Is there another way to transform a string of characters into a function other than the following?
Example: x+2
f <- function(x) {
eval( parse(text=("x+2")) )
}
print( f(2) )
4
I do not find an alternative way.
Thanks you
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1If you provide more context it's possible people could come up with some solutions that would fit. E.g., why doesn't this suit your needs?zack– zack2018-06-15 02:25:15 +00:00Commented Jun 15, 2018 at 2:25
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4I have problems in shiny. I am transforming a text into a function: f <- function(x){eval(parse ( text=( input$text ))) },but when I execute the code it takes time to do the calculations. If instead of transforming the "text into function", I write the function directly, then I have no problems.albert– albert2018-06-15 02:30:04 +00:00Commented Jun 15, 2018 at 2:30
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2 Answers
How about creating the function by pasting your string in as the body, then evaluating the whole thing?
str <- "x+2"
f <- eval(parse(text=paste("function(x) {",str,"}")))
f(2)
## 4
? That way the evaluation cost is paid once, up front, rather than every time the function is called ...
Another possibility is to create a function of x with an empty body, then fill in the parsed expression as the body ...
f <- function(x) {}
body(f) <- parse(text="x+2")
f(2)
By the way, I hope you're being careful about what strings you evaluate, e.g. what happens if str is something like "system('rm -rf /')" ... ?
Comments
I might have totally missed your point, but here's what I've got for you:
f1 <- function(x="x") {
a <- as.expression(paste0(x,"+2"))
print(class(a))
return( eval(parse(text=a)) )
}
f1(2)
4