NCHOOSE2 (v2.1 - jun 2008)

all combinations of two elements

Description

NCHOOSE2 - all combinations of two elements
 
Y = NCHOOSE2(X) returns all combinations of two elements of the array X. It is the fast, vectorized version of NCHOOSEK(X,2). X can be any type of array.
 
Example:
nchoose2([10 20 30 40])
% -> 10 20
% 10 30
% 10 40
% 20 30
% 20 40
% 30 40

nchoose2({'a','b','c','d','e'})
% -> 'a' 'b'
% 'a' 'c'
% ...
% 'c' 'e'
% 'd' 'e'
 
See also nchoosek, perms
 combn, nchoose, allcomb (on the File Exchange)

(v2.0, may 2008)

Acknowledgements

The author wishes to acknowledge the following in the creation of this submission:
nCtwo
This submission has inspired the following:
VChooseK

• MATLAB release: MATLAB 7 (R14)

[Urs (us) Schwarz]

scholarly example of smart indexing, which should be studied by ML novices as well as cognoscenti alike...
us

[Urs (us) Schwarz]

this did not copy/paste into the last review:

unfortunately, NCHOOSE2 errors with a single input, eg, nchoose2('a') - an unnecessary flaw, which quickly shall be corrected by jos, i'm sure...
us

[Jos (author)]

Thanks us. this problem will be fixed in v2.1

[m p]

eheh...

[David S]

much faster than nchoosek! thanks

[Jan Simon]

In the comments it is stated, that NCHOOSE2 is for "R13+". This means > R13.
To run this in R13, V must be a column vector:
  V = reshape(N-1:-1:2, [], 1);
  ri = cumsum([1; V]);
  ind(ri, 2) = [0; -V] + 1;
However, obviously nobody missed the R13 compatibility in the last 1.5 years. So I think there is no need to change the algorithm, but perhaps the comment to "R>=14".