Re: [R] counting the occurrences of vectors

[Spencer Graves]

     I see a case where "f1" gives the wrong answer: 

     b <- array(c("a:b", "a", "c", "b:c"), dim=c(2,2))
     a <- b[c(1,1),]
     For these two matrices, f1(a,b) == c(2,2), while f2(a,b) == 
c(2,0).  If b does not contain ":", e.g., if it is numeric, then this 
pathology can not occur.  However, if "f1" is used with objects of class 
character or string that could contain the "collapse" character, it 
could give an incorrect answer without warning. 

     hope this helps.  spencer graves
Ravi Varadhan wrote:
Thanks to Gabor, Marc, and Spencer for their elegant solutions.  Gabor's first 
solution worked the best for me.
Best,
Ravi.
________________________________
From: [EMAIL PROTECTED] on behalf of Gabor Grothendieck
Sent: Sat 7/3/2004 12:12 PM
To: [EMAIL PROTECTED]
Subject: Re: [R] counting the occurrences of vectors

Ravi Varadhan <rvaradha <at> jhsph.edu> writes:
 

Hi:
I have two matrices, A and B, where A is n x k, and B is m x k, where n >> m
   

k.  Is there a computationally fast way to
     

count the number of times each row (a k-vector) of B occurs in A?  Thanks
   

for any suggestions.
 

Best,
Ravi.
   

Here are two approaches.  The first one is an order of magnitude faster
than the second.
R> # test matrices
R> set.seed(1)
R> a <- matrix(sample(3,1000,rep=T),nc=5)
R> b <- matrix(sample(3,100,rep=T),nc=5)
R> f1 <- function(a,b) {
+ a2 <- apply(a, 1, paste, collapse=":")
+ b2 <- apply(b, 1, paste, collapse=":")
+ c(table(c(a2,unique(b2)))[b2] - 1)
+ }
R> f2 <- function(a,b) {
+ ta <- t(a)
+ apply(b,1,function(x)sum(apply(ta == x,2,all)))
+ }
R> gc(); system.time(ans1 <- f1(a,b))
        used (Mb) gc trigger (Mb)
Ncells 458311 12.3     818163 21.9
Vcells 124264  1.0     786432  6.0
[1] 0.03 0.00 0.03   NA   NA
R> gc(); system.time(ans2 <- f2(a,b))
        used (Mb) gc trigger (Mb)
Ncells 458312 12.3     818163 21.9
Vcells 124270  1.0     786432  6.0
[1] 0.1 0.0 0.1  NA  NA
R> all.equal(ans1, ans2)
[1] TRUE
______________________________________________
[EMAIL PROTECTED] mailing list
https://www.stat.math.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html

        [[alternative HTML version deleted]]
______________________________________________
[EMAIL PROTECTED] mailing list
https://www.stat.math.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
 

______________________________________________
[EMAIL PROTECTED] mailing list
https://www.stat.math.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html