[R] howto get the number of columns and column names of multiply data frames

Don MacQueen macq at llnl.gov
Sun Aug 9 22:26:36 CEST 2009

## You can use get()

for ( i in 1:n) {
   nm <- paste('dataframe',i,sep='')
   cat( ncol( get(nm)), 'columns in',nm,'\n') )

## or
nms <- ls(pattern='dataframe')
   for (nm in nms) cat( ncol(get(nm)) , 'columns in',nm,'\n') )

(Assuming I have balanced parantheses, that is -- 
my email software doesn't check that like Emacs 

Storing the dataframes as elements of a list, as 
Steve Lianoglou suggested, lets you avoid using 
the get() function.

You could also use the count.fields() function to 
check whether the files have the correct number 
of columns even before you  read the data it. Or 
make a pass through the files reading in only the 
first line as data, and comparing those as data 
rather than as a names attribute of a dataframe.


At 5:29 PM +0200 8/9/09, Frank Schäffer wrote:
>I' ve read in several files with measurements into R data frames(works
>flawlessly). Each dataframe is named by the location of measurement and
>contains hundreds of rows and about 50 columns like this
>date measurment_1  .... mesurement_n
>For further processing I need to check whether or not ncol and colnames are
>the same for all dataframes.
>Also I need to add a new column to each dataframe with contain the name of the
>dataframe, so that this column can be treated as factor in later processing
>(after merging some seleted dataframes to one)
>I tried out
>for (i in 1:length(ls()){
>	print(ncol(ls()[i])
>but this does not work because r returns a "character" for i and therefore
>"NULL" as result.
>Reading the output of ls() into a list also does not work.
>How can I accomplish this task??
>Best regards and thanks
>R-help at r-project.org mailing list
>PLEASE do read the posting guide http://*www.*R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.

Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA

More information about the R-help mailing list