convert-number-to-words

categories
string,array

Convert a given number to words

The key is to process a number's digits from left to right. Each digit is expressed in the format {digit-word} {place} (e.g. two thousand, three hundred). The combination of all the words for each digit is the result.

So, to process each digit and get its place, say starting with the thousands:

place = 1000
while number > 0:
    digit = number / place;
    
    number %= place # First digit done. Cut it from the number
    place /= 10 # The next place to process

But it gets tricky when the remaining number is less than 100. English is quirky when expressing these numbers.

When 20 <= number < 100, instead of a suffix we have some special prefixes to use: twenty, thirty, etc. But the same rule flow applies, we express the first digit in "tens" words, then append the conversion of the last digit.

So 99 becomes ninety-nine, 68 becomes sixty-eight, you get the drill.

When 0 <= number < 20, we throw the digit-by-digit processing and use the single-word terms for these numbers: one, two ... eleven, twelve, and so on.

In summary, one way to approach this is to process the more straightforward digits first when the number >= 100 in one loop. Then whatever is left, process it after with the special rules.

place = 1000
while number >= 100:
    digit = number / place

    result += # the digit + suffix 

    number %= place # First digit done. Cut it from the number
    place /= 10 # The next place to process

while number > 0:
    if number >= 20:
        result += tens[number / 10]
        number %= 10
        # add separator '-' or ' ' to result if there's still left
    else:
        result += twenty_below[number]
        number = 0