[R] extract parts of a list before symbol

[Evan Cooch]

Many thanks to all. Wasn't even aware of the names function. That does 
the trick for present purposes.

On 5/26/2023 12:02 AM, avi.e.gross using gmail.com wrote:
> All true Jeff, but why do things the easy way! LOL!
>
> My point was that various data structures, besides the list we started with,
> store the names as an attribute. Yes, names(listname) works fine to extract
> whatever parts they want. My original idea of using a data.frame was because
> it creates names when they are absent.
>
> And you are correct that if the original list was not as shown with only all
> items of length 1, converting to a data.frame fails.
>
> >From what you say, it is a harder think to write a function that returns a
> "name" for column N given a list. As you note, you get a null when there are
> no names.  You get empty strings when one or more (but not all) have no
> names. But it can be done.
>
> The OP initially was looking at a way to get a text version of a variable
> they could use using perhaps regular expressions to parse.  Of course that
> is not as easy as just looking at the names attribute in one of several
> ways. But it may help in a sense to deal with the cases mentioned above.
> The problem is that str() does not return anything except to stdout so it
> must be captured to do silly things.
>
>> test <- list(a=3,b=5,c=11)
>> str(test)
> List of 3
>   $ a: num 3
>   $ b: num 5
>   $ c: num 11
>
>> str(test[1])
> List of 1
>   $ a: num 3
>
>> str(test[2])
> List of 1
>   $ b: num 5
>
>> str(list(a=1, 2, c=3))
> List of 3
>   $ a: num 1
>   $  : num 2
>   $ c: num 3
>
>> str(list(1, 2, 3))
> List of 3
>   $ : num 1
>   $ : num 2
>   $ : num 3
>
>> text <- str(list(a=1, 2, c=3)[1])
> List of 1
>   $ a: num 1
>
>> text <- capture.output(str(list(a=1, 2, c=3)))
>> text
> [1] "List of 3"   " $ a: num 1" " $  : num 2" " $ c: num 3"
> So you could use some imaginative code that extracts what you want. I
> repeat, this is not a suggested way nor the best, just something that seems
> to work:
>
>> sub("(^[\\$ ]*)(\\w+|)(:.*$)", "\\2", text[2:length(text)])
> [1] "a" ""  "c"
>
> Obviously the first line of output needs to be removed as it does not fit
> the pattern.
>
> Perhaps in this case a way less complex way is to use summary() rather than
> str as it does return the output as text.
>
>> summary(list(a=1, 2, c=3)) -> text
>> text
>    Length Class  Mode
> a 1      -none- numeric
>    1      -none- numeric
> c 1      -none- numeric
>
> This puts the variable name, if any, at the start but parsing that is not
> trivial as it is not plain text.
>
> Bottom line, try not to do things the hard way. Just carefully use names()
> ...
>
> -----Original Message-----
> From: R-help <r-help-bounces using r-project.org> On Behalf Of Jeff Newmiller
> Sent: Thursday, May 25, 2023 10:32 PM
> To: r-help using r-project.org
> Subject: Re: [R] extract parts of a list before symbol
>
> What a remarkable set of detours, Avi, all deriving apparently from a few
> gaps in your understanding of R.
>
> As Rolf said, "names(test)" is the answer.
>
> a) Lists are vectors. They are not atomic vectors, but they are vectors, so
> as.vector(test) is a no-op.
>
> test <- list( a = 1, b = 2, c=3 )
> attributes(test)
> attributes(as.vector(test))
>
> (Were you thinking of the unlist function? If so, there is no reason to
> convert the value of the list to an atomic vector in order to look at the
> value of an attribute of that list.)
>
> b) Data frames are lists, with the additional constraint that all elements
> have the same length, and that a names attribute and a row.names attribute
> are both required. Converting a list to a data frame to get the names is
> expensive in CPU cycles and breaks as soon as the list elements have a
> variety of lengths.
>
> c) All data in R is stored as vectors. Worrying about whether a data value
> is a vector is pointless.
>
> d) All objects can have attributes, including the name attribute. However,
> not all objects must have a name attribute... including lists. Omitting a
> name for any of the elements of a list in the constructor will lead to
> having a zero-length character values in the name attribute where the names
> were omitted. Omitting all names in the list constructor will cause no names
> attribute to be created for that list.
>
> test2 <- list( 1, 2, 3 )
> attributes(test2)
>
> e) The names() function returns the value of the names attribute. If that
> attribute is missing, it returns NULL. For dataframes, the colnames function
> is equivalent to the names function (I rarely use the colnames function).
> For lists, colnames returns NULL... there are no "columns" in a list,
> because there is no constraint on the (lengths of the) contents of a list.
>
> names(test2)
>
> f) The names attribute, if it exists, is just a character vector. It is
> never necessary to convert the output of names() to a character vector. If
> the names attribute doesn't exist, then it is up to the user to write code
> that creates it.
>
> names(test2) <- c( "A", "B", "C" )
> attributes(test2)
> names(test2)
> # or use the argument names in the list function
>
> names(test2) <- 1:3 # integer
> names(test2) # character
> attributes(test2)$names <- 1:3 # integer
> attributes(test2) # character
> test2[[ "2" ]] == 2  # TRUE
> test2$`2`  == 2 # TRUE
>
>
>
> On May 25, 2023 6:17:37 PM PDT, avi.e.gross using gmail.com wrote:
>> Evan,
>>
>> List names are less easy than data.frame column names so try this:
>>
>>> test <- list(a=3,b=5,c=11)
>>> colnames(test)
>> NULL
>>> colnames(as.data.frame(test))
>> [1] "a" "b" "c"
>>
>> But note an entry with no name has one made up for it.
>>
>>
>>> test2 <- list(a=3,b=5, 666, c=11)
>>> colnames(data.frame(test2))
>> [1] "a"    "b"    "X666" "c"
>>
>> But that may be overkill as simply converting to a vector if ALL parts are
>> of the same type will work too:
>>
>>> names(as.vector(test))
>> [1] "a" "b" "c"
>>
>> To get one at a time:
>>
>>> names(as.vector(test))[1]
>> [1] "a"
>>
>> You can do it even simple by looking at the attributes of your list:
>>
>>> attributes(test)
>> $names
>> [1] "a" "b" "c"
>>
>>> attributes(test)$names
>> [1] "a" "b" "c"
>>> attributes(test)$names[3]
>> [1] "c"
>>
>>
>> -----Original Message-----
>> From: R-help <r-help-bounces using r-project.org> On Behalf Of Evan Cooch
>> Sent: Thursday, May 25, 2023 1:30 PM
>> To: r-help using r-project.org
>> Subject: [R] extract parts of a list before symbol
>>
>> Suppose I have the following list:
>>
>> test <- list(a=3,b=5,c=11)
>>
>> I'm trying to figure out how to extract the characters to the left of
>> the equal sign (i.e., I want to extract a list of the variable names, a,
>> b and c.
>>
>> I've tried the permutations I know of involving sub - things like
>> sub("\\=.*", "", test), but no matter what I try, sub keeps returning
>> (3, 5, 11). In other words, even though I'm trying to extract the
>> 'stuff' before the = sign, I seem to be successful only at grabbing the
>> stuff after the equal sign.
>>
>> Pointers to the obvious fix? Thanks...
>>
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>>
>> ______________________________________________
>> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> 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.