[R] Using a function with apply Error: undefined columns selected

[Jim Lemon]

Hi John,
First, apply isn't guaranteed to work on data frames. There are two
easy ways to do something like this, but we had better have a data
frame:

guppy<-data.frame(taste=rnorm(10,5),
 crunch=rnorm(10,5),satiety=rnorm(10,5))

If you just want to apply a function to all or a subset of columns of
a data frame, a for loop can be used:

fract2.1<-function(col,data) {
 p<-sum(data[,col],na.rm=TRUE)/sum(!is.na(data[,col]))
 return(p)
}
for(col in 1:ncol(guppy)) print(fract2.1(col,guppy))

If you really do want to use an "*apply" function, then the function
has to be written for each column, not the entire data frame:

fract2.2<-function(x) return(sum(x,na.rm=TRUE)/sum(!is.na(x)))
sapply(guppy,fract2.2)

and if you want a subset of the columns, you will have to do it before
you let sapply get into it.

Jim



On Fri, Apr 8, 2016 at 8:39 AM, John Sorkin <jsorkin at grecc.umaryland.edu> wrote:
> I am trying to write a function that can be used to apply to process all the columns of a data.frame. If you will run the code below, you will get the error message undefined columns selected. I hope someone will be able to teach me what I am doing wrong.
> Thank you,
> John
>
> # create data frame.
> guppy
>
> fract2 <- function(col,data) {
>   cat("Prove we have passed the data frame\n")
>   print(data)
>
>   # Get the name of the column being processed.
>   zz<-deparse(substitute(col))
>   cat("Column being processed\n")
>   print(zz)
>   p<-sum(data[,zz]!="")/length(data[,zz])
>   return(p)
> }
>
> apply(guppy,2,fract2,data=guppy)
> John David Sorkin M.D., Ph.D.
> Professor of Medicine
> Chief, Biostatistics and Informatics
> University of Maryland School of Medicine Division of Gerontology and Geriatric Medicine
> Baltimore VA Medical Center
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