Misc 01

New functions:

• explode: Break a string in to a list of one letter strings.
• implode: Take a list of one letter strings and join them together.

(explode "peace") => (list "p" "e" "a" "c" "e")
(implode (list "w" "a" "r")) => "war"

Usually you would write a helper function that works on one letter strings, then use these functions to convert to ordinary strings.

Exercises Revised

A 1-String is a String of length 1. Examples: "a", "4", "$".

1. Write is-digit?: 1-String -> boolean

    (check-expect (is-digit? "3") #true)
    (check-expect (is-digit? "w") #false)
   

2. Write only-digits: (Listof 1-String) -> (Listof 1-String).

    (check-expect (only-digits (list "1" "w" "3"))
                  (list "1" "3"))
   

3. Write d-in-s: String -> String. The digits-in-string function gives a string containing only the digits in its input.

    (check-expect (d-in-s "x1w3") "13")
   

4. Write fln: 1-String -> Number. The “first and last number” function makes a number out of the first and last digits you find. Expect more than one step to do this. (Use helper functions.)

    (check-expect (fln (list "1" "x" "3" "5" "y"))
                  15)
   
    ;; tricky case for experts
    (check-expect (fln (list "x" "6" "y"))
                  66)