R help - Dec 2008 - Object name vectcor as function input argument?

Dear R-helpers:

I am new to R and ran into the following question and would appreicate
your advice very much.

My question: How to use a character vector that records object names as
function input argument?

I asked this question very recently and was advised to use get(). get()
works when passing one single object name.
but it does not work when passing multiple object names.

For example, I want to rbind many dfs into one df.
Below, I use 3 data frames for illustration.
df.1 <- data.frame(v1=rnorm(5), v2=rnorm(5))
df.2 <- data.frame(v1=rnorm(5), v2=rnorm(5))
df.3 <- data.frame(v1=rnorm(5), v2=rnorm(5))

all.dfs <- c("df.1","df.2","df.3")
# all.dfs is the a character vector recording all object names and I would
like to use all.dfs as
# an input argument for a function that performs rbind

# The following works, but I do not know how to use all.dfs as its input
argument
output <- do.call("rbind",list(df.1,df.2,df.3))

# The desired function has the following form:

output <- desired.function (all.dfs)


# Show some hw I have done below:
# I tried the following things and they do not work
do.call("rbind",list(all.dfs))

one.string <- paste(all.dfs,collapse=",")

do.call("rbind",list(one.string))
do.call("rbind",list(get(one.string)))
do.call("rbind",list(parse(one.string)))
# By the way, the following loop.fun works but it is Not what I like because
I may have a large number of dfs
loop.fun <- function (all.dfs)
{
for (i in 1:length(all.dfs) )
ifelse ( i==1, output <- get(all.dfs[i]), output <-
rbind(output,get(all.dfs[i])) )
return(output)
}

output <- loop.fun(all.dfs)



#Your help is highly appreciated. Many thanks in advance.

-Sean Zhang, Ann Arbor

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On Sat, 27 Dec 2008, Sean Zhang wrote:
> Dear R-helpers:
>
> I am new to R and ran into the following question and would appreicate
> your advice very much.
>
> My question: How to use a character vector that records object names as
> function input argument?
>
> I asked this question very recently and was advised to use get(). get()
> works when passing one single object name.
> but it does not work when passing multiple object names.
>
> For example, I want to rbind many dfs into one df.
> Below, I use 3 data frames for illustration.
> df.1 <- data.frame(v1=rnorm(5), v2=rnorm(5))
> df.2 <- data.frame(v1=rnorm(5), v2=rnorm(5))
> df.3 <- data.frame(v1=rnorm(5), v2=rnorm(5))
>
> all.dfs <- c("df.1","df.2","df.3")
Try this:
> rb.obj <- quote(rbind())
> rb.obj[ all.dfs ] <- lapply( all.dfs, as.name )
> eval(rb.obj)
HTH,

Chuck
> # all.dfs is the a character vector recording all object names and I would
> like to use all.dfs as
> # an input argument for a function that performs rbind
>
> # The following works, but I do not know how to use all.dfs as its input
> argument
> output <- do.call("rbind",list(df.1,df.2,df.3))
>
> # The desired function has the following form:
>
> output <- desired.function (all.dfs)
>
>
> # Show some hw I have done below:
> # I tried the following things and they do not work
> do.call("rbind",list(all.dfs))
>
> one.string <- paste(all.dfs,collapse=",")
>
> do.call("rbind",list(one.string))
> do.call("rbind",list(get(one.string)))
> do.call("rbind",list(parse(one.string)))
> # By the way, the following loop.fun works but it is Not what I like
because
> I may have a large number of dfs
> loop.fun <- function (all.dfs)
> {
> for (i in 1:length(all.dfs) )
> ifelse ( i==1, output <- get(all.dfs[i]), output <-
> rbind(output,get(all.dfs[i])) )
> return(output)
> }
>
> output <- loop.fun(all.dfs)
>
>
>
> #Your help is highly appreciated. Many thanks in advance.
>
> -Sean Zhang, Ann Arbor
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> 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.
>
Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901

On Sat, Dec 27, 2008 at 1:32 PM, Sean Zhang <seanecon@gmail.com> wrote:
> My question: How to use a character vector that records object names as
> function input argument?
> ...
> I asked this question very recently and was advised to use get(). get()
> works when passing one single object name. but it does not work when
passing
> multiple object names....
>
In general, to apply a function to a list of objects, you can use `lapply`
(similar to map/collect/transform in other programming languages).  Thus,
lapply(all.dfs,get) == list( get(all.dfs[[1]], get(all.dfs[[2]]), ... ).

So for your original problem, you can write:

     do.call(rbind,lapply(all.dfs,get))

Is this what you need?

           -s

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