2021-09-30

Char vs List of Char

Recently seen problem: attempt to put an @ sign after the first letter of a word:

f :: [Char] -> [Char]  -- same as String -> String
f "" = ""
f (x:xs) = x : "@" : xs

This code does not work because "@" is a list containing a single character, which could also be written ['@'], instead of the desired single character '@'. The correct function follows.

f (x:xs) = x : '@' : xs

Practice

1. money :: Int -> String.

    money 123456789 == "$123,456,789"
    money 1234 == "$1,234"
   

   Advanced: treat the last two digits as cents, so examples look like this:

    moneyCents 123498 == "$1,234.98"
    moneyCents 12350 == "$123.50" -- Advanced only!
   

   Simplified: produce the corresponding list of Int:

    moneySimple 12345678 == [12,345,678]
   

   Very simplified:

    moneySuperSimple [1,2,3,4,5,6,7,8] == [12,345,678]
   

2. What is the type of the expression:

    q2 = [ ("Ping", 8), ("Pong", 7), ("Table", 19), ("Tennis", 11)]
   

3. Given (x,y) in a list [(Int, Int)] produce a list of ordered pairs (x,y) for which $y < x^2+30$.

    q3 [(10,125), (8, 104), (5, 9), (20, 500)] == [(10,125), (5,9)]