R: eval(parse()) error message: cannot open file even though "text=" is specified in parse

Question

I'm running analysis for a list of countries several times and during each iteration the result should be added to a vector. Below I show a simplified example without the loop for just one country. Even though I thoroughly looked for solutions, I could not find an answer.

#this is my simplified country vector with just 1 country country<-c("Spain") #This is the vector that should hold the results of multiple iterations #for now, it contains only the result of the first iteration Spain.condition1<- 10 #reading result vector in a variable (this is automized in a loop in my code) resultVector<-paste(country,"condition1",sep=".") #when I call the content of the vector with parse, eval #I see the content of the vector as expected eval(parse(text=resultVector)) #however, when I try to add a second result to it eval(parse(text=resultVector))[2]<-2 #I get following error message: #Error in file(filename, "r") : cannot open the connection #In addition: Warning message: #In file(filename, "r") : # cannot open file 'Spain.condition1': No such file or directory

Could anyone help me or put me in the right direction?

See also: stackoverflow.com/q/2679193/892313

Brian Diggs
– Brian Diggs

2012-07-23 15:58:23 +00:00
Commented Jul 23, 2012 at 15:58 — Brian Diggs
– Brian Diggs, Commented Jul 23, 2012 at 15:58

David Robinson · Accepted Answer · 2012-05-16 03:00:45Z

Assigning to eval isn't guaranteed to work. This is one of multiple reasons it's usually not a good idea to use eval.

Why not just store countries and their conditions in a named list, something like this:

conditions = list() conditions[["Spain"]] = list() conditions[["Spain"]][["condition1"]] <- 10 conditions[["Spain"]][["condition1"]][2] <- 2 conditions[["Spain"]][["condition1"]] # [1] 10 2

ETA: To work with a loop (I don't know precisely what the structure of your problem is, but here's the general idea):

countries = c("Spain", "England", "France", "Germany", "USA") # and so on conditions = c("Sunny", "Rainy", "Snowing") # or something data = list() for (country in countries) { data[[country]] <- list() for (condition in conditions) { data[[country]][[condition]] <- 4 # assign appropriate value here } }

It can also be constructed from a tab-delimited file, or generated in whatever way is appropriate for your problem- R is more than capable.

Thanks! This is a nice solution, however, I have 16 countries and at least 3 conditions, so I would have to add at least 48 lines of code to incorporate your solution. I'm transitioning from SAS to R and in SAS this could be done very elegantly with a macro variable loop.
Oh, this can be done very easily with a loop. I'm adding an explanation to my answer.

Dason · Accepted Answer · 2012-05-16 03:16:31Z

David's solution is a lot better but you could do this using get and assign.

country <- "Spain" Spain.condition1 <- 10 resultVector <- paste(country, "condition1", sep=".") eval(parse(text=resultVector)) #[1] 10 # Now this is one way to modify that object # Note that we *need* to assign to a temporary object # and just using get(resultVector)[2] <- 2 won't work tmp <- get(resultVector) tmp[2] <- 2 assign(resultVector, tmp) Spain.condition1 #[1] 10 2 # We could alternatively do this with eval # Even if it is a bad idea eval(parse(text = paste0(resultVector, "[2] <- 3"))) Spain.condition1 #[1] 10 3

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