How to vectorize vector indexing?
9 views (last 30 days)
Show older comments
I want to vectorize the operation of indexing an element in a vector. That is, given a vector of vector indices, I want to pick the corresponding elements out of a matrix where each row is the vector to index.
e.g.
For a matrix
[1 2 3 4;
5 6 7 8;
9 10 11 12]
and the vector of row indices
[2 3 1]
I want to return
[2;
7;
9]
Can this be done with a one-liner?
0 Comments
Accepted Answer
Matt Fig
on 16 Apr 2011
Another approach:
% Data
A = [1 2 3 4;
5 6 7 8;
9 10 11 12];
c = [2 3 1];
%Extraction:
E = A((1:size(A,1))+(c-1)*size(A,1)).'
If size(A,1) is already known (say m=3), then the obvious simplification results...
E = A((1:m)+(c-1)*m).'
2 Comments
Matt Fig
on 16 Apr 2011
Notice also that this:
ROW + (COL-1)*size(A,1)
is just an in-lining of SUB2IND for 2D matrices, something every chronic MATLABer should have memorized (IMO)...
More Answers (1)
Paulo Silva
on 16 Apr 2011
a=[1 2 3 4;
5 6 7 8;
9 10 11 12];
v=[2 3 1];
diag(a(1:end,v))
Another way
arrayfun(@(x,y)a(x,y),1:3,v)'
0 Comments
See Also
Categories
Find more on Matrix Indexing in Help Center and File Exchange
Products
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!