Get column index from label in data frame

Question

Say we have the following data frame:

> df ABC 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 index (2) from 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 get columns called "B" without those containing B, for example. "ABC".

Answer 2

This will do the following:

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

Answer 3

I wanted to see all the indexes for colnames because I needed to do a complex column permutation, so I printed the code names as a data frame. Growth names are indexes.

 as.data.frame(colnames(df)) 1 A 2 B 3 C 

Answer 4

This seems to be an efficient way to list vars with column number:

 cbind(names(df)) 

Output:

  [,1] [1,] "A" [2,] "B" [3,] "C" 

Sometimes I like to copy position variables to my code, so I use this function:

 varnums<- function(x) {w=as.data.frame(c(1:length(colnames(x))), paste0('# ',colnames(x))) names(w)= c("# Var/Pos") w} varnums(df) 

Output:

 # Var/Pos # A 1 # B 2 # C 3