[R] how to assign a value to a specific position of a list

[Jeff Newmiller]

The only thing "compelling" about your example is that you have the 
pre-conceived (wrong) notion that you have to store your objects under 
separate names in your global environment. That is not only wrong, it 
handicaps you for future unified processing of these data.

I see a lot of constructs in this code that I would change if there was a 
reproducible example here... but just reading code out of context all I 
feel like doing is replacing your uses of assign.

EEM <- list()
for (i in dir()) {
   EEM[[ i ]] <- list()
   fn <- list.files(path = i, pattern = "ASC")
   tr <- sort(as.numeric(unique(unlist(lapply(strsplit(fn, "-"),"[[",
                                              2)))))
   for (j in 1:length(tr)) {
     fn_tr <- list.files(path = i, pattern = paste(i, tr[j], "...ASC",
                                                   sep="-"))
     EEM_tmp <- matrix(NA,ncol = length(fn_tr),nrow = 371)
     for (k in 1:length(fn_tr)) {
       data_tmp <- read.csv(paste(i,fn_tr[k],sep="/"), header = FALSE)
       if (dim(data_tmp)[1] != 371) next
       EEM_tmp[,k] <- data_tmp[,2]
     }
     EEM[[ i ]][[ j ]] <- EEM_tmp
   }
}


On Tue, 2 May 2017, Jinsong Zhao wrote:

> Thank you very much, and your reply is helpful.
>
> I don't like assign, and even don't use parse in my previous codes. However, 
> in the case I encountered, assign and parse may be the right tools. Here is 
> the code I used:
>
> # in the workspace, there are tens directory.
> # In each directory, there are lots of *.ASC file,
> # with second column is the data.
> # Each *.ASC file has a name with pattern i-tr-??.ASC.
> # i is the directory name, tr is a group name, and ?? are the index.
> # I have to collect all tr-?? into a matrix,
> # and put all i-tr-?? into a list EEM_i.
>
> for (i in dir()) {
>   assign(paste("EEM_",i,sep=""), list())
>   fn <- list.files(path = i, pattern = "ASC")
>   tr <- sort(as.numeric(unique(unlist(lapply(strsplit(fn, "-"),"[[", 2)))))
>   for (j in 1:length(tr)) {
>      fn_tr <- list.files(path = i, pattern = paste(i, tr[j], "...ASC", 
> sep="-"))
>      EEM_tmp <- matrix(NA,ncol = length(fn_tr),nrow = 371)
>      for (k in 1:length(fn_tr)) {
>         data_tmp <- read.csv(paste(i,fn_tr[k],sep="/"), header = FALSE)
>         if (dim(data_tmp)[1] != 371) next
>         EEM_tmp[,k] <- data_tmp[,2]
>      }
>      eval(parse(text=paste("EEM_",i,"[[",j,"]]<-","EEM_tmp", sep="")))
>   }
> }
>
> Any alternatives or improvements? Thanks a lot.
>
> Best,
> Jinsong
>
> On 2017/4/30 23:48, peter dalgaard wrote:
>> assign(paste("list_", i, "[[1]]", sep = ""), 5) creates a new variable with 
>> a funny name.
>> 
>> You'd have to parse() and eval() to make that work, something like
>> 
>> eval(parse(text=paste("list_",i,"[[1]]<-",5, sep="")))
>> 
>> However,
>> -------
>>> fortunes::fortune("parse")
>> 
>> If the answer is parse() you should usually rethink the question.
>>    -- Thomas Lumley
>>       R-help (February 2005)
>> -------
>> 
>> It is much easier to handle this using a data structure containing a list 
>> of lists:
>> 
>> l <- rep(list(list()), 10)
>> for ( i in 1:10 )
>>    l[[i]][[1]] <- 5
>> 
>>> On 30 Apr 2017, at 17:17 , Jinsong Zhao <jszhao at yeah.net> wrote:
>>> 
>>> Hi there,
>>> 
>>> I have a problem with assign(). Here is the demo code:
>>> 
>>> for (i in 1:10) {
>>>   # create a list with variable name as list_1, list_2, ..., etc.
>>>   assign(paste("list_", i, sep = ""), list())
>>>   # I hope to assign 5 to list_?[[1]], but I don't know how to code it.
>>>   # list_1[[1]] <- 5 # works, however
>>>   assign(paste("list_", i, "[[1]]", sep = "", 5) # does not work
>>> }
>>> 
>>> How to do? Is there any alternatives? Many thanks!
>>> 
>>> Best,
>>> Jinsong
>>> 
>
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>

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