[R] passing (or obtaining) index or element name of list to FUN in lapply()
Charles C. Berry
cberry at tajo.ucsd.edu
Wed Jun 13 23:18:43 CEST 2007
This sounds like a job for mapply, viz:
> a.list <- list(a=cbind(1:4,rnorm(4)),b=cbind(4:1,rnorm(4)))
>
> plot.x.main.y <- function(x,y,...) plot( x, main=y, ... )
> mapply( plot.x.main.y , a.list, names(a.list) )
Chuck
On Wed, 13 Jun 2007, Stephen Tucker wrote:
> Hi Professor Ripley,
>
> Thanks for the response. I apologize, my examples were not too real (though
> your solutions are indeed clever)... I was trying to ask more generally
> whether the element name or index of 'listObj' could be obtained by the
> user-function 'myfunction' when used in lapply(X=listObj,FUN=myfunction);
> below I illustrate two cases in which I have come across this desire:
> (1) In 'Example 1' I essentially take the list element and do some
> transformations (optionally some number-crunching), and then plot it with the
> element name of the list for the title.
> (2) In 'Example 2' I want to read in data from the list element and write the
> contents to a file; writing a header line only when operating on the first
> element of the list.
>
> ## data specification
> data1 <- "var1 var2
> -0.44 0.17
> 1.03 0.93
> 0.85 0.39"
> data2 <- "var1 var2
> -0.16 0.97
> 0.93 0.23
> 0.80 0.42"
> L <- list(data1=data1,data2=data2)
>
> ##=== Example 1 (want element name) ===
> ## function definition
> plottingfunc <- function(i,x) {
> plot(read.table(textConnection(x[[i]]),header=TRUE),main=names(x)[i])
> }
> ## function application
> par(mfrow=c(2,1))
> lapply(seq(along=L),plottingfunc,x=L)
>
> ##=== Example 2 (want element index) ===
> ## function definition
> readwritefunc <- function(i,x,fout) {
> data <- read.table(textConnection(x[[i]]),header=TRUE)
> if(i==1) cat(paste(colnames(data),collapse=","),"\n",file=fout)
> write.table(data,file=fout,sep=",",col=FALSE,
> row=FALSE,quote=FALSE,append=TRUE)
> }
> ## function application
> fout <- file("out.dat",open="w")
> lapply(seq(along=L),readwritefunc,x=L,fout=fout)
> close(fout)
>
> Since the above code works, I suppose this is more of a question of
> aesthetics since I thought the spirit of lapply() was to operate on the
> elements of a list and not its indices - I thought perhaps there is a way to
> get the index number and element name from within the user-function.
>
> Also, I recall a lesson on 'loop avoidance' from an earlier version of MASS;
> this was in the days of S-PLUS dominance and perhaps less applicable now to R
> as you mentioned... But old habits die hard; my amygdala still invokes a fear
> response at the thought of a loop... (and as of recently, I have been
> infatuated with the notion of adhering, albeit loosely, to the 'functional
> programming' paradigm which makes me doubly fearful of loops)
>
> Thanks and best regards,
>
> Stephen
>
> --- Prof Brian Ripley <ripley at stats.ox.ac.uk> wrote:
>
>> On Tue, 12 Jun 2007, Stephen Tucker wrote:
>>
>>> Hello everyone,
>>>
>>> I wonder if there is a way to pass the index or name of a list to a
>>> user-specified function in lapply(). For instance, my desired effect is
>>> something like the output of
>>>
>>>> L <- list(jack=4098,sape=4139)
>>>> lapply(seq(along=L),function(i,x) if(i==1) "jack" else "sape",x=L)
>>> [[1]]
>>> [1] "jack"
>>>
>>> [[2]]
>>> [1] "sape"
>>
>> as.list(names(L))
>>
>>>> lapply(seq(along=L),function(i,x) if(names(x)[i]=="jack") 1 else 2,x=L)
>>> [[1]]
>>> [1] 1
>>>
>>> [[2]]
>>> [1] 2
>>
>> as.list(seq_along(L))
>>
>> lapply() can be faster than a for-loop, but usually not by much: its main
>> advantage is clarity of code.
>>
>> I think we need a real-life example to see what you are trying to do.
>>
>>> But by passing L as the first argument of lapply(). I thought there was a
>>> tangentially-related post on this mailing list in the past but I don't
>> recall
>>> that it was ever addressed directly (and I can't seem to find it now).
>> The
>>> examples above are perfectly good alternatives especially if I wrap each
>> of
>>> the lines in "names<-"() to return lists with appropriate names assigned,
>> but
>>
>> Try something like
>>
>> L[] <- lapply(seq_along(L),function(i,x) if(i==1) "jack" else "sape",x=L)
>>
>>> it feels like I am essentially writing a FOR-LOOP - though I was
>> surprised to
>>> find that speed-wise, it doesn't seem to make much of a difference
>> (unless I
>>> have not selected a rigorous test):
>>>
>>>> N <- 10000
>>>> y <- runif(N)
>>> ## looping through elements of y
>>>> system.time(lapply(y,
>>> + function(x) {
>>> + set.seed(222)
>>> + mean(rnorm(1e4,x,1))
>>> + }))
>>> [1] 21.00 0.17 21.29 NA NA
>>> ## looping through indices
>>>> system.time(lapply(1:N,
>>> + function(x,y) {
>>> + set.seed(222)
>>> + mean(rnorm(1e4,y[x],1))
>>> + },y=y))
>>> [1] 21.09 0.14 21.26 NA NA
>>>
>>> In Python, there are methods for Lists and Dictionaries called
>> enumerate(),
>>> and iteritems(), respectively. Example applications:
>>>
>>> ## a list
>>> L = ['a','b','c']
>>> [x for x in enumerate(L)]
>>> ## returns index of list along with the list element
>>> [(0, 'a'), (1, 'b'), (2, 'c')]
>>>
>>> ## a dictionary
>>> D = {'jack': 4098, 'sape': 4139}
>>> [x for x in D.iteritems()]
>>> ## returns element key (name) along with element contents
>>> [('sape', 4139), ('jack', 4098)]
>>>
>>> And this is something of the effect I was looking for...
>>>
>>> Thanks to all,
>>>
>>> Stephen
>>>
>>> ______________________________________________
>>> R-help at stat.math.ethz.ch mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>
>> --
>> Brian D. Ripley, ripley at stats.ox.ac.uk
>> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
>> University of Oxford, Tel: +44 1865 272861 (self)
>> 1 South Parks Road, +44 1865 272866 (PA)
>> Oxford OX1 3TG, UK Fax: +44 1865 272595
>>
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
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