R: user interactively supplies mathmatical function

Question

I am developing an R script that numerically solves and plots 2 person simultaneous move non-cooperative games, where the user interactively supplies the player's payoff functions. Eventually this will be a shiny app, but for now I am using readline(). I am just wondering if there is a better way to have the user supply the payoff functions?

user_input <- readline(prompt="enter a function of x and y (e.g. x^2+y^2-x*y): ")
func_text <- paste0("as.function(alist(x=,y=,",user_input,"))")
the_func <- eval(parse(text=func_text))
print(paste0("the value of your function at (x,y)=(10,10) is: ",the_func(10,10)))

Bonus: how could I test that the supplied string-function is valid before eval?

I would give serious consideration to choosing a different way to get and parse user input. What's to stop the user inputting a line that wipes your hard drive? — Allan Cameron, Nov 25 '20 at 22:42
@AllanCameron was going to say the same thing. It could be a (very) bad day.. — stevec, Nov 25 '20 at 22:42
@AllanCameron perhaps you should not put the code there, but say what it does. If someone runs it trying to find out, that would be very bad — stevec, Nov 25 '20 at 22:43

Abdessabour Mtk · Accepted Answer · 2020-11-25T22:50:40.530

2

changing the function definition to this solves the issue

user_input <- "j"
while(!grepl("^([xy+/*-]|\\d)+$", user_input)) user_input <- readline(prompt="enter a function of x and y (e.g. x^2+y^2-x*y): ")
the_func <-  eval(parse(text=paste0("function(x,y) ",user_input)))

Following the comments by @AllanCameron and @stevec and the issues they raised it seems that input validation is a must that's what the while loop is there for.

edited Nov 25 '20 at 22:50

answered Nov 25 '20 at 22:41

Abdessabour Mtk

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I added ```\\^\$\$``` to allow for polynomial functions. Cheers, – Richard Martin Nov 25 '20 at 23:49
note that in R `**` <=> `^` , I'd suggest using this regex then `^([xy+/*^()-]|\\d)+$` @RichardMartin – Abdessabour Mtk Nov 26 '20 at 01:08

R: user interactively supplies mathmatical function

1 Answers1