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

[Chuck Cleland]

John Kane wrote:
> 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?  

grep("bat", names(Df))

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

Why not do it this way?

Df$bat <- replace(Df$bat, Df$bat >=50, 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.
> 
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-- 
Chuck Cleland, Ph.D.
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