From: Robert Klemme <shortcutter@...>
Date: 2012-03-09T20:50:03+09:00
Subject: Re: in > comparison of fixnum with nil failed, what's wrong with my code?

Samuel Mensah wrote in post #1050813:
> I'm trying to write a code which tells me the number of inverted pairs
> in an array of numbers so that an array like [4,9,6,3,7] will have 9,6
> as one inversion, 6,3 as another inversion, but 4,9 won't be inverted
> cos when counting from 1 to infinity, 4 comes before 9...

Do you also want to count 9,3 or only adjacent pairs?  From your code it 
seems you want to look at all pairs a[i], a[j] where j > i.

> my code gives
> me the error of "6:in '>': Comparison of Fixnum with nil failed
> (ArgumentError) and here's the code
>
> 1   array=[8,4,2,5,6,9,7,1,3,10]
> 2   startnum=0
> 3   nextnum=1
> 4   count=0
> 5     while array[startnum] != array[8]

That's a weak comparison: better compare startnum with array.size. 
Also, why do you use a constant here?

> 6       if array[startnum] > array[nextnum]
> 7          count=count+1
> 8          nextnum=nextnum+1
> 9            if array[nextnum] == array[9]

Same here: no constant and compare array indexes not array contents.

> 10               startnum=startnum+1
> 11               nextnum=startnum+1
> 12           end
> 13      else
> 14         nextnum=nextnum+1
> 15         if array[nextnum] == array[9]

Again: no constant and compare array indexes not array contents.

> 16             startnum=startnum+1
> 17         end
> 18      end
> 19   end
> 20
> 21   puts count
>
> how do I fix this?

You need to make sure that you do not index outside the valid range. 
With a setup like this always compare indexes with array size and not 
contents of the array.

I have attached a few solutions for both cases.

Kind regards

robert

Attachments:
http://www.ruby-forum.com/attachment/7140/cnt.rb


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