Representing text: ASCII and Unicode

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課程筆記

What is Character Encoding?

Character encoding is a convention that uses a numeric value to represent each character in a writing script.
It covers natural language symbols, plus control characters and whitespace.
The numeric values are called code points; together they form a code space or code page.
Encoded text can be stored, transmitted, and transformed by computers.

Early Character Encodings

Early codes (e.g., Morse code, Baudot code) could only represent a limited set of characters, often just upper-case letters, numerals, and basic punctuation.
Morse code (1840s) used four symbols (short signal, long signal, short space, long space) to create variable-length codes.
Baudot code (1870) was a five-bit encoding, later standardized as ITA2 in 1930.
Punch cards (late 19th century) encoded data by hole positions; later, alphabetic data used multiple punches per column.
IBM's BCD (binary-coded decimal) encodings were six-bit and used in early computers like the IBM 702.

ASCII: The American Standard Code for Information Interchange

ASCII is a widely used character encoding that maps each character to a 7-bit binary code (0–127).
It includes upper and lower case letters, digits, punctuation, and control characters.
ASCII can represent only 128 characters, which is enough for English but not for other languages.
Because it uses 7 bits, ASCII text takes 1 byte per character (with the 8th bit often unused or used for parity).

Limitations of ASCII

ASCII cannot represent characters from non-English languages (e.g., accented letters, Cyrillic, Arabic, Chinese).
It also cannot represent emoji or many special symbols.
To overcome this, various extended ASCII encodings (like ISO/IEC 8859) were created, but they were incompatible with each other.
This led to the need for a universal encoding system.

Unicode: A Universal Character Set

Unicode is a comprehensive encoding system that aims to represent every character from all writing systems, plus emoji and symbols.
Each character has a unique code point (e.g., U+0041 for 'A').
Unicode is extensible, meaning new characters can be added.
It has replaced most earlier encodings because it solves the compatibility problem.

Text becomes computer-readable by mapping each character to a code point and then to binary.

Unicode Encoding Forms: UTF-8 and UTF-16

UTF-8 is the most popular encoding on the World Wide Web (used by 98.9% of surveyed sites as of January 2026).
UTF-8 uses variable-length encoding: characters take 1 to 4 bytes; ASCII characters take 1 byte, so it is backward-compatible with ASCII.
UTF-16 uses 2 or 4 bytes per character and is common in application programs and operating systems.
Both UTF-8 and UTF-16 can represent all Unicode characters.

How Text Size Relates to Length

The file size of text depends on the encoding and the characters used.
In ASCII, each character is 1 byte, so size in bytes equals the number of characters.
In UTF-8, characters may be 1 to 4 bytes; for example, 'A' is 1 byte, 'é' is 2 bytes, and an emoji is 4 bytes.
In UTF-16, most characters are 2 bytes, but some (like emoji) are 4 bytes.
Thus, the same text can have different byte sizes in different encodings.

Why Unicode Matters

Unicode allows computers to handle multiple languages and scripts in a consistent way.
It enables global communication and software that works worldwide.
It supports emoji and other symbols, making digital text more expressive.
Without Unicode, text would be garbled when moving between systems using different encodings.

Unicode covers the world writing systems and emoji in one standard, where ASCII covers only basic Latin.

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1.What does ASCII stand for?
Easy
- AAmerican Standard Code for Information Interchange
- BAmerican System for Computer and Internet Interchange
- CAutomated Standard Code for Information Interchange
- DAmerican Standard Computer Interface Interchange
2.Which character encoding system is the most popular on the World Wide Web as of January 2026?
Easy
- AASCII
- BUTF-8
- CUTF-16
- DEBCDIC
3.ASCII can represent characters from all written languages in the world.
Easy
True or false?
4.Unicode is an extensible encoding system that has replaced most earlier character encodings.
Easy
True or false?
5.Why is Unicode preferred over ASCII for representing text in modern applications?
Medium
- AUnicode is faster to process than ASCII.
- BUnicode uses fewer bits per character.
- CUnicode can represent characters from many different languages and scripts.
- DUnicode is simpler and easier to implement.
6.Which of the following are examples of character encoding systems? (Select all that apply)
Medium
- AMorse code
- BBaudot code
- CPunch card
- DASCII
- EUnicode
7.ASCII was designed to represent all characters in all human languages.
Easy
True or false?
8.Match each character encoding system to its correct description.
Easy
- ASCII
- Unicode
- Baudot code
- An early 5-bit code used in telegraphy
- A 7-bit code for English characters
- An extensible system covering most world scripts

複習筆記

## Representing Text: ASCII and Unicode ### Character Encoding Basics - **Character encoding** is a convention that uses a numeric value (a **code point**) to represent each character in a writing script. - Code points collectively form a **code space** or **code page**. - Encodings can include letters, digits, punctuation, control characters, whitespace, and symbols from constructed languages. - Encoded text can be stored, transmitted, and processed by computers. ### ASCII - **ASCII** (American Standard Code for Information Interchange) is a character encoding that uses **7 bits** to represent **128 characters** (0–127). - It includes upper and lower case English letters, digits, punctuation, and control characters. - **Limitation**: ASCII can only represent a limited set of characters (mainly English), so it cannot handle other scripts (e.g., Greek, Arabic) or emoji. ### Unicode - **Unicode** is a modern, extensible encoding system that aims to represent every character from all writing systems, plus symbols and emoji. - It uses **code points** that can be encoded in different ways, such as **UTF-8** and **UTF-16**. - **UTF-8** is the most popular encoding on the World Wide Web (used by 98.9% of surveyed websites as of January 2026). - UTF-8 uses **variable-length** encoding: common characters (like ASCII) use 1 byte, while others use 2, 3, or 4 bytes. ### Text Size in Bytes - The size of text in bytes depends on the encoding and the characters used. - For example, in UTF-8: - 'A' (ASCII) = 1 byte - 'é' (Latin-1) = 2 bytes - '中' (CJK) = 3 bytes - Emoji (e.g., 😀) = 4 bytes - So, the number of bytes is not always equal to the number of characters. ### Key Takeaway - ASCII is limited to 128 characters; Unicode (especially UTF-8) supports virtually all scripts and symbols, making it the standard for modern computing and the web.

Character encoding: A convention of using a numeric value to represent each character of a writing script.
Code point: The numerical value that represents a character in a character encoding.
Code space / code page: The collective set of code points in a character encoding.
ASCII: American Standard Code for Information Interchange; a 7-bit character encoding that represents 128 characters, including English letters, digits, and punctuation.
Unicode: A well-defined and extensible encoding system that has replaced most earlier character encodings, supporting a vast range of characters from world scripts, symbols, and emoji.
UTF-8: A variable-length Unicode encoding that uses 1 to 4 bytes per character; the most popular encoding on the World Wide Web.
UTF-16: A Unicode encoding that uses 2 or 4 bytes per character; commonly used in application programs and operating systems.

字卡

What is a character set in computing?

A character set is a collection of characters (letters, digits, symbols, control characters) that a computer can represent, each mapped to a unique numeric code.

What is a code point?

A code point is the numeric value assigned to a character in a character encoding, such as 65 for 'A' in ASCII.

What does ASCII stand for?

ASCII stands for American Standard Code for Information Interchange.

How many bits does ASCII use to represent each character?

ASCII uses 7 bits per character, allowing 128 different characters (2⁷ = 128).

What is the range of ASCII code points?

ASCII code points range from 0 to 127, covering English letters, digits, punctuation, and control characters.

What is a major limitation of ASCII?

ASCII can only represent 128 characters, which is insufficient for languages with non-Latin scripts (e.g., Chinese, Arabic) and many symbols like emoji.

What is Unicode?

Unicode is a comprehensive character encoding standard that assigns a unique code point to virtually every character in all human languages, including emoji and historical scripts.

What are UTF-8 and UTF-16?

UTF-8 and UTF-16 are Unicode transformation formats that encode Unicode code points into binary. UTF-8 uses variable-length encoding (1-4 bytes), while UTF-16 uses 2 or 4 bytes per character.

Which Unicode encoding is most popular on the web?

UTF-8 is the most popular, used by 98.9% of surveyed websites as of January 2026.

How does the size of text in bytes relate to its length in Unicode?

In UTF-8, the number of bytes per character varies: ASCII characters use 1 byte, but characters from other scripts may use 2-4 bytes, so byte size is not always equal to character count.

What is a control character?

A control character is a non-printable character in a character set that performs a function, such as newline, tab, or carriage return.

What is a code space or code page?

A code space (or code page) is the complete set of code points defined by a character encoding, such as 0-127 for ASCII.

What was the Baudot code?

The Baudot code is an early 5-bit character encoding invented by Émile Baudot in 1870, used in telegraphy, and later standardized as ITA2.

What is EBCDIC?

EBCDIC (Extended Binary-Coded Decimal Interchange Code) is an 8-bit character encoding developed by IBM in 1963 for the System/360, supporting more characters than earlier 6-bit codes.

Why were early character encodings limited?

Early encodings were limited by hardware constraints, often using only 5 or 6 bits, which could represent only a small set of characters like uppercase letters and digits.

What is the difference between ASCII and Unicode?

ASCII is a 7-bit encoding for 128 characters, while Unicode is a comprehensive standard with over a million code points, supporting many scripts and symbols. ASCII is a subset of Unicode.

How many characters can Unicode represent?

Unicode can represent over 1.1 million code points, though not all are assigned yet.

What is a variable-length encoding?

A variable-length encoding uses different numbers of bits or bytes for different characters, like UTF-8 which uses 1-4 bytes per character, to save space for common characters.

What is the purpose of character encoding?

Character encoding allows computers to store, transmit, and process text by mapping each character to a numeric code that can be represented in binary.

What is the ASCII code for the letter 'A'?

The ASCII code for 'A' is 65 in decimal, or 1000001 in binary.

How many bits are in a byte?

A byte consists of 8 bits.

Why is UTF-8 popular on the web?

UTF-8 is backward-compatible with ASCII, uses less space for common Latin characters, and can represent all Unicode characters, making it efficient and widely supported.

What is a character encoding?

A character encoding is a convention that maps each character in a character set to a numeric value (code point) so that text can be stored and processed digitally.

What is the difference between a character set and a character encoding?

A character set is the collection of characters and their code points, while a character encoding is the scheme for representing those code points in binary (e.g., UTF-8).

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