hello", "Hello"), which is what OP wanted.	[reply] [d/l] [select]
Re^2: Please explain 'list' assignment. by Bagarre (Sexton) on Nov 03, 2006 at 19:20 UTC
Just came back to say that didnt work :) The map works... and makes sense. Entities should not be multiplied unnecessarily.	[reply]
Re^2: Please explain 'list' assignment. by Bagarre (Sexton) on Nov 03, 2006 at 19:12 UTC
Perfect. Thank you :) Entities should not be multiplied unnecessarily.	[reply]
Re: Please explain 'list' assignment. by NetWallah (Canon) on Nov 04, 2006 at 07:06 UTC
TIMTOWTDI - I prefer the option below, because it allows you to expand the list, without counting the number of variables, is easy to read, and is economical in the number of keystrokes required : `$_='Hello' for my ($x,$y,$z);` [download] "A closed mouth gathers no feet." --Unknown	[reply] [d/l]