r - Apply a function on multiple columns in each row -

below data frame have. column 2 days expiration of nearest contract, column 3 days expiration of next nearest contract. i'm trying create vector gives me percentage of column 2 needed give me weighted average days expiration of 28 days each row.

   date        daysxone       daysxtwo 1 2006-01-03       15        43 days 2 2006-01-04       14        42 days 3 2006-01-05       13        41 days 4 2006-01-06       12        40 days 5 2006-01-09        9        37 days 6 2006-01-10        8        36 days 

i've tried:

 f <- function(x){  df$daysxone*(x) + (df$daysxtwo*(1-(x)) -28} 

and i've tried few things uniroot(), i'm stuck

thanks!

or go data.table:

library(data.table) df <- data.frame(daysxone = c(15,14,13,12,9,8), daysxtwo = c(43, 42,41,40, 37,36)) setdt(df)[,perc := (28-daysxtwo)/(daysxone - daysxtwo),]     daysxone daysxtwo      perc 1:       15       43 0.5357143 2:       14       42 0.5000000 3:       13       41 0.4642857 4:       12       40 0.4285714 5:        9       37 0.3214286 6:        8       36 0.2857143