All Combinations of k Elements from Vector v

Finds all unordered combinations of k elements from vector v.

combs(v,k)

Arguments

v

Any numeric vector

k

Number of elements to choose from vector v. Integer smaller or equal than length of v.

Value

combs(v,k) (where v has length n) creates a matrix with \(\frac{n!}{(n-k)! k!}\) (n choose k) rows and k columns containing all possible combinations of n elements taken k at a time.

Author

Jarek Tuszynski (SAIC) jaroslaw.w.tuszynski@saic.com

See also

I discovered recently that R packages already have two functions with similar capabilities: combinations from gTools package and Also similar to Matlab's nchoosek function (http://www.mathworks.com/access/helpdesk/help/techdoc/ref/nchoosek.html)

Examples

  combs(2:5, 3) # display examples
#>      [,1] [,2] [,3]
#> [1,]    2    3    4
#> [2,]    2    3    5
#> [3,]    2    4    5
#> [4,]    3    4    5
  combs(c("cats", "dogs", "mice"), 2)
#>      [,1]   [,2]  
#> [1,] "cats" "dogs"
#> [2,] "cats" "mice"
#> [3,] "dogs" "mice"
  
  a = combs(1:4, 2)
  b = matrix( c(1,1,1,2,2,3,2,3,4,3,4,4), 6, 2)
  stopifnot(a==b)