Array from vector on the basis of a second array

Question

I have a vector v. I need to form an array a containing elements specified according to another array b. Each row in a (let's denote it by r) should contain all elements from v, with starting and ending indices corresponding to the first and last elements given in the matching column in b. For instance:

A(1, :) = v(b(1, 1):b(2, 1));
A(2, :) = v(b(1, 2):b(2, 2));
A(3, :) = v(b(1, 3):b(2, 3));

and so on. Obviously b(2,:) = b(1,:) + constant.

Can I do this without a loop in MATLAB?

If you can add your coding to the question, that would help in answering. — shunyo, Sep 30 '13 at 17:33
related question (and possibly a duplicate): [Matlab's colon operator: values between two vectors](http://stackoverflow.com/questions/14341798/matlabs-colon-operator-values-between-two-vectors) — Eitan T, Oct 01 '13 at 05:49

score 1 · Accepted Answer · answered Sep 30 '13 at 17:53

1

Try this:

N=8; P=3; M=5;
v = rand(N,1);
b = zeros(2,M);
b(1,:) = [1 2 4 5 6];
b(2,:) = b(1,:) + P - 1;
A = cell2mat(arrayfun(@(i0,i1) v(i0:i1),b(1,:),b(2,:),'UniformOutput',false))'

answered Sep 30 '13 at 17:53

chappjc

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However, just to point out, as discussed [here](http://stackoverflow.com/questions/12522888/arrayfun-can-be-significantly-slower-than-an-explicit-loop-in-matlab-why), although loops take more space, they are faster. – umayfindurself Sep 30 '13 at 18:40
Out of curiosity, why did you need to do this without loops? – chappjc Sep 30 '13 at 18:51
@user2000581 Mind you that `bsxfun` is usually faster than both :) – Eitan T Oct 01 '13 at 05:57

score 1 · Answer 2 · answered Oct 08 '13 at 14:33

1

You can use linear indexing and bsxfun to directly access the elements:

A = v(bsxfun(@plus, b(1,:).', 0:b(2,1)-b(1,1)));

answered Oct 08 '13 at 14:33

Mohsen Nosratinia

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Array from vector on the basis of a second array

2 Answers2