Retrieving element index in spfun, cellfun, arrayfun, etc. in MATLAB

Question

Is there any way to retrieve the index of the element on which a function called by cellfun, arrayfun or spfun acts? (i.e

Answer 1

It's simple. Just make a cell like: C = num2cell(1:length(S)); then: out=arrayfun(@(x,c) c*x,S,C)

Answer 2

You can create a sparse matrix with linear_index filling instead of values.

Create A:

A(find(S))=find(S)

Then use A and S without spfun, e.g.: A.*S. This runs very fast.

Answer 3

No, but you can supply the linear index as extra argument.

Both cellfun and arrayfun accept multiple input arrays. Thus, with e.g. arrayfun, you can write

a = [1 1 2 2];
lin_idx = 1:4;
out = arrayfun(@(x,y)x*y,a,lin_idx);

This doesn't work with spfun, unfortunately, since it only accepts a single input (the sparse array).

You can possibly use arrayfun instead, like so:

S = spdiags([1:4]',0,4,4);
lin_idx = find(S);

out = spones(S);
out(lin_idx) = arrayfun(@(x,y)x*y,full(S(lin_idx)),lin_idx);
%# or
out(lin_idx) = S(lin_idx) .* lin_idx;

Note that the call to full won't get you into memory trouble, since S(lin_idx) is 0% sparse.