[R] apply formula over columns by subset of rows in a dataframe (to get a new dataframe)

[Massimo Bressan]

thank you, what a nice compact solution with ave() 

I learned something new about the subtleties of R 

let me here summarize the alternative solutions, just in case someonelse might be interested... 

thanks, bye 

# 

# my user function (an example) 
mynorm <- function(x) {(x - min(x, na.rm=TRUE))/(max(x, na.rm=TRUE) - min(x, na.rm=TRUE))} 

# my dataframe to apply the formula by blocks 
mydf<-data.frame(blocks=rep(c("a","b","c"),each=5), v1=round(runif(15,10,25),0), v2=round(rnorm(15,30,5),0)) 

# blocks (factors) to be used for splitting 
b <- mydf$blocks 

# 1 - split-lapply-unsplit with anonimous function to return a new df 
s <- split(mydf, b) 
l<- lapply(s, function(x) data.frame(x, v1mod=mynorm(x$v1))) 
mydf_new <- unsplit(l, mydf$blocks) 

# 2 - split-lapply-unsplit with function trasnform to return a new df 
l <- split(mydf, b) 
l <- lapply(l, transform, v1.mod = mynorm(v1)) 
mydf_new <- unsplit(l, b) 

# 3 - ave() encapsulating split-lapply-unsplit approach 
mydf_new<-transform(mydf, v1.mod = ave(v1, blocks, FUN=mynorm)) 

# 





Da: "William Dunlap" <wdunlap at tibco.com> 
A: "Massimo Bressan" <massimo.bressan at arpa.veneto.it> 
Cc: "David L Carlson" <dcarlson at tamu.edu>, "r-help" <r-help at r-project.org> 
Inviato: Venerdì, 13 maggio 2016 19:22:21 
Oggetto: Re: [R] apply formula over columns by subset of rows in a dataframe (to get a new dataframe) 

ave() encapsulates the split/lapply/unsplit stuff so 
transform(mydf, v1.mod = ave(v1, blocks, FUN=mynorm)) 
also gives what you got above. 

Bill Dunlap 
TIBCO Software 
wdunlap tibco.com 

On Fri, May 13, 2016 at 7:44 AM, Massimo Bressan < massimo.bressan at arpa.veneto.it > wrote: 


yes, thanks 

you pointed me in the right direction: split/unplist was the trick 

I completely left behind that possibility! 

here the final version 

############ 

mynorm <- function(x) {(x - min(x, na.rm=TRUE))/(max(x, na.rm=TRUE) - min(x, na.rm=TRUE))} 

mydf<-data.frame(blocks=rep(c("a","b","c"),each=5), v1=round(runif(15,10,25),0), v2=round(rnorm(15,30,5),0)) 

g <- mydf$blocks 
l <- split(mydf, g) 
l <- lapply(l, transform, v1.mod = mynorm(v1)) 
mydf_new <- unsplit(l, g) 

############ 

thanks again 

massimo 

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Massimo Bressan 

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