From: Martin DeMello <martindemello@...>
Date: 2003-07-12T15:20:24+09:00
Subject: Re: cross-corr time/array lag problem

Qubert <qubert@austin.rr.com> wrote:
> I am starting to find the "Way" now, but I have a problem.
> I am running cross correlations using Ruby, so first I set up the time
> series data to be in columns with rows being the same time.
> Now I am trying to lag one column to the other by:
> a = [0, 1, 2, 3, 4, 5]
> b = a.each_index do |j| 
>   a[j-1]
> end
> 
> But afterward a = b with no lag.
> a print a[j]," ",a[j-1] shows this works
> but b is being stored as the original "a" not a modified "a" with an
> hour lag.
> 
> Why is b not being set with a lag?  I can not .collect because I need
> to manupilate the index not the contents of the array.

each_index doesn't have an implicit collect, and its return value is the
array itself (not sure why, but I can't remember needing a return value
anyway). What you need is:

b = []
a.each_index {|j| b[j] = a[j-1]}

or, 

b = a.dup.unshift(nil)

or (drumroll)

b = a.map_with_index {|e, i| a[i-1]}

had we a map with index which we don't (:

though beware of wraparound - a[-1] returns the last element.

martin