[R] Odp: Still confused with lapply

[Petr PIKAL]

Hi

r-help-bounces at r-project.org napsal dne 19.11.2010 15:41:08:

> 
> Hello,
> 
> I would like to replace a for loop with lapply in order to speed up the
> treatment of my data (I've read lapply can help to do it).

Not exactly. For loop if correctly written can be as quick as lapply. See 
P.Burns' R inferno.

> 
> At the end of the message, you will find a simple example (function is 
more
> complex and data frames is more than 1 million of rows) of what I would 
like
> to do, that is applying the same calculation to all the rows of the data
> frame (knowing that I cannot change the structure of the function).

Why? You do not have access to the code of a function?

> 
> I hope you understand my french explainations ...
> Thanks in advance for your help,
> Have a nice week-end,
> Ptit Bleu.
> 
> 
> df1<-data.frame(c(1,2,3,4), c(9,6,5,8), c(5,4,8,6))
> names(df1)<-c("C1", "C2", "C3")
> 
> fcttest<-function(a1,a2,a3) {
>   v<-a1*a2
>   w<-a2+a3
> return(c(v,w))
> }
> 
> # The following for loop works
> result<-data.frame()
> for (i in 1:length(df1[,1])) {
> result<-rbind(result,fcttest(df1[i,1],df1[i,2],df1[i,3]))
> }

Let assume you want to use some function for first two columns and some 
for the columns 2 and 3. Than why not just do it?

> matrix(fcttest(df1[,1],df1[,2],df1[,3]),ncol=2)
     [,1] [,2]
[1,]    9   14
[2,]   12   10
[3,]   15   13
[4,]   32   14

Regards
Petr



> 
> # But the command 
> lapply(df1, fcttest)
> # gives (in french, sorry)
> # Erreur dans FUN(X[[1L]], ...) : l'élément 2 est vide ;
> # la partie de la liste d'arguments de '*' en cours d'évaluation était :
> #  (a1, a2)
> 
> 
> 
> -- 
> View this message in context: 
http://r.789695.n4.nabble.com/Still-confused-
> with-lapply-tp3050414p3050414.html
> Sent from the R help mailing list archive at Nabble.com.
> 
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