find the only columns with empty values in entire file - perl

Question

With the below csv data
name,place,animal
a,,
b,,
a,,
,b,
The name field is available in 3 rows but not available in 1 row
The place field is available in 1 row but not in 3 row
The animal field is empty in all the rows -> Get these column names1
i would like to get the column names only if it empty in all the rows.
I am trying to write a perl script for the same but not sure how to attack this problem.
step 1: Check all the columns in first row, if any column is not empty ,dont search it in next row
step2: keep repeating step1 in a loop and finally we will get the output.and this brings down the complexity as we are not bothered about columns that have value even once.
i will implement the code and post it here.
But if u have any new ideas, please advise me
Thanks

Answer 1

For CSV files with no quotes and escaping, just keep a hash of empty columns so far. Reading the file line by line, remove any non-empty column from the hash:
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
chomp( my #column_names = split /,/, <> );
my %empty;
#empty{ #column_names } = ();
while (<>) {
chomp;
my #columns = split /,/;
for my $i (0 .. $#columns) {
delete $empty{ $column_names[$i] } if length $columns[$i];
}
}
say for keys %empty;
For real CSV files, use Text::CSV_XS, but the method is the same: populate a hash by column names, then remove the non empty ones:
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
use Text::CSV_XS qw{ csv };
my %empty;
csv(in => shift,
out => \ 'skip',
headers => sub { undef $empty{ $_[0] }; $_[0] },
on_in => sub {
my (undef, $columns) = #_;
delete #empty{ grep length $columns->{$_}, keys %$columns }
},
);
say for keys %empty;

Answer 2

As rows are processed update an ancillary array which keeps track of each field's truth-value
If any field in a new row is non-empty the corresponding element of the array flips to true; otherwise it stays false. In the end indices of array's false elements identify indices of empty columns.
use warnings;
use strict;
use feature 'say';
use Text::CSV;
my $file = 'cols.csv';
my $csv = Text::CSV->new( { binary => 1 } )
or die "Cannot use CSV: " . Text::CSV->error_diag ();
open my $fh, '<', $file or die "Can't open $file: $!";
my #col_names = #{ $csv->getline($fh) };
my #mask;
while (my $line = $csv->getline($fh)) {
#mask = map { $mask[$_] || $line->[$_] ne '' } (0..$#$line);
}
for (0..$#mask) {
say "Column \"$col_names[$_]\" is empty" if not $mask[$_];
}
Syntax: $#$line is the index of the last element of arrayref $line (like $#ary is for #ary)