Selecting elements from an array in MATLAB
arrays, indexing, matlab
Solution
Converting to linear indices seems like a legitimate way to go:
indices = tuples(:, 1) + size(a,1)*(tuples(:,2)-1);
selection = a(indices);
Note that this is also implement in the Matlab built-in solution `sub2ind`, as in nate'2 answer:
a(sub2ind(size(a), tuples(:,1),tuples(:,2)))
however,
a = rand(50);
tuples = [1,1; 1,4; 2,5];
start = tic;
for ii = 1:1e4
indices = tuples(:,1) + size(a,1)*(tuples(:,2)-1); end
time1 = toc(start);
start = tic;
for ii = 1:1e4
sub2ind(size(a),tuples(:,1),tuples(:,2)); end
time2 = toc(start);
round(time2/time1)
which gives
ans =
38
so although `sub2ind` is easier on the eyes, it's also ~40 times slower. If you have to do this operation often, choose the method above. Otherwise, use `sub2ind` to improve readability.
Problem
I know that in MATLAB, in the 1D case, you can select elements with indexing such as `a([1 5 3])`, to return the 1st, 5th, and 3rd elements of a. I have a 2D array, and would like to select out individual elements according to a set of tuples I have. So I may want to get `a(1,3), a(1,4), a(2,5)` and so on. Currently the best I have is `diag(a(tuples(:,1), tuples(:,2))`, but this requires a prohibitive amount of memory for larger a and/or tuples. Do I have to convert these tuples into linear indices, or is there a cleaner way of accomplishing what I want without taking so much memory?