R help - Dec 2013 - Using assign with mapply

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)

If I do 

   : assign(kkk$vars[1], kkk$vals[1])

it works

   : var1
   [1] 10

However, if I try with mapply this is what I get:

   : mapply(assign, kkk$vars, kkk$vals)
   var1 var2 var3 
     10   20   30 
   : var2
   Error: object 'var2' not found

Maybe I have not undestand how mapply and assign work. Do you have
any comments?

Thanks,

  -Sergio.

On Dec 6, 2013, at 11:27 AM, Julio Sergio Santana wrote:
> 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)
> 
> If I do 
> 
>   : assign(kkk$vars[1], kkk$vals[1])
> 
> it works
> 
>   : var1
>   [1] 10
> 
> However, if I try with mapply this is what I get:
> 
>   : mapply(assign, kkk$vars, kkk$vals)
>   var1 var2 var3 
>     10   20   30 
>   : var2
>   Error: object 'var2' not found
> 
> Maybe I have not undestand how mapply and assign work. Do you have
> any comments?
I think you will find that the value returned from the mapply call was a three
element list with the desired names and values  ... except you then gave that
enclosing list no name and it will be garbage-collected. If you want to have
'assign' do its magic into the global environment, then you need to
supply 'mapply' a MoreArgs argument on the other side of the ellipsis:

Usage:
mapply(FUN, ..., MoreArgs = NULL, SIMPLIFY = TRUE,
       USE.NAMES = TRUE)

So what happens if you try this:

mapply(assign,  kkk$vars, kkk$vals, MoreArgs = list(envir = .GlobalEnv)

-- 

David Winsemius
Alameda, CA, USA

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.