Using map to assign to a hash

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

I have two lines of similar code, the first works fine while the second produces a syntax error. What is happening?

%hash =  map { $_, "$var/$_" } @array;   # Works great
%hash =  map { "$var/$_", $_ } @array;   # Syntax Error!
[download]

This one works as expected with only slight modification:

%hash =  map { "$var/".$_, $_ } @array;  # Also works.
[download]

Thanks

Comment on Using map to assign to a hash Select or Download Code

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Re: Using map to assign to a hash by jeffa (Bishop) on Jun 08, 2004 at 15:30 UTC
The answer is in the docs for map: `perldoc -f map`. "{" starts both hash references and blocks, so "map { ..." could be either the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn’t look ahead for the closing "}" it has to take a guess at which its dealing with based what it finds just after the "{". Usually it gets it right, but if it doesn’t it won’t realize something is wrong until it gets to the "}" and encounters the missing (or unexpected) comma. The syntax error will be reported close to the "}" but you’ll need to change something near the "{" such as using a unary "+" to give perl some help ... The docs then list some examples, i prefer the syntax BrowserUk replied with, myself. You can also use parens, FWIW: `%hash = map (("$var/$_", $_), @array);` [download] jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l]
Re: Using map to assign to a hash by BrowserUk (Patriarch) on Jun 08, 2004 at 15:30 UTC
Looks like a bug to me, but if you put parens around the list it makes it 'go away'; `%hash = map { ( "$var/$_", $_ ) } @array;` [download] Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail	[reply] [d/l]
Re: Using map to assign to a hash by Grygonos (Chaplain) on Jun 08, 2004 at 17:00 UTC
Also worth noting, putting parens only around the key definition fixes it as well. ie `map{("$var/$_"),$_}@array` [download] So simply adding extra clarity to the key definition is the main fix (as stated by jeffa in his perldoc ref). Grygonos	[reply] [d/l]