Accessing an element of an array using an array as its index.

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Ayden Clay 2020-4-8

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评论： J. Alex Lee 2020-4-9

Hello,

I am writing up an interpolation function and need to define the vertices of the hypercube surrounding the current values.

My current code to do this is:

% (3) Calculate the A values that are the vertices of the N-D hypercube.
a = zeros(1,2^size(CURRENT,1)-1);
curridx = zeros(1,size(CURRENT,1));
idx = zeros(1,size(CURRENT,1));
for cnt = 1:(2^size(CURRENT,1))
    change = find((mod(cnt-1,2.^(0:size(CURRENT,1)-1))==0)==1);
    curridx(change) = 1*curridx(change)==0;
    for cnt1 = 1:size(CURRENT,1)
        idx(cnt1) = lidx(cnt1)*(curridx(cnt1)==0)+uidx(cnt1)*(curridx(cnt1)==1);
    end
    Subs = num2cell(idx);
    a(cnt) = DATA(Subs{:}); 
end

With the profiler showing that the lines

    Subs = num2cell(idx);
    a(cnt) = DATA(Subs{:}); 

Are 32.1% and 23.8% of the total run-time respectively. Does anybody know a more efficient method?

Thank you!

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darova 2020-4-9

did you try my suggestions?

Ayden Clay 2020-4-9

I did, sadly idx is a 7x1 number, and isn't readily inputtable into data.

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Answer 1

J. Alex Lee 2020-4-9

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在 MATLAB Online 中打开

Interesting...I guess your question title should be edited to "use array as subscripts into multidimensional array". I dug into the sub2ind() to extract how to use subscripts to get a linear index and came up with this test

siz = [4,3,5,7]
subs = [1,2,4,6]
subsc = num2cell(subs)
ndx = sub2ind(siz,1,2,4,6)
ndx = sub2ind(siz,subsc{:})
ndx = subs(1);
k = cumprod(siz);
for i = 2:length(subs)
	ndx = ndx + (subs(i)-1)*k(i-1);
end
ndx

So you should be able to do

function ndx = sub2ind_a(siz,subs)
% no-check version of sub2ind, with input array of subs
ndx = subs(:,1);
k = cumprod(siz);
for i = 2:size(subs,2)
	ndx = ndx + (subs(:,i)-1)*k(i-1);
end
end

And in your code:

siz = size(DATA); % somewhere at the top
% and in your loop
a(cnt) = DATA(sub2ind_a(siz,idx));

For readability I would change your variable name "idx" to "subs_a" or something.

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Ayden Clay 2020-4-9

This absolutely works and is around 5% faster, which is wonderful.

Thank you greatly!

J. Alex Lee 2020-4-9

The difference between indexing on the linearized subscripts versus using num2cell and subscripting is about 10x faster on my laptop...maybe you are spending too much time determining the subscripts themselves (e.g., see darova's comments about the looping)

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