# Subtract according to the values in another column

8 views (last 30 days)
Aragorn23 on 11 Jul 2019
Edited: Matt J on 11 Jul 2019
Hi everyone,
I have two columns, with the same amount of elements.
C1 = 1-1-1-1-1-2-2-2-2-2-2-3-3-3-4-4-4-4
C2 = 5-10-12-35-3-1-4-26-5-11-12-8-2-22-8-2-5-7
I would like to subtrate the values of C2 according to C1(e.g., 3-5; 12-1; 22-8; 7-8), and present the results like that:
1: -2;
2: 11
3: 14
4: -1
How can I do that?

Geoff Hayes on 11 Jul 2019
Why is it 3-5 for the first row and 12-1 cfor the second row (etc.)? It isn't clear to me where these pairs of number are coming from...
Guillaume on 11 Jul 2019
Like Geoff, I don't understand how the result is obtained. Pleas clarify
And, please, use valid matlab notation in your examples, so we can just copy/paste it into matlab. With the way you've written it it's difficult to know which element of C1 correspond to which element of C2
C1 = [1 1 1 1 1 2 2 2 2 2 2 3 3 3 4 4 4 4] %this is valid matlab syntax
C2 = [5 10 12 35 3 1 4 26 5 11 12 8 2 22 8 2 5 7] %We can just copy/paste that into matlab
edit: And now that I've written it in a readable way, it's clear that what you want is last-first for each index. Hence, the importance of formatting your post properly.
Aragorn23 on 11 Jul 2019
Maybe I didn't present the data in the right way.
C1 C2
1 5
1 10
1 12
1 35
1 3
2 1
2 4
2 26
2 5
2 11
2 12
3 8
3 2
3 22
4 8
4 2
4 5
4 7
C1 = [1 1 1 1 1 2 2 2 2 2 2 3 3 3 4 4 4 4];
C2 = [5 10 12 35 3 1 4 26 5 11 12 8 2 22 8 2 5 7];
I would like to subtract the first and the last element of C2 according to C1.

Guillaume on 11 Jul 2019
Edited: Guillaume on 11 Jul 2019
one way:
C1 = [1 1 1 1 1 2 2 2 2 2 2 3 3 3 4 4 4 4] %this is valid matlab syntax
C2 = [5 10 12 35 3 1 4 26 5 11 12 8 2 22 8 2 5 7] %We can just copy/paste that into matlab
transitions = find(diff([-Inf, C1, Inf]) ~= 0);
result = [C1(transitions(1:end-1)); C2(transitions(2:end)-1) - C2(transitions(1:end-1))].'
edit: Note that this assumes that your grouping indices (C1) are not mixed (It wouldn't work with C1 = [1 1 1 2 2 1 1 2] for example)