[R] Passing a Data Frame Name as a Variable in a Function

[Alan Yong]

Dear R-help,
I have df.1001 as a data frame with rows & columns of values.

I also have other data frames named similarly, i.e., df.*.

I used DFName from:

DFName <- ls(pattern = glob2rx("df.*"))[1]

& would like to pass on DFName to another function, like:

length(DFName[, 1])

however, when I run:

> length(DFName[, 1])
Error in DFName[, 1] : incorrect number of dimensions

and

length(df.1001[, 1])
[1] 104

do not provide the same expected answer.

How can I successfully pass the data frame name of df.1001 as a variable named 
DFName in a function?

Thanks,
Alan

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Re: [R] Passing a Data Frame Name as a Variable in a Function

[Alan Yong]

Much thanks to everyone for their recommendations!  I agree that fishing in the 
global environment isn't ideal & only shows my budding understanding of R.

For now, I will adapt Chel Hee's "length(eval(parse(text=DFName))[,1])" 
solution then fully explore Jeff's suggestion to put the data frames into a 
list.

At best, I am implementing an ineloquent approach to:

(1) Add a column to each data frame with a string that is parsed from the 
appendage of the data frame name, i.e., string is "1001" from data frame object 
of "df.1001"; then,
(2) Bind the rows of all the files.

Any more feedback will be much appreciated!

Cheers,
Alan

On Jan 29, 2015, at 3:11 AM, peter dalgaard wrote:

> 
> On 29 Jan 2015, at 07:34 , Jeff Newmiller  wrote:
> 
>> This approach is fraught with dangers.
>> 
>> I recommend that you put all of those data frames into a list and have your 
>> function accept the list and the name and use the list indexing operator 
>> mylist[[DFName]] to refer to it. Having functions that go fishing around in 
>> the global environment will be hard to maintain at best, and buggy at worst.
> 
> Agreed. However, just to help understand the issue: 
> 
> DFName is a length-one vector of character strings, not the object that has 
> the name contained in the string. I.e. you can do nchar(DFName) and 
> presumably get the value 7, but there is no operation on df.1001 that can 
> tell you the length of its name. You can (but shouldn't, as per Jeff's note) 
> get from name to object using get()/assign() and also via some concoctions 
> involving combinations of eval(), parse(), as.name(), and substitute(). 
> 
> -- 
> Peter Dalgaard, Professor,
> Center for Statistics, Copenhagen Business School
> Solbjerg Plads 3, 2000 Frederiksberg, Denmark
> Phone: (+45)38153501
> Email: pd@cbs.dk  Priv: pda...@gmail.com
> 
> 
> 
> 
> 
> 
> 
> 

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Re: [R] Passing a Data Frame Name as a Variable in a Function

[Alan Yong]

Much thanks, Hadley!

Cheers,
Alan

On Jan 29, 2015, at 12:36 PM, Hadley Wickham wrote:

> On Thu, Jan 29, 2015 at 11:43 AM, Alan Yong  wrote:
>> Much thanks to everyone for their recommendations!  I agree that fishing in 
>> the global environment isn't ideal & only shows my budding understanding of 
>> R.
>> 
>> For now, I will adapt Chel Hee's "length(eval(parse(text=DFName))[,1])" 
>> solution then fully explore Jeff's suggestion to put the data frames into a 
>> list.
> 
> If you have to go down this route, at least do nrow(get(DFName))
> 
>> (1) Add a column to each data frame with a string that is parsed from the 
>> appendage of the data frame name, i.e., string is "1001" from data frame 
>> object of "df.1001"; then,
>> (2) Bind the rows of all the files.
> 
> I'd highly recommend learning a little functional programming such as
> the use of lapply (e.g. http://adv-r.had.co.nz/Functionals.html).
> Then you can easily do:
> 
> csvs <- dir(pattern = "\\.csv$")
> all <- lapply(csvs, read.csv)
> one <- do.call("rbind", all)
> 
> to find all the csv files in a directory, load into a list and then
> collapse into a single data frame.
> 
> You're much better off learning how to do this than futzing around
> with named objects in the global environment.
> 
> Hadley
> 
> -- 
> http://had.co.nz/

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