CODEWITHSUNDEEP
rmatrixcombinations
J.Z.
J.Z.

Reputation: 435

How to enumerate all combinations in a matrix in R?

I am trying to construct a matrix that includes all the possible combinations. For example,

a=(1:2)^3 #=c(1,8)
b=(1:3)^2 #=c(1,4,9)

And I would like to define c such that c=c(1+1,1+4,1+9,8+1,8+4,8+9). I have learned from my previous question on how to get such a c from function outer. My current question is, how can I get a matrix M as follows:

enter image description here

Thanks in advance!

Upvotes: 1

Views: 82

Answers (3)

akrun
akrun

Reputation: 887138

Or another option is CJ

library(data.table)
CJ(a, b)[, C := V1 + V2][]
#.  V1 V2  C
#1:  1  1  2
#2:  1  4  5
#3:  1  9 10
#4:  8  1  9
#5:  8  4 12
#6:  8  9 17

Upvotes: 2

Zheyuan Li
Zheyuan Li

Reputation: 73315

OK, here it is:

z <- outer(b, a, "+")
cbind(a[col(z)], b[row(z)], c(z))
#     [,1] [,2] [,3]
#[1,]    1    1    2
#[2,]    1    4    5
#[3,]    1    9   10
#[4,]    8    1    9
#[5,]    8    4   12
#[6,]    8    9   17

A slightly adapted expand.grid solution.

ref <- expand.grid(b = b, a = a)
val <- do.call("+", ref)  ## or `rowSums(ref)` with an implicit `as.matrix`
cbind(ref, c = val)
#  b a  c
#1 1 1  2
#2 4 1  5
#3 9 1 10
#4 1 8  9
#5 4 8 12
#6 9 8 17

In this case the result is a data frame rather than a matrix.

Upvotes: 5

Ronak Shah
Ronak Shah

Reputation: 388982

We can use expand.grid with outer

data.frame(expand.grid(a, b), c = c(outer(a, b, "+")))

#  Var1 Var2  c
#1    1    1  2
#2    8    1  9
#3    1    4  5
#4    8    4 12
#5    1    9 10
#6    8    9 17

where 

outer(a, b, "+") #gives

#     [,1] [,2] [,3]
#[1,]    2    5   10
#[2,]    9   12   17

Upvotes: 5

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