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matlabvectorization
Chang
Chang

Reputation: 876

MATLAB vectorization to create a matrix

I want to create a matrix like

[1 2;
 1 3;
 1 4;
 1 5;
 2 3;
 2 4;
 2 5;
 3 4;
 3 5;
 4 5 ]

when the size is 5. I aim to have sizes greater than 100. How can I create a matrix like this using vertorization in MATLAB?

Upvotes: 3

Views: 107

Answers (2)

Eitan T
Eitan T

Reputation: 32920

You're looking for binomial coefficients, so use the built-in nchoosek command. For example, the matrix in your question can be generated by:

A = nchoosek(1:5, 2)

This results in:

A =
     1     2
     1     3
     1     4
     1     5
     2     3
     2     4
     2     5
     3     4
     3     5
     4     5

Upvotes: 7

yuk
yuk

Reputation: 19870

One solution:

[r,c]=find(tril(ones(N),-1));
result = [c,r];

As a bonus, you can get the number of rows in such matrix with

nrows = nchoosek(N,2);

Upvotes: 2

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