[R] Finding the position of a variable in a data.frame

[John Kane]

Simple problem but I don't see the answer. I'm trying
to clean up some data
I have 120 columns in a data.frame.  I have one value
in a column named "blaw" that I want to change. How do
I find the coordinates. I can find the row by doing a
subset on the data.frame but how do I find out here
"blaw " is in columns without manually counting them
or converting names(Df) to a list and reading down the
list.

Simple example

cat <- c( 3,5,6,8,0)
dog <- c(3,5,3,6, 0)
rat <- c (5, 5, 4, 9, 0)
bat <- c( 12, 42, 45, 32, 54)

Df <- data.frame(cbind(cat, dog, rat, bat))
Df
subset(Df, bat >= 50)

----results
  cat dog rat bat
5   0   0   0  54


Thus I know that my target is in row 5 but how do I
figure out where 'bat' is?  

All I want to do is be able to say
Df[5,4] <- 100

Is there some way to have function(bat) return the
column number: some kind of a colnum() function?  I
had thought that I had found somthing  in
library(gdata) matchcols but no luck.