[R] Preserving lists in a function
Don MacQueen
macq at llnl.gov
Sat Feb 27 01:51:12 CET 2010
Barry explained your first puzzle, but let me add some explanation
and examples.
> tmpfun <- function( a =3 ) {a}
> tmpfun()
[1] 3
> tmpfun(a='x')
[1] "x"
Inside the function, the value of the argument is whatever the user
supplied. The default is replaced by what the user supplies. There is
no mechanism for retaining the default structure and filling in any
missing parts. R never preserves the defaults when the user supplies
something other than the default.
For example, and using your function,
> myfunction(list1='x')
$list1
[1] "x"
$list2
$list2$variable1
[1] "variable1"
$list2$variable2
[1] "variable2"
$list2$variable3
[1] "variable3"
$list3
$list3$variable1
[1] "character"
$list3$variable2
[1] 24
$list3$variable3
[1] 0.1 0.1 0.1 0.1
$list3$variable4
[1] TRUE
> myfunction(list1=data.frame(a=1:2, b=c('x','y')))
$list1
a b
1 1 x
2 2 y
$list2
$list2$variable1
[1] "variable1"
$list2$variable2
[1] "variable2"
$list2$variable3
[1] "variable3"
$list3
$list3$variable1
[1] "character"
$list3$variable2
[1] 24
$list3$variable3
[1] 0.1 0.1 0.1 0.1
$list3$variable4
[1] TRUE
What you put in is what you get out.
I don't know that I would deal with this the way Barry did. I would
probably write code to examine the structure of what the user
supplies, compare it to the required structure, and then fill in.
myf <- function(l1, l2, l3) {
if (missing(l1)) {
## user did not supply l1, so set it = to the default
l1 <- list(v1=1, v2=2, v3=3)
} else if (!is.list(l1)) {
## user must supply a list, if not, it's an error
stop('l1 must be a list')
} else {
## user has at least supplied a list
## now write code to check the names of the list that the user supplied
## make sure the names that the user supplied are valid, if not, stop()
## if the user supplied too few elements, fill in the missing ones
## if the user supplied too many elements stop()
## if the user supplied all the correct elements, with all the
correct names, use what the user supplied
}
Looks complicated; maybe Barry's way is better...
-Don
At 5:56 PM -0500 2/26/10, Shang Gao wrote:
>Dear R users,
>
>A co-worker and I are writing a function to facilitate graph
>plotting in R. The function makes use of a lot of lists in its
>defaults.
>
>However, we discovered that R does not necessarily preserve the
>defaults if we were to input them in the form of list() when
>initializing the function. For example, if you feed the function
>codes below into R:
>
>myfunction=function(
> list1=list (variable1=1,
> variable2=2,
> variable3=3),
>
> list2=list (variable1="variable1",
> variable2="variable2",
> variable3="variable3"),
>
> list3=list (variable1="character",
> variable2=24,
> variable3=c(0.1,0.1,0.1,0.1),
> variable4=TRUE))
>
>{return(list(list1=list1,list2=list2,list3=list3))}
>
>By definition, the values associated with each variable in the lists
>would be the default unless the user impute a different value while
>executing the function. But a problem arises when a variable in the
>list is left out completely (not imputed at all). An example is
>shown below:
>
>myfunction( list1=list (variable1=1,
> variable2=2), #variable 3 deliberately left out
>
> list2=list (variable1="variable1",
> variable3="position changed",
> variable2="variable2"),
>
> list3=list (variable1="character",
> variable2=24,
> variable4=FALSE)) #variable 3 deliberately left out
>
>#The outcome of the above execution is shown below:
>
>$list1
>$list1$variable1
>[1] 1
>
>$list1$variable2
>[1] 2
>#list1$variable3 is missing. Defaults in function not assigned in
>this execution
>
>$list2
>$list2$variable1
>[1] "variable1"
>
>$list2$variable3
>[1] "position changed"
>
>$list2$variable2
>[1] "variable2"
>
>
>$list3
>$list3$variable1
>[1] "character"
>
>$list3$variable2
>[1] 24
>
>$list3$variable4
>[1] FALSE
>#list3$variable3 is missing. Defaults in function not assigned in
>this execution
>
>We later realized that the problem lies in list() commands. Hence,
>we tried to enforce the defaults on the list using these codes in
>the function definition:
>
>myfunction.alternative=function(
> list1=list (variable1=1,
> variable2=2,
> variable3=3),
>
> list2=list (variable1="variable1",
> variable2="variable2",
> variable3="variable3"),
>
> list3=list (variable1="character",
> variable2=24,
> variable3=c(0.1,0.1,0.1,0.1),
> variable4=TRUE))
>{
>defaults=vector("list", 3)
>names(defaults)=c("list1","list2","list3")
>defaults$list1=list(variable1=1,
> variable2=2,
> variable3=3)
>defaults$list2=list(variable1="variable1",
> variable2="variable2",
> variable3="variable3")
>defaults$list3=list (variable1="character",
> variable2=24,
> variable3=c(0.1,0.1,0.1,0.1),
> variable4=TRUE)
>if(length(list1$variable1)==0){list1$variable1=defaults$list1$variable1}
>if(length(list1$variable2)==0){list1$variable2=defaults$list1$variable2}
>if(length(list1$variable3)==0){list1$variable3=defaults$list1$variable3}
>
>if(length(list2$variable1)==0){list2$variable1=defaults$list2$variable1}
>if(length(list2$variable2)==0){list2$variable2=defaults$list2$variable2}
>if(length(list2$variable3)==0){list2$variable3=defaults$list2$variable3}
>
>if(length(list3$variable1)==0){list3$variable1=defaults$list3$variable1}
>if(length(list3$variable2)==0){list3$variable2=defaults$list3$variable2}
>if(length(list3$variable3)==0){list3$variable3=defaults$list3$variable3}
>if(length(list3$variable4)==0){list3$variable4=defaults$list3$variable4}
>
>return(list(list1=list1,list2=list2,list3=list3))}
>
>The outcome of execution the above function with the same commands
>produces the results that we wanted:
>> myfunction.alternative( list1=list (variable1=1,
>+ variable2=2), #variable 3 deliberately left out
>+
>+ list2=list (variable1="variable1",
>+ variable3="position changed",
>+ variable2="variable2"),
>+
>+ list3=list (variable1="character",
>+ variable2=24,
>+ variable4=FALSE)) #variable 3 deliberately left out
>$list1
>$list1$variable1
>[1] 1
>
>$list1$variable2
>[1] 2
>
>$list1$variable3
>[1] 3
> #list1$variable3 is assigned default despite being left out in the
>execution command
>
>
>$list2
>$list2$variable1
>[1] "variable1"
>
>$list2$variable3
>[1] "position changed"
>
>$list2$variable2
>[1] "variable2"
>
>
>$list3
>$list3$variable1
>[1] "character"
>
>$list3$variable2
>[1] 24
>
>$list3$variable4
>[1] FALSE
>
>$list3$variable3
>[1] 0.1 0.1 0.1 0.1
> #list3$variable3 is assigned default despite being left out in the
>execution command
>
>Even though the function works, as you can see, the codes that
>enforce the defaults are very long and bulky. Such lengthy codes
>won't be efficient if we have a write a function containing a large
>number of lists. We tried to come up with ideas to try to shorten
>the codes, but so far none of them prove to be effective.
>
>What would be your recommendation to deal with such situation? It
>would be great if you would be able to help us our with this
>problem. We appreciate your help tremendously.
>
>Thank you.
>
>Sincerely,
>Shang
>
>
> [[alternative HTML version deleted]]
>
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>and provide commented, minimal, self-contained, reproducible code.
--
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
925-423-1062
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