Characters (Edexcel GCSE Computer Science): Revision Notes

Characters

What are characters in computing?

When you type on a keyboard or see text on a screen, computers need a way to understand and store each letter, number, and symbol. Computers work with binary code (1s and 0s), so they need a system to convert characters into numbers that can be represented in binary. This is where character encoding comes in.

The most common character encoding system is called ASCII, which stands for American Standard Code for Information Interchange. Think of ASCII as a giant dictionary that tells the computer exactly which number represents each character you might want to use.

Understanding ASCII code

ASCII is a 7-bit code system, which means it uses 7 binary digits to represent each character. With 7 bits, you can create 128 different combinations ($2^7 = 128$), giving us 128 possible character codes numbered from 0 to 127.

Each character has its own unique ASCII code number. For example:

• The letter 'C' has the ASCII code 67 (which is 100 0011 in binary)
• A space character has the ASCII code 32 (which is 010 0000 in binary)

The complete set of these 128 codes is called the character set - this is the list of all binary codes that the computer's hardware and software can recognise and work with.

How ASCII codes are organised

ASCII codes aren't just random numbers - they're carefully organised into groups based on what type of character they represent. This makes the system logical and easier to work with.

The ASCII codes are grouped as follows:

• Codes 0-32: Control codes like SHIFT and SPACE - these control how text behaves rather than displaying visible characters
• Codes 33-47: Symbols such as punctuation marks (!, ", #, $, %, &, ', (, ), *, +, comma, -, ., /)
• Codes 48-57: The digits 0 through 9 in order
• Codes 58-64: More symbols including @, <, >, and =
• Codes 65-90: Uppercase letters A through Z in alphabetical order
• Codes 91-96: Symbols like square brackets [ ], backslash , and underscore _
• Codes 97-122: Lowercase letters a through z in alphabetical order
• Codes 123-127: Final symbols including curly braces { }, and DEL

This organisation is really clever! Notice how all the uppercase letters are together in one block, and all the lowercase letters are in another block. The digits are also all together. This makes it easy to work with characters programmatically.

Using ASCII codes in Python

Python provides two very useful functions for working with ASCII codes:

The ord() function

The ord() function takes a character and returns its ASCII code number.

code = ord("c")  # Returns 99

The chr() function

The chr() function does the opposite - it takes an ASCII code number and returns the corresponding character.

char = chr(100)  # Returns "d"

These functions are incredibly useful when you need to process text or manipulate characters in your programmes.

# Convert a character to its ASCII code
letter = 'A'
ascii_code = ord(letter)
print(f"The ASCII code for '{letter}' is {ascii_code}")  # Output: 65

# Convert an ASCII code back to a character
code_number = 72
character = chr(code_number)
print(f"ASCII code {code_number} represents '{character}'")  # Output: 'H'

Finding ASCII codes for other characters

One of the most useful features of ASCII is that related characters have consecutive codes. This means if you know the code for one character, you can easily work out the codes for nearby characters.

For example, since uppercase and lowercase letters are arranged alphabetically:

• If you know 'a' is ASCII code 97, then 'c' must be 99 (97 + 2, since 'c' is 2 letters after 'a')
• If you know 'S' is ASCII code 83, then 'W' must be 87 (83 + 4, since 'W' is 4 letters after 'S')

This pattern works because the ASCII codes follow the alphabetical order exactly. The same principle applies to digits - '0' is code 48, so '5' is code 53.

Worked example: processing strings with ASCII

Let's look at a practical example of using ASCII codes in programming:

for index in range(len(myString)):
    print(ord(myString[index]))

Real-world applications

ASCII codes are everywhere in computing:

• File formats: Text files use ASCII to store readable content
• Programming: Many programming languages use ASCII for variable names and code
• Data transmission: When sending text over networks, ASCII ensures compatibility
• Databases: Text data in databases is often stored using ASCII encoding

Exam tips

Understanding ASCII is crucial for computer science exams, and questions often test both theoretical knowledge and practical application.

Key things to remember for exams:

• Uppercase A = 65, lowercase a = 97 (there's a difference of 32)
• Digits start at 48 ('0' = 48, '1' = 49, etc.)
• Letters are consecutive (if you know one, you can calculate others)
• Space character is code 32