Question

I’m trying to open a CSV file, look up a string, and then return the 2nd column of the csv file, but only the the first instance of it. I’ve gotten as far as the following, but unfortunately, it returns every instance. I’m a bit flummoxed.

Can the gods of Ruby help? Thanks much in advance.

M

for the purpose of this example, let’s say names.csv is a file with the following:

foo, happy
foo, sad
bar, tired
foo, hungry
foo, bad


#!/usr/local/bin/ruby -w

require 'rubygems'
require 'fastercsv'
require 'pp'

  FasterCSV.open('newfile.csv', 'w') do |output|
    FasterCSV.foreach('names.csv') do |lookup|
      index_PL = lookup.index('foo')
      if index_PL
        output << lookup[2]
      end
    end
  end

ok, so, if I want to return all instances of foo, but in a csv, then how does that work?
so what I’d like as an outcome is happy, sad, hungry, bad. I thought it would be:

  FasterCSV.open('newfile.csv', 'w') do |output|
    FasterCSV.foreach('names.csv') do |lookup|
      index_PL = lookup.index('foo')
      if index_PL
        build_str << "," << lookup[2]
      end
      output << build_str
    end
  end

but it does not seem to work

Answer 1

Replace foreach with open (to get an Enumerable) and find:

FasterCSV.open('newfile.csv', 'w') do |output|
    output << FasterCSV.open('names.csv').find { |r| r.index('foo') }[2]
end

The index call will return nil if it doesn’t find anything; that means that the find will give you the first row that has 'foo' and you can pull out the column at index 2 from the result.

If you’re not certain that names.csv will have what you’re looking for then a bit of error checking would be advisable:

FasterCSV.open('newfile.csv', 'w') do |output|
    foos_row = FasterCSV.open('names.csv').find { |r| r.index('foo') }
    if(foos_row)
        output << foos_row[2]
    else
        # complain or something
    end
end

Or, if you want to silently ignore the lack of 'foo' and use an empty string instead, you could do something like this:

FasterCSV.open('newfile.csv', 'w') do |output|
    output << (FasterCSV.open('names.csv').find { |r| r.index('foo') } || ['','',''])[2]
end

I’d probably go with the “complain if it isn’t found” version though.