Get the column number in R given the column name [duplicate]

Question

Possible Duplicate:
Get column index from label in a data frame

I need to get the column number of a column given its name.

Supose we have the following dataframe:

df <- data.frame(a=rnorm(100),b=rnorm(100),c=rnorm(100)) 

I need a function that would work like the following:

getColumnNumber(df,"b") 

And it would return

[1] 2 

Is there a function like that?

Thanks!

Answer 1

which( colnames(df)=="b" ) 

Should do it.

Answer 2

One fast and neat method is :

> match("b",names(df)) [1] 2 

That avoids the vector scan that == and which do. If you have a lot of columns, and you do this a lot, then you might like the fastmatch package.

> require(fastmatch) > fmatch("b",names(df)) [1] 2 

fmatch is faster than match, but on subsequent calls it's not just faster, it's instant.