Matrix max by row

It seems to be very system dependent.  Here's another take:

m <- matrix(runif(100000),1000,100)
junk <- gc()
print(system.time(for(i in 1:100) X1 <- do.call(pmax,data.frame(m))))

user  system elapsed 
   1.53    0.01    1.57

junk <- gc()
print(system.time(for(i in 1:100) X2 <- apply(m,1,max)))

user  system elapsed 
   1.81    0.00    1.83

Now what happens if you work with data frames rather than matrices:

m <- data.frame(matrix(runif(100000),1000,100))
junk <- gc()
print(system.time(for(i in 1:100) X1 <- do.call(pmax, m)))

user  system elapsed 
   0.31    0.00    0.31

junk <- gc()
print(system.time(for(i in 1:100) X2 <- apply(m,1,max)))

user  system elapsed 
   3.22    0.03    3.34

Go figure!  


Bill Venables
http://www.cmis.csiro.au/bill.venables/ 


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Rolf Turner
Sent: Monday, 30 March 2009 1:39 PM
To: Bert Gunter
Cc: 'Wacek Kusnierczyk'; r-help at r-project.org
Subject: Re: [R] Matrix max by row

I tried the following:

m <- matrix(runif(100000),1000,100)
junk <- gc()
print(system.time(for(i in 1:100) X1 <- do.call(pmax,data.frame(m))))
junk <- gc()
print(system.time(for(i in 1:100) X2 <- apply(m,1,max)))

and got

    user  system elapsed
   2.704   0.110   2.819
    user  system elapsed
   1.938   0.098   2.040

so unless there's something that I am misunderstanding (always a serious
consideration) Wacek's apply method looks to be about 1.4 times  
*faster* than
the do.call/pmax method.

	cheers,

		Rolf Turner

If speed is a consideration,availing yourself of the built-in pmax()
function via

do.call(pmax,data.frame(yourMatrix))

will be considerably faster for large matrices.

If you are puzzled by why this works, it is a useful exercise in R  
to figure
it out.

Hint:The man page for ?data.frame says:
"A data frame is a list of variables of the same length with unique  
row
names, given class 'data.frame'."

Cheers,
Bert

Bert Gunter
Genentech Nonclinical Statistics

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r- 
project.org] On
Behalf Of Wacek Kusnierczyk
Sent: Saturday, March 28, 2009 5:22 PM
To: Ana M Aparicio Carrasco
Cc: r-help at r-project.org
Subject: Re: [R] Matrix max by row

Ana M Aparicio Carrasco wrote:

I need help about how to obtain the max by row in a matrix.
For example if I have the following matrix:
2 5 3
8 7 2
1 8 4

The max by row will be:
5
8
8

matrix(apply(m, 1, max), nrow(m))

vQ

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