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Indexing an array with a vector

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Jori
Jori 2015-8-7
评论： Jori 2015-8-7
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
Walter Roberson 2015-8-7
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.
Jori
Jori 2015-8-7
Thanks, that is a nice remark.
In these computations, data(n,2:end) has values starting at 0. I need to add 1 to adjust for the minimal index 1 in Matlab. This is followed by the "-1" that you had and so the "-1" disappears in line 25 of the profiler figure.
I verified the results, and this is indeed the correct indexing.

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采纳的回答

Walter Roberson
Walter Roberson 2015-8-7
%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
Azzi Abdelmalek 2015-8-7
编辑：Azzi Abdelmalek 2015-8-7
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{:}])
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Azzi Abdelmalek
Azzi Abdelmalek 2015-8-7
编辑：Azzi Abdelmalek 2015-8-7
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)
Jori
Jori 2015-8-7
编辑：Jori 2015-8-7
The size of v is 1 x N. I do not know N beforehand.
Using idx = sub2ind(size(A),v) does not work. So I would need to use idx = sub2ind(size(A),v(1),v(2),...,v(N)), but I do not know the value of N beforehand. Thus, I would need to use
v = num2cell(v);
idx = sub2ind(size(A),v{:});
A(idx)
Which brings me back to the original problem.

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