R: How can I make a function with a function input with data.table

Question

I faced a problem when exercising with data.table. Here is my problem. I wrote a simple subtraction function:

minus <- function(a, b){
      return(a - b)
  }

My dataset is a simple data.table:

dt <- as.data.table(data.frame(first=c(5, 6, 7), second=c(1,2,3)))
dt
  first second
1     5      1
2     6      2
3     7      3

I would like to write another function,

myFunc <- function(dt, FUN, ...){
      return(dt[, new := FUN(...)])
  }

The usage is simply:

res <- myFunc(dt, minus, first, second)

and the result would be the following:

res
   first second new
1:     5      1   4
2:     6      2   4
3:     7      3   4

How can I archive such a goal? Thanks!

A5C1D2H2I1M1N2O1R2T1 · Accepted Answer · 2015-11-25 08:39:18Z

2

Maybe there's a better way, but you can try something like this:

myFunc <- function(indt, FUN, ...) {
  FUN <- deparse(substitute(FUN))    # Get FUN as a string
  FUN <- match.fun(FUN)              # Match it to an existing function
  dots <- substitute(list(...))[-1]  # Get the rest of the stuff
  # I've used `copy(indt)` so that it doesn't affect your original dataset
  copy(indt)[, new := Reduce(FUN, mget(sapply(dots, deparse)))][]
}

(Note that this is very specific to how you've created your minus() function.)

Here it is in action:

res <- myFunc(dt, minus, first, second)
dt  ## Unchanged
#    first second
# 1:     5      1
# 2:     6      2
# 3:     7      3

res
#    first second new
# 1:     5      1   4
# 2:     6      2   4
# 3:     7      3   4

edited Nov 25, 2015 at 8:39

answered Nov 25, 2015 at 8:33

A5C1D2H2I1M1N2O1R2T1

194k31 gold badges417 silver badges497 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

morningfin Over a year ago

Thank you! What if I would like to change the new column name based on input?

A5C1D2H2I1M1N2O1R2T1 Over a year ago

@Nal-rA, maybe use setnames?

morningfin Over a year ago

Yeah, that is doable. Thank you again!

morningfin Over a year ago

What if my minus function change to minus <- function(a, b, c)? Your solution does not seem to work.

A5C1D2H2I1M1N2O1R2T1 Over a year ago

You might need something with do.call instead of Reduce. Not in front of my computer right now...

|

wasyl · Accepted Answer · 2015-11-27 13:11:18Z

0

Here is a solution with do.call:

myFunc <- function(dt, FUN, ...){
  arg.names <- as.character(match.call()[-(1:3)])
  copy(dt)[, "new" := do.call(FUN, lapply(arg.names, function(x) get(x)))]
}
#test
myFunc(dt, minus, first, second)
#   first second new
#1:     5      1   4
#2:     6      2   4
#3:     7      3   4

answered Nov 27, 2015 at 13:11

wasyl

655 bronze badges

Collectives™ on Stack Overflow

R: How can I make a function with a function input with data.table

2 Answers 2

8 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

8 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related