How to add new column to an dataframe (to the front not end)?

Question

How to add a new variable to an existing data frame, but I want to add to the front not end. eg. my dataframe is

b c d
1 2 3
1 2 3
1 2 3

I

Answer 1

Add column "a"

> df["a"] <- 0
> df
  b c d a
1 1 2 3 0
2 1 2 3 0
3 1 2 3 0

Sort by column using colum name

> df <- df[c('a', 'b', 'c', 'd')]
> df
  a b c d
1 0 1 2 3
2 0 1 2 3
3 0 1 2 3

Or sort by column using index

> df <- df[colnames(df)[c(4,1:3)]]
> df
  a b c d
1 0 1 2 3
2 0 1 2 3
3 0 1 2 3