[R] Problem with pmax and matrix to calculate row maxima

[Enrico Schumann]

Hi Wolfgang,

how about a loop?

matRandom <- matrix(runif(n=600000), ncol=6)

## variant 1
system.time(test1 <- pmax(matRandom[,1], matRandom[,2], matRandom[,3],
                           matRandom[,4], matRandom[,5], matRandom[,6]))

User      System verstrichen
0.01        0.00        0.01


## variant 2
system.time(test2 <- apply(matRandom, 1, max))

User      System verstrichen
0.56        0.00        0.56


## variant 3
system.time({
   test3 <- matRandom[ ,1L]
   ## add a check that ncol(matrix) > 1L
   for (i in 2:ncol(matRandom))
     test3 <- pmax(test3, matRandom[ ,i])

})
User      System verstrichen
0.01        0.00        0.01



 > all.equal(test1,test2)
[1] TRUE

 > all.equal(test1,test3)
[1] TRUE


Regards,
Enrico

Am 12.10.2011 13:06, schrieb Wolfgang Wu:
> I am having the following problem. I want to calculate the maximum of each row in a matrix. If I pass in the matrix split up by each column then this is no problem and works great. However I don't know how many columns I have in advance. In the example below I have 3 columns, but the number of columns is not fix. So how do I do this?
>
>
>      matRandom<- matrix(runif(n=30), ncol=3);
>      #Does not work
>      pmax(matRandom)
>      #Does work
>      pmax(matRandom[,1], matRandom[,2], matRandom[,3])
>
>
> I am aware that I can do it with the apply function, but the calculation is time sensitive so fast execution is important.
>
>
>      #Apply might be too slow
>
>      matRandom<- matrix(runif(n=300000), ncol=3);
>      system.time(test<- pmax(matRandom[,1], matRandom[,2], matRandom[,3]))
>      system.time(test<- apply(matRandom, 1, max))
>
>
>> matRandom<- matrix(runif(n=300000), ncol=3);
>> system.time(test<- pmax(matRandom[,1], matRandom[,2], matRandom[,3]))
>     user  system elapsed
>     0.02    0.00    0.02
>> system.time(test<- apply(matRandom, 1, max))
>>      user  system elapsed
>     2.37    0.00    2.38
>
>
>
>
> Thanks for your help.
>
> Regards.
>
>
> Wolfgang Wu
>
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-- 
Enrico Schumann
Lucerne, Switzerland
http://nmof.net/