Combine matrix elements

0
votes

I wish to get all combinations of elements from a matrix of size m * n.

Sample Example:

1   3   5
2   6   7

Expected Output:

2 , 1
2 , 3
2 , 5
6 , 1
6 , 3
6 , 5
7 , 1
7 , 3
7 , 5

The rules:

  1. Every combination starts from bottom of matrix and proceeds towards top. It may switch columns though.
  2. Every combination should have number of elements equal to number of rows.
  3. A combination can't have an element from the same row present twice.
  4. Number of columns and rows could vary. So solution has to be generic.

I tried doing this but not getting the exact result

m <- rbind(c(1, 3, 5), c(2, 6, 7))
do.call(expand.grid, split(m, rep(nrow(m):1, ncol(m))))
1
rmatrixcombinatorics
This sounds like a homework question. Please take some time revising How do I ask and answer homework questions?. In short, SO is not a free tutorial/homework service. We are happy to help provided you demonstrate your own effort at solving the problem. Please revise your question to include your code attempt. – Maurits Evers
Perhaps you can show your attempt with expand.grid. – John Coleman
m <- rbind(c(1,3,5),c(2,6,7)) do.call(expand.grid, split(m, rep(nrow(m):1, ncol(m)))) – Nrj
its not a homework or anything.. i came accross this problem and was figuring out how to do this in r – Nrj
@Nrj Please don't post critical clarifications as comments. Edit your post to include (1) your code attempt, and (2) a clear description of where you got stuck and why. – Maurits Evers

1 Answers

0
votes

There are many ways,

do.call(
    rbind,lapply(m[2,], function(x){ cbind(x, c(m[1,])) })
)

data:

m<-
structure(c(1L, 2L, 3L, 6L, 5L, 7L), .Dim = 2:3, .Dimnames = list(
    NULL, c("V1", "V2", "V3")))

result:

#   x  
#V1 2 1
#V2 2 3
#V3 2 5
#V1 6 1
#V2 6 3
#V3 6 5
#V1 7 1
#V2 7 3
#V3 7 5

general solution:

do.call(
    rbind,lapply(m[nrow(m),], function(x){ cbind(x, c(m[-nrow(m),])) })
)