How to calculate between the elements in an array for consecutive indexes.

I have a time array(T) and two index arrays.(example)

idx_1 = [32,33,34,35,36,37,38,39,40]  % 1 set of consecutive numbers
idx_2 = [32,33,35,36,37,38,40,41,42]  % 3 sets of consecutive numebers

What I want to do is to subtract the last vlaue from the first value among the consecutive indexes.

so in the case of idx_1, I want

T(40)-T(32)

and for idx_2,

T(42)-T(40) + T(38)-T(35) + T(33)-T(32)

I have a lot of the T arrays and I can find the index for what I need, but I'm stuck in making the code for this problem.

If anyones can help, thanks in advance :)

Answer 1

[John D'Errico]

idx_1 = [32,33,34,35,36,37,38,39,40];  % 1 set of consecutive numbers
idx_2 = [32,33,35,36,37,38,40,41,42];  % 3 sets of consecutive numebers

First, see what happen when you form the diff. If diff > 1, that indicates something special. Do you see that?

diff([-inf,idx_2,inf])

ans = 1×10

Inf 1 2 1 1 1 2 1 1 Inf

Note that I appended -inf and to inf each end, so that find will trigger for the start and end of each sub-sequence.

loc1 = find(diff([-inf,idx_1,inf])>1)

loc1 = 1×2

1 10

loc2 = find(diff([-inf,idx_2,inf])>1)

loc2 = 1×4

1 3 7 10

So there is ONE sub-sequence in the first set, and three in the second. How will we use them? It looks like you wish to form a sum, so this will do it:

sum(T(idx1(loc1(2:end) - 1)) - T(idx1(loc1(1:end-1))))
sum(T(idx2(loc2(2:end) - 1)) - T(idx2(loc2(1:end-1))))

[Naomi Gray]

This is exactily what I wanted to do! Thank you!