R: how to get the value or object from a name in a custom function?-CodePudding

I am trying to create a custom function for batch analysis, at the beginning it is like:

> myfunction <- function(DATA, col1, col2, col3){
>   print(class(col3))  #"name"
>   print(is.object(col3))  #FALSE
>   library(plyr) 
>   output <- ddply(DATA, .(eval(col1), eval(col2)), summarize, N=sum(eval(col3)),...)
>   ...
> }

> myfunction(DATA=df, col1=quote(colA), col2=quote(colB), col3=quote(colC))  #colA[chr], colB[chr], colC[numeric] are column names in dataframe df

but this is what I get:

> Error in eval("col3") : object 'col3' not found
> 10. eval("col3")
> 9. eval(cols[[col]], .data, parent.frame())
> 8. eval(cols[[col]], .data, parent.frame())
> 7. .fun(piece, ...)
> 6. (function (i)  { piece <- pieces[[i]] if (.inform) { ...
> 5. loop_apply(n, do.ply)
> 4. llply(.data = .data, .fun = .fun, ..., .progress = .progress,  .inform = .inform, .parallel = .parallel, .paropts = .paropts)
> 3. ldply(.data = pieces, .fun = .fun, ..., .progress = .progress,  .inform = .inform, .parallel = .parallel, .paropts = .paropts)
> 2. ddply(DATA, .(eval(col1), eval(col2)), summarize, N = sum(eval(col3)))
> 1. myfunction(DATA = df, col1 = quote(colA), col2= quote(colB),  col3 = quote(colC))

I dont understand why error comes up until col3 while nothing goes wrong with col1 and col2.

As class(col3) in the custom function show me col3 is a "name", I replace the eval() with get() but it doesn't work.

Can anyone tell me how to get the object behind the name col3?

Or am I in the wrong way from the start and need to change my mindset allover?

CodePudding user response：

It may be easier to use dplyr instead of plyr. Here, is one way to change the function

myfunction <- function(DATA, col1, col2, col3){

   
    plyr::ddply(DATA, c(col1, col2),
         .fun = function(.data) c(N = sum(.data[[col3]], na.rm = TRUE)))

 }

-testing

> myfunction(mtcars, "cyl", "vs", "mpg")
 cyl vs     N
1   4  0  26.0
2   4  1 267.3
3   6  0  61.7
4   6  1  76.5
5   8  0 211.4
# outside the function
> plyr::ddply(mtcars, c("cyl", "vs"), summarize, N = sum(mpg))
  cyl vs     N
1   4  0  26.0
2   4  1 267.3
3   6  0  61.7
4   6  1  76.5
5   8  0 211.4