Default column names for `read_csv` and for data frames created from other data
If you read data from a file with `read_csv` the default column names of the resulting data frame are set to `X.1` to `X.N` (and to `X1` to `XN` for versions >= 0.9), which are strings.  

If you create a data frame from exiting arrays or lists or something the column names default to `0` to `N` and are integers.

Should these default names be the same? I think this would be more consistent and would avoid confusion (Not sure how this is handled in `R` if this a part of this design decision). 

See also [this question](http://stackoverflow.com/q/12747417/1301710) and the answers on stackoverflow.
