Question

I am designing a function that consumes a natural number (like 0, 1, 2, ….etc) and returns the string representing it in unary (base 1) using the symbol I and also displaying the number in decimal after its unary form. So if I type in 2, I would get "II (2)". 5, I would get "IIIII (5)" .

I’ve done it this way:

  (define (number->unary c )
      (cond
       [(= c  1 )  "I (1)" ]
       [(= c  2 ) "II (2)" ]
       [(= c  3) "III (3)"]
       [(= c  4) "IIII (4)"]
       [(= c  5) "IIIII (5)"]
       [(= c  6) "IIIIII (6)"]
       [(= c  7) "IIIIIII (7)"]
       [(= c  8) "IIIIIIII (8)"]
       [(= c  9) "IIIIIIIII (9)"]
       [(= c  10) "IIIIIIIIII (10)"]))

but is there an easier way to do this problem? I’ve been reading about number->string, string-appends, and replicates but I am not sure where to start.

Answer 1

There’s an easier way indeed – using a loop for generating the appropriate number of "I" characters. Think of it, what would happen if c equals 100? you shouldn’t have to create a hundred conditions for this!

Here’s one possible way, using a helper procedure for implementing iteration with a recursive procedure called loop:

(define (number->unary c)
  (loop c (string-append " (" (number->string c) ")")))

(define (loop num str)
  (if (zero? num)
      str
      (loop (sub1 num) (string-append "I" str))))