Indexing an array with a vector
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I would like to index an N-dimensional array with a vector of length N. In particular, for the 2-dimensional case I am currently doing the following.
A = rand(10);
v = randi(10,1,2);
v = num2cell(v);
A(v{:})
However, this approach seems horribly inefficient. Is there not a smarter way to convert the vector v to a proper index (comma-separated) for the array A?
Edit Let's say v = randi(10,1,2) = [ 3 5 ]. In that case, I want to obtain A(3,5).
4 Comments
Walter Roberson
on 7 Aug 2015
You can improve performance slightly by assigning the transpose to p1_off ahead of time, equivalent to if I had written
Aoff = cumprod([1 As(1:end-1)]) .';
and then just use * Aoff rather than * Aoff.'
Also the "-1" was important to add to the indices. It probably can be rolled into the calculation of the offset at the expense of clarity.
Accepted Answer
Walter Roberson
on 7 Aug 2015
%setup
As = size(A);
Aoff = cumprod([1 As(1:end-1)]);
%calculation at run-time
vidx = (v-1) * Aoff.' + 1;
A(vidx)
This will work when v is an M x N array of indices, producing a column array of M values. If your array has trailing singular dimensions being indexed then the code will not work as-is (the last entry of Aoff needs to be duplicated for all trailing singular dimensions.)
The calculation here converts the index components into linear indices using some trivial matrix multiplication.
More Answers (1)
Azzi Abdelmalek
on 7 Aug 2015
Edited: Azzi Abdelmalek
on 7 Aug 2015
A = rand(10);
v = randi(10,1,2)
Why converting to cell array? just type
A(v)
If v is a cell array, convert it to a double array
A = rand(10);
v = randi(10,1,2);
v = num2cell(v);
A([v{:}])
4 Comments
Azzi Abdelmalek
on 7 Aug 2015
Edited: Azzi Abdelmalek
on 7 Aug 2015
A = rand(10)
v = randi(10,1,2)
idx=sub2ind(size(A),v(1),v(2))
A(idx)
If v is nx2 array
idx=sub2ind(size(A),v(:,1),v(:,2))
A(idx)
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