Array multiplicities via sparse

6 February 2017 in matlab

Multiplicities of array elements can be efficiently computed using sparse. I’ve found this elegant solution in an old Newsgroup thread.

% generate some data
A = randi(10,1,100);
% construct the sparse matrix
S = sparse(A,1,1);
% get unique elements and multiplicities
[uA,~,mult] = find(S);

mult =
   (1,1)        8
   (2,1)       15
   (3,1)       11
   (4,1)        5
   (5,1)       10
   (6,1)        6
   (7,1)       10
   (8,1)       13
   (9,1)       13
  (10,1)        9

This method is faster than other solutions – almost twice as fast as histc.

% test array: 2000 random integers from the set 1:1000
% 1000 iterations
Elapsed time is 0.228704 seconds. % histc
Elapsed time is 1.838388 seconds. % bsxfun
Elapsed time is 0.128791 seconds. % sparse