# Find indices of points in meshgrid-like data

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Aleksander Marek 2018 年 9 月 26 日
コメント済み: Aleksander Marek 2018 年 9 月 27 日
Hi,
I'm looking for an optimal solution to the following problem:
You are given a vector containing coordinates x,y for set of points e.g.:
A = [2 3; 3 4; 1 1];
given that the full-data is structured in a matrix form (meshgrid-like)
[X,Y] = meshgrid(1:5, 1:5);
find indices B, such that:
A = [X(B), Y(B)];
I am able to solve this problem simply by looping over all points in A and finding the index with
for pt = 1:size(A,1);
index(pt) = find(A(pt,1)== X & A(pt,2) == Y);
end
However I'm not satisfied with the performance of this solution as I am running through ~50,000 data points around 300 times, and I'd love to replace the for loop with some vectorized formula.
Any help would be highly appreciated!
Thanks,
Alex

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### 採用された回答

Adam 2018 年 9 月 26 日
[~, index] = ismember( A, [X(:) Y(:)], 'rows' );
Not sure if that is any faster than a loop though. ismember is not a very performant function as, if I remember correctly, it contains quite a lot of checks that slow it down.
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Aleksander Marek 2018 年 9 月 27 日
It actually works really well, and gives improvement of about 10 times over the for loop.
Thanks a lot!

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### その他の回答 (1 件)

Bish Erbas 2018 年 9 月 26 日
A = [2 3; 3 4; 1 1];
[X,Y] = meshgrid(1:5, 1:5);
XIdxCell = arrayfun(@(x) find(X==x),A(:,1),'UniformOutput',false);
YIdxCell = arrayfun(@(x) find(Y==x),A(:,2),'UniformOutput',false);
XIdx = cell2mat(XIdxCell);
YIdx = cell2mat(YIdxCell);
A = [X(XIdx), Y(YIdx)]

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