[R] extract parts of a list before symbol

[Jeff Newmiller]

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.

-- 
Sent from my phone. Please excuse my brevity.