From: Larme <lalalarme@...>
Date: 2007-04-22T03:20:05+09:00
Subject: Array.sort problem

Dear all, I'm a newbie to ruby and today when I'm writing a simple
script to process some data, I found something I can't understand.

The data is stored in several column seperated by tab or space.  I use
the following code to get the data (assuming the data comes from
standard input and all numbers are integer)

data=[]
counter = 0
while line = STDIN.gets
  data[counter] = line.split
  data[counter].map! {|str| str.to_i}
  counter += 1
end

hence data[i] is an array hold all the numbers in the ith line, data[i]
[j] is the number on ith line and jth column. Then what I want the
script to do is sorting lines according to a specified column.  I
thought the following code should work:

result = data.sort {|x, y| x[col] <=> y[col] }

where the col determine which column the script will sort according
to. However ruby raise a error saying:
"undefined method `<=>' for nil:NilClass (NoMethodError)
        from ana.rb:16:in `sort'
        from ana.rb:16
"

I have to write the code as

result = data.sort {|x, y| x[col].to_i <=> y[col].to_i }

to let the script run properly. I'm quite confused here. I think the
elements of array data are converted to integer when the code

  data[counter].map! {|str| str.to_i}

finished. However why ruby still requires a explicit conversion when I
use the data.sort?