[R] list to dataframe conversion-testing for identical

[David L Carlson]

Yes it does have something to do with the representation of floating point
numbers. Using cbind() forces the list to become a matrix and that forces
all of the data to become character strings since one of the list elements
is character:

> set.seed(42)
> listdat1<-list(rnorm(10,20),rep(LETTERS[1:2],5),rep(1:5,2))
> str(do.call("cbind", listdat1))
 chr [1:10, 1:3] "21.3709584471467" "19.4353018286039" ...
Then you convert that to a data.frame. The default in data.frame() is to
convert characters to factors so you get 

> str(data.frame(do.call("cbind",listdat1)))
'data.frame':   10 obs. of  3 variables:
 $ X1: Factor w/ 10 levels "19.4353018286039",..: 8 1 5 7 6 2 9 3 10 4
 $ X2: Factor w/ 2 levels "A","B": 1 2 1 2 1 2 1 2 1 2
 $ X3: Factor w/ 5 levels "1","2","3","4",..: 1 2 3 4 5 1 2 3 4 5

With dat2 you used data.frame() so the numeric fields were not converted to
strings and then factors. Then you converted the dat1 factors back to
numeric. You would be fine with just

> dat1 <- data.frame(listdat1)
> colnames(dat1) <- paste0("Var", 1:3)

Or you can name the list elements and then convert

> names(listdat1) <- paste0("Var", 1:3)
> dat1 <- data.frame(listdat1)

----------------------------------------------
David L Carlson
Associate Professor of Anthropology
Texas A&M University
College Station, TX 77843-4352


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of arun
> Sent: Sunday, July 01, 2012 12:56 PM
> To: R help
> Subject: [R] list to dataframe conversion-testing for identical
> 
> HI R help,
> 
> I was trying to get identical data frame from a list using two methods.
> 
> #Suppose my list is:
> listdat1<-list(rnorm(10,20),rep(LETTERS[1:2],5),rep(1:5,2))
> #Creating dataframe using cbind
> 
> dat1<-data.frame(do.call("cbind",listdat1))
> colnames(dat1)<-c("Var1","Var2","Var3")
> #Second dataframe conversion
> 
> dat2<-
> data.frame(Var1=listdat1[[1]],Var2=listdat1[[2]],Var3=listdat1[[3]])
> 
> #Structure is different in two datasets
>  >str(dat1)
> 'data.frame':    10 obs. of  3 variables:
>  $ Var1: Factor w/ 10 levels "18.6153321029756",..: 5 2 6 8 7 9 1 4 3
> 10
>  $ Var2: Factor w/ 2 levels "A","B": 1 2 1 2 1 2 1 2 1 2
>  $ Var3: Factor w/ 5 levels "1","2","3","4",..: 1 2 3 4 5 1 2 3 4 5
> > str(dat2)
> 'data.frame':    10 obs. of  3 variables:
>  $ Var1: num  20.3 19.2 20.5 20.9 20.5 ...
>  $ Var2: Factor w/ 2 levels "A","B": 1 2 1 2 1 2 1 2 1 2
>  $ Var3: int  1 2 3 4 5 1 2 3 4 5
> 
> #Converting structure of dat1 to match da2 structure
> dat1<-within(dat1,{Var1<-as.numeric(as.character(Var1))
>     Var3<-as.integer(Var3)})
> 
> head(dat1)
>       Var1 Var2 Var3
> 1 20.27193    A    1
> 2 19.17586    B    2
> 3 20.53197    A    3
> 4 20.93615    B    4
> 5 20.53498    A    5
> 6 21.02044    B    1
> > head(dat2)
>       Var1 Var2 Var3
> 1 20.27193    A    1
> 2 19.17586    B    2
> 3 20.53197    A    3
> 4 20.93615    B    4
> 5 20.53498    A    5
> 6 21.02044    B    1
> 
> 
> #New structure identical(str(dat1),str(dat2))
> 'data.frame':    10 obs. of  3 variables:
>  $ Var1: num  19.9 19 21.2 20.7 20.4 ...
>  $ Var2: Factor w/ 2 levels "A","B": 1 2 1 2 1 2 1 2 1 2
>  $ Var3: int  1 2 3 4 5 1 2 3 4 5
> 'data.frame':    10 obs. of  3 variables:
>  $ Var1: num  19.9 19 21.2 20.7 20.4 ...
>  $ Var2: Factor w/ 2 levels "A","B": 1 2 1 2 1 2 1 2 1 2
>  $ Var3: int  1 2 3 4 5 1 2 3 4 5
> [1] TRUE
> 
> 
> 
> #structure is identical and dataframe looks to be same, but it is not
> identical.
> > identical(dat1,dat2)
> [1] FALSE
> 
> 
> Is it something to do with the floating point?
> 
> Thanks,
> 
> A.K.
> 
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