Re: [R] extract parts of a list before symbol
Evan,
Yes, once you know a bit about the details, all kinds of functions are
available to solve problems without going the hard way.
But the names() function is taught fairly widely and did you also pick up on
the fact that it can be used on both sides so it also sets the names?
> # Create a list with mixed case names
> named <- list(age=69, Month="October", YEAR=1492)
> # Save the names to be reset later
> old_names <- names(named)
> # Show current names for the items in named
> print(names(named))
[1] "age" "Month" "YEAR"
> # Replace the current names with an upper case version starting with
"var_"
> names(named) <- paste0("var_", toupper(names(named)))
> # Show current namesfor the items in named
> print(names(named))
[1] "var_AGE" "var_MONTH" "var_YEAR"
> # Reset the names to the original names
> names(named) <- old_names
> # Show current namesfor the items in named
> print(names(named))
[1] "age" "Month" "YEAR"
Look out for other such functions that work on both the LHS and RHS.
> evens <- seq(0,20,2)
> evens
[1] 0 2 4 6 8 10 12 14 16 18 20
> length(evens)
[1] 11
> length(evens) <- 5
> evens
[1] 0 2 4 6 8
If you look just a bit under the hood, R stores all kinds of things about a
variable that you can change with the right function. You can change the
dimensions of a vector for example with dim() as well as get the current
dimensions.
-Original Message-
From: R-help On Behalf Of Evan Cooch
Sent: Friday, May 26, 2023 10:38 AM
To: r-help@r-project.org
Subject: Re: [R] extract parts of a list before symbol
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.gr...@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 On Behalf Of Jeff Newmiller
> Sent: Thursday, May 25, 2023 10:32 PM
> To: r-help@r-project.org
> Subject: Re: [
Re: [R] extract parts of a list before symbol
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.gr...@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 On Behalf Of Jeff Newmiller
Sent: Thursday, May 25, 2023 10:32 PM
To: r-help@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
Re: [R] extract parts of a list before symbol
You probably want `names(test)`.
On Thu, May 25, 2023 at 7:58 PM Evan Cooch wrote:
> 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...
>
> __
> R-help@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.
>
[[alternative HTML version deleted]]
__
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and provide commented, minimal, self-contained, reproducible code.
Re: [R] extract parts of a list before symbol
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 On Behalf Of Jeff Newmiller
Sent: Thursday, May 25, 2023 10:32 PM
To: r-help@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 # int
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.gr...@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 On Behalf Of Evan Cooch
>Sent: Thursday, May 25, 2023 1:30 PM
>To: r-help@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...
>
>__
>R-help@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.
>
>__
>R-help@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.
__
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
Re: [R] extract parts of a list before symbol
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 On Behalf Of Evan Cooch
Sent: Thursday, May 25, 2023 1:30 PM
To: r-help@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...
__
R-help@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.
__
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
Re: [R] extract parts of a list before symbol
On Thu, 25 May 2023 13:29:52 -0400
Evan Cooch wrote:
> 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...
Perhaps names(test) ???
cheers,
Rolf Turner
--
Honorary Research Fellow
Department of Statistics
University of Auckland
Stats. Dep't. (secretaries) phone:
+64-9-373-7599 ext. 89622
Home phone: +64-9-480-4619
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
[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...
__
R-help@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.
