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

You can do this with split/unsplit:

mydf.split <- split(mydf, mydf$blocks)
str(mydf.split)

List of 3
 $ a:'data.frame':      5 obs. of  3 variables:
  ..$ blocks: Factor w/ 3 levels "a","b","c": 1 1 1 1 1
  ..$ v1    : num [1:5] 19 15 17 22 16
  ..$ v2    : num [1:5] 35 31 35 31 39
 $ b:'data.frame':      5 obs. of  3 variables:
  ..$ blocks: Factor w/ 3 levels "a","b","c": 2 2 2 2 2
  ..$ v1    : num [1:5] 12 24 25 22 18
  ..$ v2    : num [1:5] 31 19 35 32 38
 $ c:'data.frame':      5 obs. of  3 variables:
  ..$ blocks: Factor w/ 3 levels "a","b","c": 3 3 3 3 3
  ..$ v1    : num [1:5] 17 14 21 21 22
  ..$ v2    : num [1:5] 27 25 23 23 27

mydf.split2 <- lapply(mydf.split, function(x) data.frame(x, 

+      v1mod=mynorm(x$v1)))

str(mydf.split2)

List of 3
 $ a:'data.frame':      5 obs. of  4 variables:
  ..$ blocks: Factor w/ 3 levels "a","b","c": 1 1 1 1 1
  ..$ v1    : num [1:5] 19 15 17 22 16
  ..$ v2    : num [1:5] 35 31 35 31 39
  ..$ v1mod : num [1:5] 0.571 0 0.286 1 0.143
 $ b:'data.frame':      5 obs. of  4 variables:
  ..$ blocks: Factor w/ 3 levels "a","b","c": 2 2 2 2 2
  ..$ v1    : num [1:5] 12 24 25 22 18
  ..$ v2    : num [1:5] 31 19 35 32 38
  ..$ v1mod : num [1:5] 0 0.923 1 0.769 0.462
 $ c:'data.frame':      5 obs. of  4 variables:
  ..$ blocks: Factor w/ 3 levels "a","b","c": 3 3 3 3 3
  ..$ v1    : num [1:5] 17 14 21 21 22
  ..$ v2    : num [1:5] 27 25 23 23 27
  ..$ v1mod : num [1:5] 0.375 0 0.875 0.875 1

mydf2 <- unsplit(mydf.split2, mydf$blocks)
str(mydf2)

'data.frame':   15 obs. of  4 variables:
 $ blocks: Factor w/ 3 levels "a","b","c": 1 1 1 1 1 2 2 2 2 2 ...
 $ v1    : num  19 15 17 22 16 12 24 25 22 18 ...
 $ v2    : num  35 31 35 31 39 31 19 35 32 38 ...
 $ v1mod : num  0.571 0 0.286 1 0.143 ...

-------------------------------------
David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77840-4352

-----Original Message-----
From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Massimo Bressan
Sent: Friday, May 13, 2016 6:56 AM
To: r-help at r-project.org
Subject: [R] apply formula over columns by subset of rows in a dataframe (to get a new dataframe)

hi 

I need to apply a user defined formula over some selected columns of a dataframe by subsetting group of rows (blocks) and get back a new dataframe 

I?ve been managed to get the the calculations right but I?m not satisfied at all by the form of the results 

please refer to my reproducible example 

########## 
# 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)) 


#my attempts (not satisfied by final output) 

tapply(mydf$v1, mydf$blocks, mynorm) 

byf<-factor(mydf$blocks) 
aggregate(mydf[2:3], list(byf), mynorm) 
aggregate(mydf[2:3], list(mydf$blocks), mynorm, simplify = FALSE) 

########### 

please can anyone give me some hints on how to properly proceed? 

I need a dataframe with all variables as final result 
sorry but I?m sort of definitely stuck with this? 

thanks 



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