Mutate with custom function in R does not work

Question

I have a data frame, containing a column called: "Frequency". Frequency has values like "Year", "Week", "Month" etc. Now I want to create a new column based on the Frequency column where Year's new corresponding value will be 1, Month's corresponding value will be 12 and Week corresponding value will be 48. I tried to make a function for this as "getValue" and tried to make a new column applying a mutation (dplyr) on that funcion. But unfortunately I am getting the following warning and all values being transformed by the value of "1".

getValue <- function(input) {

          if (input == 'Year')
          {
            result <- 1
          } 
          else if(input == 'Month')
          {
            result <- 12
          } 
          else if(input == 'Week')
          {
            result <- 48
          } 

          return(result)
        }

      Data =  mutate(gateway, YearlyHit = getValue(gateway$Frequency))
      Data

Here is the warning message I am getting -

Warning message:
In if (input == "Year") { :
  the condition has length > 1 and only the first element will be used

How can I achieve the required result in R?