Iterating through 2 Arrays and Performing a Calculation

A1 = [5 10 15 20 25 30 35 40]

A2 = [ 7 22 38]

Goal: Iterate through A2 and subtract from appropriate value in A1, store this value in a separate array.

Example:

7 in A2 is between 10 and 5 where

t1= 5

t2 = 10

Subtract A2 from t1.

7-5 = 2 (store this value in a separte array.

Move on to the second element in A2, 22. 22 is between 20 and 25. Substract (22-20) = 2; store this value

[Sindar]

Is A1 always evenly spaced? Is it always sorted? Will A2 ever have elements greater than all elements in A1?

[Graduate Student]

A2 is not evenly space but it sorted. The next value is always bigger than the previous.

I'm not sure what you mean by third question. A2 will aways be larger than T1. So when you subtract, it will always be a positive integer.

[Sindar]

Ah, sorry, I asked the wrong question: Will A2 ever have elements less than all the elements in A1? (so t1 doesn't exist). Also, how many elements, roughly, will A1 and A2 have?

[Graduate Student]

There are about ~1000 elements in each array although each array is a different length from each other. No, A2 will not be less than A1. Thanks!

Answer 1

[Sindar]

loop version:

for ind=1:length(A2)
    idx = find(A1 < A2(ind),1,'last');
    A3(ind) = A2(ind) - A1(idx);
end

Another (probably much worse) way of looking at it for smallish arrays:

A1 = [3 10 15 20 25 30 35 40];
A2 = [ 7 22 38];
% create a matrix of all combinations of A1(i)-A2(j)
tmp = A2-A1';
% set any less than zero to infinity
tmp(tmp<0)=Inf;
% find the minimum for each column
A3 = min(tmp)