SUMM

Function analogous to DIFF, but takes sums instead.

%SUMM Sum of consecutive pairs in an array, analogous to DIFF.
% SUMM(X), for a vector X, is [X(2)+X(1) X(3)+X(2) ... X(n)+X(n-1)].
% SUMM(X), for a matrix X, is the matrix of row sums,
% [X(2:n,:) + X(1:n-1,:)].
% SUMM(X), for an N-D array X, is the sum along the first
% non-singleton dimension of X.
% SUMM(X,N) is the N-th order sum along the first non-singleton
% dimension (denote it by DIM). If N >= size(X,DIM), SUMM takes
% successive sums along the next non-singleton dimension.
% SUMM(X,N,DIM) is the Nth sum function along dimension DIM.
% If N >= size(X,DIM), SUMM returns an empty array.
%
% Examples:
%
% If X = [3 7 5
% 0 9 2]
% then summ(X,1,1) is [3 16 7], summ(X,1,2) is [10 12
% 9 11],
% summ(X,2,2) is the 2nd order sum along the dimension 2, and
% summ(X,3,2) is the empty matrix.
%
% Note: There's probably a better way to do this. In testing, this
% is consistently three times slower than diff. But it was quick
% enough.
%
% This was inspired by the CSSM question:
% http://www.mathworks.com/matlabcentral/newsreader/view_thread/259947