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I'm trying to develop an API to normalize Japanese numbers into arabic numbers. Since I'm not familiar with Japanese numbers, there are three questions for which I did not find an answer:

  1. How do we write negative numbers in kanji ? for example, what would be the syntax of -1234 ? Does one use the minus sign as in English, or some other sign like 負 ?
  2. For the decimal separator, I read many things, and the source do not have the same answer: some say that it can be as in English, so for example "三千2百2十.三" will be equal to "3220.3". Some other source say that the separator is "點" or "点" and hence "七十五點四〇二五" is equal to "75.4025". So should I support the three characters as decimal separator ?
  3. About the zero, which one is really used ? "零" or "〇" ?
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I'm majoring in Computer Sciences and studying in Japan. Here are what I normally hear them being used:

  1. In Japanese, the term to describe the negative number or value in general is 負の数. However, when you are saying a specific number like -1, -2 or -1234, we use マイナス1, マイナス2, マイナス1234 respectively.
  2. Both 点 and 點 describe the decimal separator. However, the kanji 点 is being used more often since it is a modern Kanji whereas 點 is an old and out-dated kanji. Since you are developing an API to normalize it, you can opt to go for both with 点 being the default.
  3. Kanji for zero is 零(Rei). That is correct but when we write it out, it's レイ or れい. In my class, both the lecturers and students tend to use ゼロ(loan words of zero) or マル(Maru, which as you wrote above, is 〇 but we don't convert it into a circle). We hardly use 零 unless you are writing about the history of something or someone else's name. It may be used in other field of areas but in programming and our daily life, hardly anyone writes Rei as 零. With that being said, there is a possibility Rei is being written as kanji on the computer as the computer provides the option to choose the Kanji.
  • I think OP is asking for the decimal separator itself ("・"), not the name/reading to describe the separator ("点"). No one writes 3.5 as "3point5". – naruto Jun 26 '18 at 5:23
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Kanji is no longer actively used to represent complicated numbers in most part of our daily life, and it's not a suitable way to describe numbers in technical contexts. Kanji numbers are still used in vertical writing. When people really need to write negative fractional numbers in modern vertically-written materials (e.g. novels and newspapers), -43.05, for example, will usually look like this:

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Katakana マイナス indicates the number is negative, and a centered dot ("中黒") is used in place of the decimal sign. (By the way, 点 "ten" is the reading of this sign, not the actual symbol you write.) The pseudo-kanji is usually used in this context. This is a relatively recent convention only used in vertical writing. The same thing written horizontally (マイナス四三・〇五) looks pretty unnatural.

Lastly, even if you're interested only in positive integers, there are still several ways to write them. Please see my previous answer.

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