mutate expression - how to pass name of column stored as character in the different column?

[RSzt]

How to pass name of column stored as character in the different column to the mutate expression, so that expression can use different column for every row?

df <- data.frame(which_col = c("A", "A", "B", "B"),
                 A = c(1, 2, 3, 4),
                 B = c(100, 200, 300, 400),
                 C = c(10, 10, 10, 10),
                 stringsAsFactors = F)

output <- df %>%
  dplyr::mutate(result = which_col * C) # of course this doesn't work

expected_output <- data.frame(which_col = c("A", "A", "B", "B"),
                              A = c(1, 2, 3, 4),
                              B = c(100, 200, 300, 400),
                              C = c(10, 10, 10, 10),
                              result = c(10, 20, 3000, 4000),
                              stringsAsFactors = F)

So if which_col == "A" expression is result = A * C, while for which_col == "B" expression is result = B * C

I know that case_when might be the option, but in case of many possible columns and if expression is more complicated, this would create many duplicates.

[JohnMount]

I have a work-up on the related question of using one column to pick from other columns (inspired by an earlier RStudio community question): here.