RogerBW's Blog

Perl Weekly Challenge 32 04 November 2019

I’ve been doing the Perl Weekly Challenges (I missed 31 because of getting ready for Essen, and didn’t have time to do this one in Perl6). This week’s was about counting entities and generating ASCII bar charts.

Create a script that either reads standard input or one or more files specified on the command-line. Count the number of times [each item occurs] and then print a summary, sorted by the count of each entry.

For extra credit, add a -csv option to your script, which would generate:

Those of us who speak Unix recognise this as the extremely useful formulation |sort|uniq -c|sort -nr, which I use often enough that I can type it as though it were a long and familiar word. (Sort the lines, count how often each one occurs, sort that list numerically in descending order.)

But in Perl the most obvious approach is to build a hash keyed on the lines, so we do:

use Getopt::Std;
use Text::CSV_XS;

my %o;
getopts('c',\%o);

my %s;
while (<>) {
  chomp;
  $s{$_}++;
}

Then sort by the key values, descending, and all is done.

my $csv = Text::CSV_XS->new;

foreach my $k (sort {$s{$b} <=> $s{$a} ||
                       $a cmp $b} keys %s) {
  if ($o{c}) {
    $csv->say(*STDOUT,[$k,$s{$k}]);
  } else {
    print "$k $s{$k}\n";
  }
}

The use of Text::CSV_XS is possibly a heavier-weight approach than this problem really requires, but I’ve been bitten by the vagaries of CSV “standard” formatting before. If someone had asked me to do this for a real problem, I’d use the module so that when their specific requirements for CSV files turned out to be subtly different from what I’d produced I could just tweak the module parameters rather than re-invent things from scratch.

(It's entirely standard until you need to include a comma within a data field. Or a quotation mark of some sort. Or a non-ASCII character. Or transfer files between Unix and the outside world. Or…)

Write a function that takes a hashref where the keys are labels and the values are integer or floating point values. Generate a bar graph of the data and display it to stdout.

If you fancy then please try this as well: (a) the function could let you specify whether the chart should be ordered by (1) the labels, or (2) the values.

I know that NeilB, who contributed these questions, maintains a module to produce tabular output…

Terminal width is always a slightly fiddly thing, so I allow the caller to specify it; then I scale the largest bar to the full width of the terminal (minus the width of the longest label, and the decoration), and the others grow or shrink accordingly. Yes, there’s a bug here if the allowed width is too narrow for the labels and decoration; and this function doesn’t allow for negative values either. The third parameter should be non-zero if you want ordering by labels.

use List::Util qw(max);

sub generate_bar_graph {
  my $data=shift;
  my $width=shift || $ENV{COLUMNS} || 80;
  my $labelordering=shift or 0;
  my @k=keys %{$data};
  if ($labelordering) {
    @k=sort @k;
  } else {
    @k=sort {$data->{$b} <=> $data->{$a}} @k;
  }
  my $kl=max(map {length($_)} @k);
  my $bw=$width-$kl-3;
  my $scale=$bw/max(values %{$data});
  my $format='%-'.$kl.'s | %-'.$bw."s\n";
  foreach my $k (@k) {
    printf($format,$k,'#' x ($scale*$data->{$k}));
  }
}

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