R help - Jul 2008 - name returned by lapply

Hi there,

I have a very simple question. If I execute the following code:


test <- function(x) {
	name <- paste(x,"info_within_function")
	c(1,2,3,4,5)
}

ret <- lapply(1:10, test)


, I end up with a list and each entry is just numbered [[1]], [[2]], ... [[10]]

How can I force the result entries gettings names which are determined within 
the function. For this example, I'd like the entries named like:

"1 info_within_function" instead of [[1]]
"2 info_within_function" instead of [[2]]
and so on.
(that's why I put this strange "name <- ..." line inside)

Can anybody help me what I have to do to get the returned list entries like
this?

Thanks a lot!

Antje

On Fri, 2008-07-18 at 13:45 +0200, Antje wrote:
> Hi there,
> 
> I have a very simple question. If I execute the following code:
> 
> 
> test <- function(x) {
> 	name <- paste(x,"info_within_function")
> 	c(1,2,3,4,5)
> }
> 
Here your test is returning c(1,2,3,4,5), and is doing nothing with
name, other than to create the object, 'name'.

## ones presumes your real example is more complicated than this so
## I'll stick with the function test, but just have it return the
## values you asked for
test <- function(x) {
	c(1,2,3,4,5)
}

##  we then apply this
ret <- lapply(1:10, test)

## now that we have a list, we change the names to what you want
names(ret) <- paste(1:10, "info_within_function")

This does cause a problem though as you can't access the components
directly via:

ret$1 info_within_function

You need to quote them:

ret$`1 info_within_function`

And even if you replace the space with a "_", the same problem
persists:

names(ret) <- paste(1:10, "info_within_function", sep =
"_")
ret$1_info_within_function
ret$`1_info_within_function`

So depending on why you want the list components named this way, you
might consider altering the name so that it doesn't start with a number
and doesn't contain spaces, otherwise you'll need to quote the names
when trying to access the components of the list by name.

Does that help?

G
> ret <- lapply(1:10, test)
> 
> 
> , I end up with a list and each entry is just numbered [[1]], [[2]], ...
[[10]]
> 
> How can I force the result entries gettings names which are determined
within
> the function. For this example, I'd like the entries named like:
> 
> "1 info_within_function" instead of [[1]]
> "2 info_within_function" instead of [[2]]
> and so on.
> (that's why I put this strange "name <- ..." line inside)
> 
> Can anybody help me what I have to do to get the returned list entries like
this?
> 
> Thanks a lot!
> 
> Antje
> 
> ______________________________________________
> R-help at r-project.org 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.
-- 
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 Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
 ECRC, UCL Geography,          [f] +44 (0)20 7679 0565
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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
>>>
>> ______________________________________________
>> R-help at r-project.org 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.
>