CODEWITHSUNDEEP
rcombinations
pat
pat

Reputation: 627

find combination pairs of attribute variables

I looked around for a solution but could not find an exact one.

Given:

a<-c('a','b','c')
b<-c('d','e','f')
d<-c('g','h')

as a toy subset of a much larger set, I want to be able to find unique pairs between attribute (vector) sets. If I use

combn(c(a,b,d),2)

It would return ALL pairwise combinations of all of the attribute elements. e.g. 

combn(c(a,b,d),2)

returns c(a,b) c(a,d) c(a,d) c(a,e)...

But I only want pairs of elements between attributes. So I would not see a,b or a,c but a,d a,e a,f b,d b,e,b,f etc...

I could sort of do it with expand.grid(a,b,d)..

   Var1 Var2 Var3
1     a    d    g
2     b    d    g
3     c    d    g
4     a    e    g
5     b    e    g
6     c    e    g
7     a    f    g
8     b    f    g
9     c    f    g
10    a    d    h
11    b    d    h
12    c    d    h
13    a    e    h
14    b    e    h
15    c    e    h
16    a    f    h
17    b    f    h
18    c    f    h

but now I have an n-col dimensional set of the combinations. Is there any way to limit it to just attribute pairs of elements, such as combn(x,2)

The main goal is to find a list of unique pairwise combinations of elements between all attribute pairs, but I do not want combinations of elements within the same attribute column, as it is redundant in my application.

Upvotes: 2

Views: 3966

Answers (2)

flodel
flodel

Reputation: 89057

First, create a list where each element is a pair of your original vectors, e.g. list(a, b):

L <- list(a, b, d)
L.pairs <- combn(seq_along(L), 2, simplify = FALSE, FUN = function(i)L[i])

Then run expand.grid for each of these pairs and put the pieces together:

do.call(rbind, lapply(L.pairs, expand.grid))
#    Var1 Var2
# 1     a    d
# 2     b    d
# 3     c    d
# [...]
# 19    d    h
# 20    e    h
# 21    f    h

Upvotes: 1

Matthew Lundberg
Matthew Lundberg

Reputation: 42629

Taking combinations of pairs in each row in the grid, then filtering to get unique entries, we have this:

unique(do.call(c, apply(expand.grid(a,b,d), 1, combn, m=2, simplify=FALSE)))

A list of combinations is returned:

> L <- unique(do.call(c, apply(expand.grid(a,b,d), 1, combn, m=2, simplify=FALSE)))[1:5]
> length(L) ## 21
> L
## [[1]]
## Var1 Var2 
##  "a"  "d" 
## 
## [[2]]
## Var1 Var3 
##  "a"  "g" 
## 
## [[3]]
## Var2 Var3 
##  "d"  "g" 
## 
## [[4]]
## Var1 Var2 
##  "b"  "d" 
## 
## [[5]]
## Var1 Var3 
##  "b"  "g"

Upvotes: 2

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