I am parsing a CSV file and I would like to turn the columns into individual arrays contained in a hash which I can access by name (names are contained in the header line of the input file).

Example input file (MyCSV.csv):

Name,Cost,Inventory
pickles,2.99,12
vegemite,4.00,8
nuclear sub,22.50,4

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What I have done thus far:

open IN, "MyCSV.csv" or die "Cannot open input file: $!\n";

my ($line, @colNames, $size, %columns, $j);
$j = 0;

chomp($line =<IN>);

# Read the column header
@colNames = split(',',$line);
$size = scalar @colNames;

while($line = <IN>)
{
    chomp($line);
    push @{$columns{$colNames[$j++ % $size]}}, $_ for(split(',', $line
+));
}

# Now I can play with whole columns of data by name
print “Total Inventory = ”, List::Util::sum(@{$columns{'Inventory'}}),
+ “\n”;
print “Total Cost = ” , List::Util::sum(@{$columns{'Cost'}}), “\n”;

[download]

A couple of quick notes:

  1. I know I should be using Text::CSV or Text::CSV_XS, but this is just for testing
  2. I am fairly new to Perl, but I really like it and I think there is a better more Perl-tastic way of doing this
  3. I tried preallocating the arrays in the hash (using $#@{$columns{$colName}} = 100, where $colName came from iterating over @colNames), but I ran into problems where push started adding at position 101 and leaving the preallocated slots empty

I really like this approach because I don't have to create $size arrays, I just need to know what the columns are called (which is fairly standardized for my data files). I thought there might be a way of transposing the rows/columns after reading the file completely into memory then I could just store Text::CSV(_XS)->fields() as my columns of data, but I didn't see such a facility in Text::CSV(_XS).

The runtime on this isn't horrible, but it is still linear in the number of columns (though not so linear in the number of rows). I have done time trials on 16 to 128 columns with 10,000 rows and the runtime is between 0.2 and 0.9 seconds, respectively, but I feel it could be better. Any thoughts are greatly appreciated.

Thanks.
- Tim

In reply to CSV Columns: Friend or foe? by dekul

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