Using assign with mapply
The take home message that you should be learning from your struggles is to "Not Use The 'assign' Function!" and "Do Not Use Global Variables Like This". R has lists (and environments) that make working with objects that are associated with each other much simpler and fits better with the functional programming style of R. For example you can create a list from your data frame quickly and easily with code like: mydata <- as.list(kkk$vals) names(mydata) <- kkk$vars or mydata <- setNames( as.list(kkk$vals), kkk$vars ) then you will have you variables (with names) inside the list (mydata in this example, but name it whatever you want) This list can then be passed out of a function or otherwise used. To access a specific variable by name you can do: mydata$var1 or mydata[['var2']] or with(mydata, var3) but you can also do things like (and this is often a follow-up question to questions like yours): varname <- 'var3' mydata[[ varname ]] and you can also use lapply and sapply to do the same action on every variable in your list: sapply( mydata, function(x) x + 5 ) instead of having to loop through a bunch of global variables. And if you want to save or delete these, now you just save or delete the entire list rather than having to loop through the set of global variables. If you tell us more about how you want to use these variables we can give more suggestions, but the main point is that in the long run you will be happier learning to use lists (and possibly environments) in place of trying to create and work with global variables like you asked about. On Mon, Dec 16, 2013 at 8:55 AM, Julio Sergio Santana
<juliosergio at gmail.com> wrote:
Julio Sergio Santana <juliosergio <at> gmail.com> writes:
I have a data frame whose first colum contains the names of the variables
and whose second colum contains the values to assign to them:
: kkk <- data.frame(vars=c("var1", "var2", "var3"),
vals=c(10, 20, 30), stringsAsFactors=F)
For those interested in the problem this is how I solved the problem:
I want to have something similar to:
#
# var1 <- 10
# var2 <- 20
# var3 <- 30
my first trial was:
mapply(assign, kkk$vars, kkk$vals)
## var1 var2 var3
## 10 20 30
#
This is, however, what I got:
var1
## Error: object 'var1' not found
David Winsemius suggested me something similar to
mapply(assign, kkk$vars, kkk$vals, MoreArgs = list(pos = 1))
# or:
mapply(assign, kkk$vars, kkk$vals, MoreArgs = list(envir = .GlobalEnv))
var1
## [1] 10
This almost works, but what if this construction is used inside a function?
example <- function () {
var1 <- 250
kkk <- data.frame(vars=c("var1", "var2", "var3"),
vals=c(10, 20, 30), stringsAsFactors=F)
mapply(assign, kkk$vars, kkk$vals, MoreArgs = list(pos = 1))
print (var2)
print (var1)
}
example()
## [1] 20
## [1] 250
var1, which was defined inside the function, isn't modified
To fix this, I defined the function as follows:
example <- function () {
var1 <- 250
kkk <- data.frame(vars=c("var1", "var2", "var3"),
vals=c(10, 20, 30), stringsAsFactors=F)
mapply(assign, kkk$vars, kkk$vals,
MoreArgs = list(pos = sys.frame(sys.nframe())))
# sys.nframe() is the number of the frame created inside the function
# and sys.frame() establishes it as the one assign uses to set values
print (var2)
print (var1)
}
example()
## [1] 20
## [1] 10
And the purpose is got
Thanks,
-Sergio.
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