Using combn in R to create a matrix of all possible combinations

Question

This is related to a homework question I am working on. I need to perform a data manipulation of a few vectors into a matrix, and the TA suggested using the combn function:

# what I'm starting with
a = c(1, 2)
b = c(NA, 4, 5)
c = c(7, 8)

# what I need to get 
my_matrix
a    b    c
1   NA    7
1   NA    8
1    4    7 
1    4    8 
1    5    7 
1    5    8 
2   NA    7 
2   NA    8 
2    4    7 
2    4    8
2    5    7
2    5    8

my_matrix is a matrix with all possibly combinations of the elements in a, b and c, with column names a, b, and c. I understand what combn() is doing, but not exactly sure how to convert it into the matrix shown above?

Thanks in advance for any help!

It's almost like the hint to use combn was just a distraction. Thanks! — Canovice
The hint could have been subtle - the ?combn help file mentions expand.grid in its 'See also' section with the description: 'for creating a data frame from all combinations of factors or vectors.' — thelatemail

d.b d.b · Accepted Answer · 2017-02-27T01:16:54

expand.grid, mentioned in the comments to the question, is the better and much easier way to do it. But you can use combn too

#STEP 1: Get all combinations of elements of 'a', 'b', and 'c' taken 3 at a time
temp = t(combn(c(a, b, c), 3))

# STEP 2: In the first column, only keep values present in 'a'
#Repeat STEP 2 for second column with 'b', third column with 'c'
#Use setNames to rename the column names as you want
ans = setNames(data.frame(temp[temp[,1] %in% a & temp[,2] %in% b & temp[,3] %in% c,]),
                                                                  nm = c('a','b','c'))
ans
#   a  b c
#1  1 NA 7
#2  1 NA 8
#3  1  4 7
#4  1  4 8
#5  1  5 7
#6  1  5 8
#7  2 NA 7
#8  2 NA 8
#9  2  4 7
#10 2  4 8
#11 2  5 7
#12 2  5 8

Using combn in R to create a matrix of all possible combinations

1 Answers