[R] Looping through data sets to change column from character to numeric

[Rolf Turner]

On 29/07/11 11:09, Sarah Henderson wrote:
> Greetings to all --
>
> I am having a silly problem that I just can't solve.  Someone has given me
> an .RData file will hundreds of data frames.  Each data frame has a column
> named ResultValue, currently character when it should be numeric.  I want to
> loop through all of the data frames to change the variable to numeric, but I
> cannot figure out how to do it. My best guess was along the lines of:
>
> frames = ls()
> for (frame in frames){
>       assign(frame, get(frame), .GlobalEnv)
>       frame[,"ResultValue"] = as.numeric(frame[,"ResultValue"])
>       }
>

Note that ``frame'' is an object the value of which is a character string
naming (successive) data frames. Your call to assign doesn't change the
value of ``frame''.  It *could* (but doesn't actually) change the value 
of the
object *named* by the character string stored in ``frame''.

Suppose that you have data frames clyde, melvin, and irving.  On your 
first pass
through the loop the value of frame is the character string ``clyde''.

Your assign() call in effect does ``clyde <- clyde'' which doesn't 
accomplish
much. :-)

The value of frame after the assign() is still the character string 
``clyde''.
I.e. frame is a character scalar, so frame[,"ResultValue"] doesn't make any
sense.

What you need to do is something like:

     frames <- ls()
     for(frame in frames) {
         assign("temp",get(frame)) # Don't think you need the 
``.GlobalEnv'', although it doesn't hurt.
         temp[,ResultValue"] <- as.numeric(temp[,"ResultValue"])
         assign(frame,temp)
     }

This is untested ....

But I hope it helps.

         cheers,

             Rolf Turner