Perl-ish way to create hash from array

gri6507 has asked for the wisdom of the Perl Monks concerning the following question:

When processing CSV files, I usually use Text::CSV to read in the first line to get the column headings and then I use that information to index into the rest of the data by column names (I know what the columns are called, but I don't know what column number they are). Currently, I use the following code, which does not feel very Perl-ish.

use warnings;
use strict;
use Text::CSV;

# setup a CSV parser
my $csvParser = Text::CSV->new({ binary => 1, sep_char => ',', empty_i
+s_undef => 1 }) or die "Cannot create new CSV parser: $!\n";
open my $csvHandle, "<", $csvFile or die "could not open $csvFile: $!"
+;

# figure out the column headings from the first line
my @csvColumns = ();
foreach my $colName (@{$csvParser->getline($csvHandle)}) {
    push @csvColumns, $colName;
}

# reverse that to allow lookup by name
my %csvColumns;
for my $colNum (0 .. scalar(@csvColumns)-1) {
    $csvColumns{$csvColumns[$colNum]} = $colNum;
}

# access some column in the CSV file by name
while (my $line = $csvParser->getline($csvHandle)) {
    print "Column Foo has value $$line[$csvColumns{Foo}];
}
[download]

Is there a more perl-like way to reverse an array into a hash that could then be used to get the index into the data?

Comment on Perl-ish way to create hash from array Download Code

Replies are listed 'Best First'.
Re: Perl-ish way to create hash from array by moritz (Cardinal) on Jul 09, 2013 at 17:29 UTC
Maybe you want to use a hash slice slice: `my %csvColumns; @csvColumns{@csvColumns} = (0..$#csvColumns);` [download] Or you can the combination of `$csvParser->column_names(@csvColumns)` and `$csvParser->getline_hr` to obtain a hash ref from the Text::CSV module directly. Perl 6 - the future is here, just unevenly distributed	[reply] [d/l] [select]
Re: Perl-ish way to create hash from array by kennethk (Abbot) on Jul 09, 2013 at 17:30 UTC
I'd consider it Perlish, because it gets the job done and is quite legible. However, you could do it on one line with a map if that feels Perlier to you: `my %csvColumns = map {$csvColumns[$_] => $_} 0 .. $#csvColumns;` [download] Or you could use an explicit iterator with a do to limit scope: `my %csvColumns = do {my $i = 0; map {$_ => $i++} @csvColumns};` [download] I know lots of folks would describe those as Perlier, though would not be using the phrase in a positive way. #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.	[reply] [d/l] [select]
Re: Perl-ish way to create hash from array by LanX (Saint) on Jul 09, 2013 at 17:29 UTC
maybe hash-slices are what you want: `DB<101> @columns=qw(firstname middlename lastname) => ("firstname", "middlename", "lastname") DB<102> @colidx{@columns}=0..$#columns => (0, 1, 2) DB<103> $colidx{middlename} => 1 DB<104> \%colidx => { firstname => 0, lastname => 2, middlename => 1 }` [download] Cheers Rolf ( addicted to the Perl Programming Language)	[reply] [d/l]
Re: Perl-ish way to create hash from array by tangent (Parson) on Jul 09, 2013 at 19:14 UTC
Maybe I'm not understanding your question but you can use Text::CSV to give you a hash reference directly: `# figure out the column headings from the first line my $headrow = $csvParser->getline($csvHandle); # tell the parser to use those headings as hash keys $csvParser->column_names($headrow); while ( my $hash = $csvParser->getline_hr($csvHandle) ) { print "Column Foo has value $hash->{Foo}"; }` [download]	[reply] [d/l]
Re: Perl-ish way to create hash from array by PrakashK (Pilgrim) on Jul 09, 2013 at 20:58 UTC
moritz and tangent have already answered your question. But, I would like to point out something in your code: Here, you are iterating through the list of column names and building a new list in `@csvColumns` array. `# figure out the column headings from the first line my @csvColumns = (); foreach my $colName (@{$csvParser->getline($csvHandle)}) { push @csvColumns, $colName; }` [download] In your `foreach` loop, you are building a (temporary) list by dereferencing the return value of the `getline` method (`@{$csvParser->getline($csvHandle)}`), then iterating through this list and copying each element into the other list (`push @csvColumns, $colName;`), and finally discarding (implicitly) the temporary list. You could instead do: `# figure out the column headings from the first line my @csvColumns = @{$csvParser->getline($csvHandle)};` [download]	[reply] [d/l] [select]
Re: Perl-ish way to create hash from array by rjt (Curate) on Jul 09, 2013 at 20:42 UTC
As of Perl 5.12, each works on arrays: `use 5.012; use warnings; my @array = qw/zero one two three four five six/; my %hash; while (my ($idx, $val) = each @array) { $hash{$idx} = $val };` [download] I wouldn't call this a "win" in this particular example over using a slice or Text::CSV's own API, but it's not bad, and in more complex situations it works handily.	[reply] [d/l]


Come for the quick hacks, stay for the epiphanies.
	PerlMonks