R help - Oct 2004 - creating named elements of lists on the fly

HI Folks,

I'm trying to create a list with named elements. Only, I don't know the
names of the elements a priori (they come from the data being calculated).
Currently, my approach is to create an environment, then assign things to the
environement, then as.list the environment to get a list.

Running the code gives, for example:
> e2 <- new.env(FALSE, NULL)
> assign("dude", 123, env=e2)
> assign("chick", 456, env=e2)
> as.list(e2)
Error in as.vector(x, "list") : cannot coerce to vector

Is this the best way to make a list like this? 

Thanks,
Ben

Dear Ben,

Is this the kind of thing you had in mind?
> lst <- list()
> element <- "a"
> lst[[element]] <- 1:5
> element <- "b"
> lst[[element]] <- letters[1:5]
> lst
$a
[1] 1 2 3 4 5

$b
[1] "a" "b" "c" "d" "e"


I hope this helps,
 John 
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Ben Shapiro
> Sent: Friday, October 08, 2004 11:46 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] creating named elements of lists on the fly 
> 
> HI Folks,
> 
> I'm trying to create a list with named elements. Only, I 
> don't know the names of the elements a priori (they come from 
> the data being calculated). Currently, my approach is to 
> create an environment, then assign things to the 
> environement, then as.list the environment to get a list. 
> 
> Running the code gives, for example:
> > e2 <- new.env(FALSE, NULL)
> > assign("dude", 123, env=e2)
> > assign("chick", 456, env=e2)
> > as.list(e2)
> Error in as.vector(x, "list") : cannot coerce to vector
> 
> Is this the best way to make a list like this? 
> 
> Thanks,
> Ben

Ben Shapiro wrote:
> HI Folks,
> 
> I'm trying to create a list with named elements. Only, I don't know
the names of the elements a priori (they come from the data being calculated).
Currently, my approach is to create an environment, then assign things to the
environement, then as.list the environment to get a list.
> 
> Running the code gives, for example:
> 
>>e2 <- new.env(FALSE, NULL)
>>assign("dude", 123, env=e2)
>>assign("chick", 456, env=e2)
>>as.list(e2)
> 
> Error in as.vector(x, "list") : cannot coerce to vector
> 
> Is this the best way to make a list like this? 
> 
> Thanks,
> Ben
> 
You can use the "[[" operator for list objects:

nm <- c("dude", "chick")
e2[[nm[1]]] <- 123
e2[[nm[2]]] <- 456

HTH,

--sundar

Ben Shapiro <bshapiro-lists-R <at> getdown.org> writes:

: 
: HI Folks,
: 
: I'm trying to create a list with named elements. Only, I don't know
the
names of the elements a priori (they
: come from the data being calculated). Currently, my approach is to create an 
environment, then assign
: things to the environement, then as.list the environment to get a list. 
: 
: Running the code gives, for example:
: > e2 <- new.env(FALSE, NULL)
: > assign("dude", 123, env=e2)
: > assign("chick", 456, env=e2)
: > as.list(e2)
: Error in as.vector(x, "list") : cannot coerce to vector
: 
: Is this the best way to make a list like this? 
: 
: Thanks,
: Ben


If you need to add them one at a time then John Fox has
already provided an answer.  If you want to create it
all at once and you have the contents in a list and
the names in a vector like this:

nams <- letters[1:3]
contents <- list(1:3, 4:5, 6:9)

# then here are two ways:

# 1
mapply("{", nams, contents, SIMPLIFY = FALSE)

# 2
names(contents) <-  nams
contents

#  Actually in your example the elements are all homogeneous so
#  you could alternately use a vector to hold the results:

# 1a
contents <- c(1, 2, 3)
mapply("{", nams, contents)

# 2a
names(contents) <- nams
contents