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).
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Walter Roberson
2015년 8월 7일
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.
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Walter Roberson
2015년 8월 7일
%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.
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Azzi Abdelmalek
2015년 8월 7일
편집: Azzi Abdelmalek
2015년 8월 7일
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{:}])
댓글 수: 4
Azzi Abdelmalek
2015년 8월 7일
편집: Azzi Abdelmalek
2015년 8월 7일
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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