Using Ruby CSV header converters

After doing some research here in my desktop, it seems to me the error is for something else.

First I put the data in my "a.txt" file as below :

First Name,Last Name
John,Doe
Jane,Doe

Now I ran the code, which is saved in my so.rb file.

so.rb

require 'csv'

CSV.foreach("C:\\Users\\arup\\a.txt", 
             :headers => true,
             :converters => :all,
             :header_converters => lambda { |h| h.downcase.gsub(' ', '_') }
           ) do |row|
    p row
end

Now running the :

C:\Users\arup>ruby -v so.rb
ruby 1.9.3p448 (2013-06-27) [i386-mingw32]
#<CSV::Row "first_name":"John" "last_name":"Doe">
#<CSV::Row "first_name":"Jane" "last_name":"Doe">

So everything is working now. Now let me reproduce the error :

I put the data in my "a.txt" file as below ( just added a , after the last column) :

First Name,Last Name,
John,Doe
Jane,Doe

Now I ran the code, which is saved in my so.rb file, again.

C:\Users\arup>ruby -v so.rb
ruby 1.9.3p448 (2013-06-27) [i386-mingw32]
so.rb:5:in `block in <main>': undefined method `downcase' for nil:NilClass (NoMethodError)

It seems, in your header row, there is blank column value which is causing the error. Thus if you have a control to the source CSV file, check there the same. Or do some change in your code, to handle the error as below :

require 'csv'

CSV.foreach("C:\\Users\\arup\\a.txt",
             :headers => true,
             :converters => :all,
             :header_converters => lambda { |h| h.downcase.gsub(' ', '_') unless h.nil? }
           ) do |row|
    p row
end

A more general answer, but if you have code that you need to process as text, and sometimes you might get a nil in there, then call to_s on the object. This will turn nil into an empty string. eg

h.to_s.downcase.gsub(' ', '_') 

This will never blow up, whatever h is, because every class in ruby has the to_s method, and it always returns a string (unless you've overridden it to do something else, which would be unadvisable).