r - Get column index from label in a data frame

Question

Say we have the following data frame:

> df
  A B C
1 1 2 3
2 4 5 6
3 7 8 9

We can select column 'B' from its index:

> df[,2]
[1] 2 5 8

Is there a way to get the index (2) from the column label ('B')?

Answer 1

you can get the index via grep and colnames:

grep("B", colnames(df))
[1] 2

or use

grep("^B$", colnames(df))
[1] 2

to only get the columns called "B" without those who contain a B e.g. "ABC".