[R] Union of columns of two matrices

Thu Aug 7 01:20:29 CEST 2008

Here is my attempt. I'm not sure if that's the most efficient way to do
it, cause I'm "cheating" using the nice features from R, namely
"duplicated()".

I assume the matrices have same number of rows.

### example ###
### background setup: simulate 2 matrices with some common columns
(A <- cbind(1:4, matrix(rnorm(16),4), 101:104))
#      [,1]       [,2]       [,3]       [,4]      [,5] [,6]
# [1,]    1 -0.5305169 -1.7243920 -0.1722617 1.7343167  101
# [2,]    2 -0.3466017  0.3737072  0.5961296 1.4493053  102
# [3,]    3 -1.7812876 -1.5707614  1.4401485 0.9683144  103
# [4,]    4 -1.7219545  0.4762025 -0.2137656 0.7008253  104
(B <-
cbind(matrix(rnorm(8),4),1:4,matrix(rnorm(12),4),101:104,c(1:3,5)))
#            [,1]       [,2] [,3]       [,4]       [,5]       [,6] [,7]
[,8]
# [1,]  1.1182879  0.5340995    1  1.0434300 -0.5105291 -1.0994476  101
1
# [2,]  0.4031942  0.3156704    2 -0.4704723  0.8367561 -1.6163610  102
2
# [3,] -1.0317547 -0.5642614    3  1.0916636  1.0411857  0.1914676  103
3
# [4,] -0.6036328  3.2339688    4  1.8505135  2.0055947 -0.0359060  104
5

### some auxiliary abstractions
matrix2list <- function(M) lapply(split(M,col(M)), function(c) c)
list2matrix <- function(L) sapply(L, function(c) c)

### Then the problem can be solved in 2 lines 
L <- c(matrix2list(A),matrix2list(B))
list2matrix(L[!duplicated(L)])

# Or even 1 line, but kind of confusing
(function(L)
list2matrix(L[!duplicated(L)]))(c(matrix2list(A),matrix2list(B)))

# output; compare to above, the duplicated columns are gone
#      1          2          3          4         5   6          1
2
# [1,] 1 -0.5305169 -1.7243920 -0.1722617 1.7343167 101  1.1182879
0.5340995
# [2,] 2 -0.3466017  0.3737072  0.5961296 1.4493053 102  0.4031942
0.3156704
# [3,] 3 -1.7812876 -1.5707614  1.4401485 0.9683144 103 -1.0317547
-0.5642614
# [4,] 4 -1.7219545  0.4762025 -0.2137656 0.7008253 104 -0.6036328
3.2339688
#               4          5          6 8
# [1,]  1.0434300 -0.5105291 -1.0994476 1
# [2,] -0.4704723  0.8367561 -1.6163610 2
# [3,]  1.0916636  1.0411857  0.1914676 3
# [4,]  1.8505135  2.0055947 -0.0359060 5

##### end example #####

I'm not sure how "duplicated" is coded in R. If those two lists are
sorted before comparing, then I guess the complexity is O(n). If not,
then it's O(n^2). [n = ncol(L)]

-gary

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Giuseppe Paleologo
Sent: Wednesday, August 06, 2008 6:33 PM
To: r-help at r-project.org
Subject: [R] Union of columns of two matrices

I was posed the following problem/teaser:

given two matrices, come up with an "elegant" (=fast & short) function
that
returns a matrix with all and only the non-duplicated columns of both
matrices; the column order does not matter. In essence, a matrix
equivalent
of union(x,y), where x and y are vectors. I could not come with anything
nice. Any ideas?

Giuseppe

-- 
Giuseppe A. Paleologo :: Email: paleologo at gmail.com :: AOL: gappy3000 ::
Skype :: gappy3000 :: Gtalk: paleologo :: Mobile: 917.331.3497
fact: 2^32,582,657-1 is a prime

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