[R] dataframe question

[Vadlamani, Satish {FLNA}]

Folks:
 Good day. Please see the code below. three_wk_out is a dataframe with columns wk1 through wk209. I want to change the format of the columns. I am trying the code below but it does not work.  I need $week in the for loop interpreted as wk1, wk2, etc. Could you please help? Thanks.
Satish

R code below
week_list <- paste("wk",c(1:209),sep="")
for (week in week_list)
{
        three_wk_out$week <- as.numeric(three_wk_out$week)
}