雖然我們大多數人可能從來沒有停下來思考過這個問題，但字母字元在表示它們所需的位元組數上並不完全相同。但為什麼呢？今天的超級使用者問答帖子回答了一位好奇的讀者的問題。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

維基百科提供的部分ASCII圖表截圖。

問題

超級使用者閱讀器khajvah想知道為什麼不同的字母表在儲存時佔用不同的磁碟空間：

When I put ‘a’ in a text file and save it, it makes it 2 bytes in size. But when I put a character like ‘ա’ (a letter from the Armenian alphabet) in, it makes it 3 bytes in size.

What is the difference between alphabets on a computer? Why does English take up less space when saved?

字母就是字母，對吧？也許不是！這個按字母順序排列的謎的答案是什麼？

答案

超級使用者貢獻者Doktoro Reichard和ernie為我們提供了答案。首先，Doktoro Reichard：

One of the first encoding schemes to be developed for use in mainstream computers is the ASCII (American Standard Code for Information Interchange) standard. It was developed in the 1960s in the United States.

The English alphabet uses part of the Latin alphabet (for instance, there are few accented words in English). There are 26 individual letters in that alphabet, not c***idering case. And there would also have to exist the individual numbers and punctuation marks in any scheme that pretends to encode the English alphabet.

The 1960s was also a time when computers did not have the amount of memory or disk space that we have now. ASCII was developed to be a standard representation of a functional alphabet across all American computers. At the time, the decision to make every ASCII character 8 bits (1 byte) long was made due to technical details of the time (the Wikipedia article menti*** the fact that perforated tape held 8 bits in a position at a time). In fact, the original ASCII scheme can be tran**itted using 7 bits, and the eighth could be used for parity checks. Later developments expanded the original ASCII scheme to include several accented, mathematical, and terminal characters.

With the recent increase of computer usage across the world, more and more people from different languages had access to a computer. That meant that, for each language, new encoding schemes had to be developed, independently from other schemes, which would conflict if read from different language terminals.

Unicode came into being as a solution to the existence of different terminals by merging all possible meaningful characters into a single abstract character set.

UTF-8 is one way to encode the Unicode character set. It is a variable-width encoding (i.e. different characters can have different sizes) and it was designed for backwards compatibility with the former ASCII scheme. As such, the ASCII character set will remain one byte in size whilst any other characters are two or more bytes in size. UTF-16 is another way to encode the Unicode character set. In comparison to UTF-8, characters are encoded as either a set of one or two 16-bit code units.

As stated in other comments, the ‘a’ character occupies a single byte while ‘ա’ occupies two bytes, denoting a UTF-8 encoding. The extra byte in the original question was due to the existence of a newline character at the end.

接下來是厄尼的回答：

1 byte is 8 bits, and can thus represent up to 256 (2^8) different values.

For languages that require more possibilities than this, a simple 1 to 1 mapping can not be maintained, so more data is needed to store a character.

Note that generally, most encodings use the first 7 bits (128 values) for ASCII characters. That leaves the 8th bit, or 128 more values for more characters. Add in accented characters, Asian languages, Cyrillic, etc. and you can easily see why 1 byte is not sufficient for holding all characters.

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