Assumptions when dealing with decimals and thousand separators

I have enjoyed the lists of typical, and erroneous, assumptions we developers make with regards to dates and time, geography, names, addresses and genders. So I will here add a short list of erroneous assumptions commonly made about decimals and separators:

  1. The decimal point and the separator is globally the same.
    The wikipedia page for the decimal mark lists at least 11 different characters and combinations.
  2. The decimal point is a dot and the thousand separator is a comma.
    This is the default in programming, thanks to most development principles having originated in USA. But, alas, in most European countries it is exactly the opposite.
  3. At least, they are the same for every country.
    Except when they are not. Canada uses different rules depending on language.
  4. What about for all situations within the same language?
    No, sorry. The Swiss use one rule currency and what is practically the exact opposite for other numbers.
  5. I got it now. At least they are always found in the ASCII character set.
    Of course not, not even close. The Japanese prefer characters found in their own character set. Eastern Arabic numerals likewise.
  6. OK, then I can assume that thousand separators are for every 3 digits, right?
    Not even close. If the Chinese or Japanese group their digits they prefer 4 digits.
  7. The rules for printed and handwritten numbers be the same?
    Sorry, some countries have different traditions depending on form. Spain, Italy, Switzerland and others are the special cases here.
  8. If there is a rule for separators for every X digits, they always apply. Definitely.
    Well, even the Americans seem to get this one wrong, as 4 digit numbers are often written without a thousand separator, but 5-or-more digit numbers with. This is however nothing compared to the Indians who find it natural to but a separator with before the final 3 digits, but for every 2 digits before that.
  9. The separators are the same for all positions within a single number.
    Yes. Except of course if the Japanese or Chinese choose to use thousand, or rather, myriad, separators, which depend on position.

See Wikipedia:Decimal_mark, Wikipedia:Japanese_numerals, Wikipedia:Eastern_Arabic_NumeralsHow the world separates its digits and How the world separates its decimals.

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