Subtract according to the values in another column

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]

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]

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]

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.

Answer 1

[Guillaume]

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)

Answer 2

[Matt J]

[~,i]=unique([C1,C1(end+1)],'stable');

 result = C2(i(2:end)-1)-C2(i(1:end-1))