From: Nathan Clark <tokenshift@...>
Date: 2011-01-27T02:21:40+09:00
Subject: Re: Matrix Referencing

> I have a Matrix defined as follows:
>
>  wvp =
> Matrix[[-40.0,0.006],[0.0,0.045],[32.0,0.18],[59.0,0.50],[70.0,0.74],
>      [100.0,1.93],[130.0,4.53]]
>
>  I want to interpolate within this 2D array, which I start to do with
> the following equation:
>
>  m = (wvp[i,2]-wvp[i-1,2])/(wvp[i,1]-wvp[i-1,1])
>
> This is readily done in C, but Ruby does not like the fact that I
> subtract a one from the index i within the context of the array, i.e.,
> [i-1,2].  Even if I try to fake it out by first declaring j = i - 1, and
> using j in place of the i-1 within the array coordinate, I get the
> following error:
>
> C:\rails_projects\TestRubyConcepts\lib\main.rb:74:in
> `humidity_correction': undefined method `-' for nil:NilClass
> (NoMethodError)
>
> It's claiming my method - is undefined.  How do I defeat this?

"undefined method `-' for nil:NilClass" means that the operand to the
left of one of the '-' operators is Nil. That will most likely be one
of the wvp[...] accesses, assuming i and j are initialized correctly.

Check each of
wvp[i,2], wvp[i-1,2], wvp[i,1], and wvp[i-1,1] and see what they return.

Nathan