[R] Extracting multiple elements from a list

[Roger D. Peng]

Sorry, that second example should be

alist <- lapply(seq(along = alist), function(i) {
                 alist[[i]]$name <- new.names[i])
                 alist
          })

-roger

Roger D. Peng wrote:
> For the first case, I actually do this pretty often and usually use 
> something like:
> 
> as.vector(sapply(alist, "[[", "vec"))
> 
> For the second case, I think you just need to use lapply():
> 
> alist <- lapply(seq(along = alist), function(i)
>         alist[[i]]$name <- new.names[i])
> 
> -roger
> 
> Waichler, Scott R wrote:
> 
>> For a long time I've wanted a way to conveniently extract multiple 
>> elements
>> from a list, which [[ doesn't allow.  Can anyone suggest an efficient
>> function to do this?  Wouldn't it be a sensible addition to R?
>>
>> For example,
>>
>> alist <- list()
>> alist[[1]] <- list()
>> alist[[1]]$name <- "first"
>> alist[[1]]$vec <- 1:4
>> alist[[2]] <- list()
>> alist[[2]]$name <- "second"
>> alist[[2]]$vec <- 5:8
>> both.vec <- c(alist[[1]]$vec, alist[[2]]$vec)
>>
>> Can I get both.vec without c() or an explicit loop?
>>
>> and
>>
>> new.names <- c("one", "two")
>> alist[[1]]$name <- new.names[1]
>> alist[[2]]$name <- new.names[2]
>>
>> Could I assign the new values in a quasi-vectorized way?
>>
>> Thanks,
>> Scott Waichler
>> Pacific Northwest National Laboratory
>> Richland, WA   USA
>>
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> 
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