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

[Steve Lianoglou]

Hi,

On Aug 9, 2009, at 11:29 AM, Frank Schäffer wrote:

> Hi,
> 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
>
> dataframe1.
> date measurment_1  .... mesurement_n
> 1
> 2
> 3
> ..
> ..
> ..
> n

Just as an aside, it's somehow considered more R-idiomatic to store  
all of these tables in a list (of tables) and access them as  
mydata[[1]], mydata[[2]], ..., mydata[[n]]. Assuming the datafiles are  
'filename.1.txt', 'filename.2.txt', etc. You might do this like so:

mydata <- lapply(paste('filename', 1:n, 'txt', sep='.'), read.table,  
header=TRUE, sep=...)

To test that all colnames are the same, you could do something like.

names1 <- colnames(mydata[[1]])
all(sapply(2:n, function(dat) length(intersect(names1,  
colnames(mydata[[n]]))) == length(names1)))

> 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??

If you still want to do it this way, see: ?get

for example:

for (varName in paste('dataframe', 1:n, sep='')) {
   cat(colnames(get(varName)))
}

HTH,
-steve

--
Steve Lianoglou
Graduate Student: Computational Systems Biology
  | Memorial Sloan-Kettering Cancer Center
  | Weill Medical College of Cornell University
Contact Info: http://cbio.mskcc.org/~lianos/contact