How to sum specific values from a loop and create a new vector?

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Philippe Corner
Philippe Corner le 23 Jan 2018
Commenté : Star Strider le 23 Jan 2018
If I have this matrix M:
a=1:1:20;
b=[0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 2 0 0];
M=[a,b];
How could I find the i values where M(:,b)>0 (where the second column "b">0), and then take the values of the same row but the first column "a" M(a,:) and make a summation from that row i and i-2, i,e, [i + (i-1)+(i-2)].. like this:
X=[(5+6+7) (12+13+14) (18+17+16)] = [18 39 51]
Besides a vector Y which contains the summation from i-5 until i-3 like this:
Y=[(2+3+4) (9+10+11) (13+14+15)] = [9 30 42]
Both of them for all i values in a vector b of a matrix.

Réponse acceptée

Star Strider
Star Strider le 23 Jan 2018
Use bsxfun and sum:
a=1:1:20;
b=[0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 2 0 0];
M=[a(:),b(:)];
idx = find(M(:,2) > 0);
X = sum(bsxfun(@plus, idx, -2:0),2)
Y = sum(bsxfun(@plus, idx, -5:-3),2)
X =
18
39
51
Y =
9
30
42
  8 commentaires
Philippe Corner
Philippe Corner le 23 Jan 2018
Amazing! The last question would be how to solve the same problem but not for the rows which M(:,2)>0, but all the rows! if I apply the same sense, the problem would be for the very beginning rows.. any idea Mr Star?
Star Strider
Star Strider le 23 Jan 2018
Thank you!
If you want to have a moving sum for all the rows taken 10 at a time, use the movsum (link) function (introduced in R2016a).

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