phone_knights.R

library(lazyeval)
library(dplyr)

phone_knight_combinations <- function(n, starting_value = 1) {
  mapping <- data.frame(this_value = c(1,1,6,6,6,7,7,2,2,9,9,4,4,4,3,3,8,8,0,0),
                        next_value = c(6,8,1,7,0,6,2,7,9,2,4,9,0,3,4,8,1,3,4,6)
  )
  combinations <- data.frame(value_1 = c(starting_value), this_value = c(starting_value))
  for (i in 2:n) {
    combinations <- merge(combinations, mapping)
    mutate_call <- lazyeval::interp(~ a, a = as.name("next_value"))
    combinations <- combinations %>% 
      mutate_(.dots = setNames(list(mutate_call), paste0("value_",i))) %>%
      select(-this_value) %>%
      rename(this_value = next_value)
  }
  return(dim(combinations)[1])
}

phone_knight_combinations(5, 1)