[R] "evaluating expressions" contained in a dataframe

[Rui Barradas]

Hello,

I've changed the way you create data.frame 'tests', like it was the 
conditions were factors, which are coded as integers. We need them of 
class character to be parsed. Note that the same could be done with the 
data.frame 'info' but it's not absolutely needed.


tests <- data.frame(rule=c("info$country == 'Greenland'", "info$age > 50"),
	stringsAsFactors=FALSE)
str(tests)
tests

#------------------ This does the trick
fun <- function(x){
	f <- function(){}  # an empty function, no body

	force(x)                 # evaluate the argument
	body(f) <- parse(text=x) # parse it and assign the function
	f  # return the function
}

# See if it works
expr1 <- fun(tests$rule[1])  # This creates a function
expr1()


Hope this helps,

Rui Barradas

Em 03-07-2012 17:24, New RUser escreveu:
> #I have a dataframe called "tests" that contain "character expressions".  These
> characters are rules that use data from within another dataframe.  Is there
> any way within R I can access the rules in the dataframe called tests, and
> then "evaluate" these "rules"?
>
>
>
> #An example may better explain what I am trying to accomplish:
>
>
>
> tests <- data.frame(matrix(data=c("info$country == 'Greenland'", "info$age
>> 50"), nrow=2, ncol=1))
>
> names(tests) <- "rule"
>
>
>
> info <- data.frame(matrix(data=NA, nrow=5, ncol=3))
>
> names(info) <- c("first", "country", "age")
>
> info$name <- c("Mary", "Paul", "Robert", "John", "Ivan")
>
> info$country <- c("GReenland", "Iceland", "Ireland", "Greenland",
> "Greenland")
>
> info$age <- c(30, 55, 66, 79, 80)
>
>
>
> #e.g. for
>
> "info$country == 'Greenland'"
>
>
>
> #I want:
>
> info$country == 'Greenland'
>
> [1] FALSE FALSE FALSE  TRUE  TRUE
>
> #e.g.
> info$country == 'Greenland'
>
> info$age > 50
>
>
>
> #I tried this, but it does not "work":
>
> eval(tests$rule[1])
>
> 	[[alternative HTML version deleted]]
>
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