Get column index from label in a data frame

The following will do it:

which(colnames(df)=="B")

I wanted to see all the indices for the colnames because I needed to do a complicated column rearrangement, so I printed the colnames as a dataframe. The rownames are the indices.

as.data.frame(colnames(df))

1 A
2 B
3 C

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".

Following on from chimeric's answer above:

To get ALL the column indices in the df, so i used:

which(!names(df)%in%c()) 

or store in a list:

indexLst<-which(!names(df)%in%c())