[R] name returned by lapply

[Antje]

Oh true, this would solve the problem too :-)
Thanks a lot for the suggestions!

Antje



Martin Morgan schrieb:
> Antje <niederlein-rstat at yahoo.de> writes:
> 
>> Thanks a lot for your help!
>>
>> I know that I cannot directly access the list created, I just was not
>> sure if there is any "format" of the return value which could provide
>> additionally a name for the returned list.
>> I tried to return the values as list with the appropriate name but
>> then I end up with a list entry as list entry...
>>
>> Okay, then I'll solve it with a loop and thanks for the hint with the article
> 
> maybe this:
> 
>> res <- lapply(1:5, function(i) list(key=paste("Hugo", i, sep="_"), val=i))
>> val <- lapply(res, "[[", "val")
>> names(val) <- lapply(res, "[[", "key")
>> val
> $Hugo_1
> [1] 1
> 
> $Hugo_2
> [1] 2
> 
> $Hugo_3
> [1] 3
> 
> $Hugo_4
> [1] 4
> 
> $Hugo_5
> [1] 5
> 
> Martin
> 
>> Ciao,
>> Antje
>>
>>
>>
>>
>>
>> Gavin Simpson schrieb:
>>> On Fri, 2008-07-18 at 14:19 +0200, Antje wrote:
>>>> Hi Gavin,
>>>>
>>>> thanks a lot for your answer.
>>>> Maybe I did not explain very well what I want to do and probably
>>>> chose a bad example. I don't mind spaces or names starting with a
>>>> number. I could even name it:
>>>>
>>>> "Hugo1", "Hugo2", ...
>>>>
>>>> My biggest problem is, that not only the values are
>>>> calculated/estimated within my function but also the names (Yes, in
>>>> reality my funtion is more complicated).
>>>> Maybe it's easier to explain like this. the parameter x can be a
>>>> coordinate position of mountains on earth. Within the funtion the
>>>> height of the mountain is estimated and it's name.
>>>> In the end, I'd like to get a list, where the entry is named like
>>>> the mountain and it contains its height (or other measurements...)
>>>>
>>>>
>>>>> ## now that we have a list, we change the names to what you want
>>>>> names(ret) <- paste(1:10, "info_within_function")
>>>> so this would not work, because I don't have the information
>>>> anymore about the naming...
>>> OK, so you can't do what you want to do in the manner you tried, via
>>> lapply as you don't have control of how the list is produced once the
>>> loop over 1:10 has been performed. At the stage that 'test' is being
>>> applied, all it knows about is 'x' and it doesn;t have access to the
>>> list being built up by lapply().
>>> The *apply family of functions help us to *not* write out formal
>>> loops
>>> in R, but here this is causing you a problem. So we can specify an
>>> explicit loop and fill in information as and when we want from within
>>> the loop
>>> ## create list to hold results
>>> n <- 10
>>> ret <- vector(mode = "list", length = n)
>>> ## initialise loop
>>> for(i in seq_len(n)) {
>>>     ## do whatever you need to do here, but this line just
>>>     ## replicates what 'test' did earlier
>>>     ret[[i]] <- c(1,2,3,4,5)
>>>     ## now add the name in
>>>     names(ret)[i] <- paste("Mountain", i, sep = "")
>>> }
>>> ret
>>> Alternatively, collect a vector of names during the loop and then
>>> once
>>> the loop is finished do a single call to names(ret) to replace all the
>>> names at once:
>>> n <- 10
>>> ret <- vector(mode = "list", length = n)
>>> ## new vector to hold vector of names
>>> name.vec <- character(n)
>>> for(i in seq_len(n)) {
>>>     ret[[i]] <- c(1,2,3,4,5)
>>>     ## now we just fill in this vector as we go
>>>     name.vec[i] <- paste("Mountain", i, sep = "")
>>> }
>>> ## now replace all the names at once
>>> names(ret) <- name.vec
>>> ret
>>> This latter version is likely to more efficient if n is big so you
>>> don't
>>> incur the overhead of the repeated calls to names()
>>> The moral of the story is to not jump to using *apply all the time
>>> to
>>> avoid loops. Loops in R are just fine, so use the tool that helps you do
>>> the job most efficiently *and* most transparently.
>>> Take a look at the R Help Desk article by Uwe Ligges and John Fox in
>>> the
>>> current issue of RNews:
>>> http://www.r-project.org/doc/Rnews/Rnews_2008-1.pdf
>>> Which goes into this in much more detail
>>> HTH
>>> G
>>>
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>