Project Euler 17

Question

I\'ve been trying to solve Euler 17 and have been running into some trouble. The definition of that problem is:

If the numbers 1 to 5 are written out

Answer 1

Explaining the discrepancy

Your code is riddled with errors:

1. This is wrong:

   maps[60] = 6
   

   Contribution to error: +100 (because it affects 60 to 69, 160 to 169, ..., 960 to 969).

2. Several teenagers are mistaken:

   >>> teen(12)
   7
   >>> teen(13)
   9
   >>> teen(15)
   8
   >>> teen(18)
   9
   

   Contribution to error: +40 (because it affects 12, 13, ..., 112, 113, ..., 918)

3. And any number of the form x10:

   >>> three_digit(110)
   17
   

   Contribution to error: 9 (because 110, 210, ... 910)

4. The number 20 is not counted (you consider i < 20 and i > 20 but not i == 20).

   Contribution to error: −6

5. The number 1000 is written "one thousand" in English but:

   >>> thousand(1000)
   8
   

   Contribution to error: −3

6. You subtract 10 at the end in an attempt to compensate for one of these errors.

   Contribution to error: −10

Total error: 100 + 40 + 9 − 6 − 3 − 10 = 130.

How you could have avoided these mistakes

By trying to work directly with letter counts you made it really hard for you to check your own work. How many letters are there in "one hundred and ten" again? Is it 17, or is it 16? It would have been much easier for you to test your work if you had adopted a strategy like this:

unit_names = """zero one two three four five six seven eight nine ten
                eleven twelve thirteen fourteen fifteen sixteen seventeen
                eighteen nineteen""".split()
tens_names = """zero ten twenty thirty forty fifty sixty seventy eighty
                ninety""".split()

def english(n):
    "Return the English name for n, from 0 to 999999."
    if n >= 1000:
        thous = english(n // 1000) + " thousand"
        n = n % 1000
        if n == 0:
            return thous
        elif n < 100:
            return thous + " and " + english(n)
        else:
            return thous + ", " + english(n)
    elif n >= 100:
        huns = unit_names[n // 100] + " hundred"
        n = n % 100
        if n == 0:
            return huns
        else:
            return huns + " and " + english(n)
    elif n >= 20:
        tens = tens_names[n // 10]
        n = n % 10
        if n == 0:
            return tens
        else:
            return tens + "-" + english(n)
    else:
        return unit_names[n]

def letter_count(s):
    "Return the number of letters in the string s."
    import re
    return len(re.findall(r'[a-zA-Z]', s))

def euler17():
    return sum(letter_count(english(i)) for i in range(1, 1001))

With this approach it's easier to check your result:

>>> english(967)
'nine hundred and sixty-seven'