List context with map

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

Some unexpected behavior, with Date::Calc, context, and map. Most of the Date::Calc functions expect a list of @args, which can easily be passed around with arrays (as opposed to separate scalars for each $arg). The function map returns a list. But consider this:

my @eDate=map{$cgi->param($_)}qw(eYear eMonth eDate);
@eDate=Add_Delta_Days(@eDate,1);
[download]

That works okay, but I should be able to skip an assignment, and go straight through Add_Delta_Days with the map, like so:

my  @eDate= Add_Delta_Days(map{$cgi->param($_)}
            qw(eYear eMonth eDate),1);
[download]

Now, to me, that's but a simple replace, replacing @eDate in the original Add_Delta_Days call with my map, which originally defines @eDate. But Date::Calc throws this:

Usage: Date::Calc::Add_Delta_Days(year, month, day, Dd)

Soooo, I tried this:


my @eDate=Add_Delta_Days((map{$cgi->param($_)}qw(eYear eMonth eDate)),
+1);
[download]

And explictly wrap the map in parens. It worked! Whats the difference, and thanks in advance for shedding the light!

ÅßÅ×ÅßÅ
"It is a very mixed blessing to be brought back from the dead." -- Kurt Vonnegut

Comment on List context with map Select or Download Code

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Re: List context with map by tachyon (Chancellor) on Nov 05, 2004 at 01:40 UTC
Whitout the parens you map is greedy and thus eats argument 4 ie the digit 1. There is a good chance you have no cgi param '1' so this becomes undef. `use CGI; my $q = CGI->new( { one => 1, two => 2, three => 3 } ); test( map{$q->param($_)\|\|"${_}_IS_NULL"}qw(one two three) , 4 ); test( ( map{$q->param($_)\|\|"${_}_IS_NULL"}qw(one two three) ), 4 ); sub test { $"=', ';print "Args( @_ )\n" } __DATA__ Args( 1, 2, 3, 4_IS_NULL ) Args( 1, 2, 3, 4 )` [download] Your obvious expectation is that the qw( ) perfoms a useful binding function to the map when in fact you are getting a flat list: `func( map{$q->param($_)} 1, 2, 3, 4 )` cheers tachyon	[reply] [d/l] [select]
Re^2: List context with map by ysth (Canon) on Nov 05, 2004 at 09:11 UTC
map is greedy* and thus eats argument 4 ie the digit 1. There is a good chance you have no cgi param '1' so this becomes undef.* perldoc CGI: If the parameter does not exist at all, then param() will return undef in a scalar context, and the empty list in a list context. so the 1 will not become undef, it will vanish entirely.	[reply]
Re: List context with map by VSarkiss (Monsignor) on Nov 05, 2004 at 03:35 UTC
Brother tachyon has the right answer above. If you'd like to figure out stuff like this for yourself, the B::Deparse module is very handy: `$ perl -MO=Deparse -e 'Add_Delta_Days( map{$cgi->param($_)} qw(eYear e +Month eDate), 1)' Add_Delta_Days(map({$cgi->param($_);} ('eYear', 'eMonth', 'eDate'), 1) +); -e syntax OK $ $ perl -MO=Deparse -e 'Add_Delta_Days( (map{$cgi->param($_)}qw(eYear e +Month eDate)), 1)' Add_Delta_Days(map({$cgi->param($_);} 'eYear', 'eMonth', 'eDate'), 1); -e syntax OK` [download] Note where it put the parentheses.	[reply] [d/l]
•Re: List context with map by merlyn (Sage) on Nov 05, 2004 at 08:08 UTC
`$cgi->param($_)` Caution! There's no guarantee that you'll get one output for one input there. You could get zero, or two, or twenty, and that'd really mess up your output. Maybe you want to add `scalar` in front of that. -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply] [d/l]
Re^2: List context with map by tachyon (Chancellor) on Nov 05, 2004 at 08:14 UTC
`\|\| '' #:-)`	[reply] [d/l]
Re: List context with map by NetWallah (Canon) on Nov 05, 2004 at 03:55 UTC
FWIW - you could also have written this by explicitly using parens at the boundary of the "map" parameters (my preference): `my @eDate=Add_Delta_Days(map({$cgi->param($_)}qw(eYear eMonth eDate)), +1);` [download] Update: Oops - just noticed that VSarkiss already shows this option. Please ignore this node. Earth first! (We'll rob the other planets later)	[reply] [d/l]