# [plt-scheme] build-vector avoids sharing problem + tensor algebras

From: "Richard C. Cobbe" <cobbe at ccs.neu.edu>
However, you may not necessarily need to use side effects to
initialize your arrays. If you can write an expression that
computes the value for location (i,j) from i and j, why not simply
use this in a nested build-vector? Something like the following:
(build-vector 50
(lambda (i)
(build-vector 50
(lambda (j)
(compute-element i j)))))
Here, compute-element may be your B(i,j); I didn't find the description
above very clear on this point.
My B(i,j) will be the value of your (compute-element i j).
The problem is that (compute-element i j) needs to reference all
earlier elements of the 2-d array. So if you had written
(define Big-Matrix (build-vector 50 .... ))
then (compute-element i j) needs to call
(vector-ref (vector-ref Big-Matrix a) b)
For b < j and also (a, b) = (i-1, j)
That is, (compute-element i j) needs the value of earlier
(compute-element a b).
--
Bill
<http://www.math.nwu.edu/~richter>