Reproducible example:
set.seed(1)
testMat <- matrix(round(runif(3*6,1,5)), nrow = 3, ncol = 6)
Output:
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 2 5 5 1 4 3
[2,] 2 2 4 2 3 4
[3,] 3 5 4 2 4 5
Here first 3 columns (1,2,3) belong to one set and next 3 columns (4,5,6) belong to another set. I want to add one column from each set and need to do it for all possible combinations.
For this example, I should get 9 resulting vectors as there are 9 combinations:
Combination 1: (1,4) = (3,4,5)
Combination 2: (1,5) = (6,5,7)
Combination 3: (1,6) = (.,.,.)
Combination 4: (2,4) = (.,.,.)
Combination 5: (2,5) = (.,.,.)
Combination 6: (2,6) = (.,.,.)
Combination 7: (3,4) = (.,.,.)
Combination 8: (3,5) = (.,.,.)
Combination 9: (3,6) = (.,.,.)
Is there any elegant way to do it, especially when the number of columns can be higher? For example 9, 12, etc. would yield combinations of 27 and 81 respectively.
EDIT: More clarification: Each 3 columns (e.g. 1:3, 4:6, 7:9, 10:12,...etc.) construct one set and the objective is to take 1 column from each set and add them together. For example if we have 6 columns at testMat we take 1 column among 1:3 and another from 4:6 add tese 2 columns together. Similarly for 9 we add 3, for 12 we add 4 columns one from each set.
Here is one way :
#Create a sequence from 1 to number of columns in the data
cols <- seq(ncol(testMat))
n <- 3
#Create a list of every n columns and use expand.grid
#to create all possible combinations
vals <- expand.grid(split(cols, ceiling(cols/n)))
#RowSums every combination using `sapply`.
sapply(asplit(vals, 1), function(x) rowSums(testMat[, x]))
# [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
#[1,] 3 6 6 6 9 9 5 8 8
#[2,] 4 4 6 5 5 7 6 6 8
#[3,] 5 7 6 7 9 8 8 10 9