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

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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!
  4 Comments
Ayden Clay
Ayden Clay on 9 Apr 2020
I did, sadly idx is a 7x1 number, and isn't readily inputtable into data.

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Accepted Answer

J. Alex Lee
J. Alex Lee on 9 Apr 2020
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.
  4 Comments
J. Alex Lee
J. Alex Lee on 9 Apr 2020
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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