← IGCSE Computer Science Paper 1

Character set

The agreed list of characters a system can use, with a unique binary code for each one, so every device reads the same code as the same character.

Part of: IGCSE Computer Science 0478 — 1.2 Text, Sound and Images — Module 4, ACARA Digital Technologies Years 7–8 — Representing Text, Images and Sound — Module 15

Understanding Character set in IGCSE Computer Science Paper 1

What it does: Adding a bit doubles the number of unique codes, so a set with more characters needs codes with more bits. A letter, a digit, a punctuation mark and a symbol are all characters in the set, and each has a different binary code. A character set gives every character a unique code. If two characters shared a code, the computer could not tell them apart. A character set covers more than letters. It includes digits like 7, punctuation like ! and symbols like £ or #. Character sets are agreed in advance, so any device that uses the same set reads a code as the same character.

Common mistakes: The number of codes n bits can store is 2 to the power n, so the line must be size = 2 ** bits. With 7 bits that gives 128 codes, but 2 * bits would give only 14. ord() turns a character into its number code. chr() does the opposite and turns a number into a character. The line should be code = ord(letter).

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This module's key terms are also in a printable crossword (Modules 28–41) and word search (Modules 28–58).

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Learning

Duo

What is a character set?

An agreed list of characters, each with its own unique binary codeA list of characters that all share the same binary code

Matches

Phone A uses one character set and Phone B uses a different one. A text sent as 01000001 is always shown as 'A' on both phones.

↓

Devices must share the same character set. If the sets differ, the same binary code can be shown as a different character, so the message is garbled.

Applying

Meanings

Term

character set

Meaning

The agreed list of characters a system can use, with a unique binary code for each one, so every device reads the same code as the same character.

Practice

// Character set: a unique code for each

Testing

Gaps

code = ("A")

Check Answer

Spots

1bits = 7

2size = 2 * bits

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Quick Questions

1. What is a character set?

A. An agreed list of characters, each with its own unique binary code ✓

B. A list of characters that all share the same binary code

A character set gives every character a unique code. If two characters shared a code, the computer could not tell them apart.

2. Why must two devices use the same character set to share a text message?

A. So the message uses fewer bits when it is sent

B. So the codes sent are read as the same characters ✓

If the sender and receiver use different sets, the same binary code could be shown as a different character, so the text would look wrong.

3. Which of these can be stored in a character set?

A. Only the letters A to Z

B. Letters, digits, punctuation and symbols ✓

A character set covers more than letters. It includes digits like 7, punctuation like ! and symbols like £ or #.

4. The digit 5 typed as text is stored as...

A. A character with its own code in the set ✓

B. No code, because it is a number

When typed as text, the digit 5 is a character. It has its own code, which is different from the binary number 5.

5. A character set needs to add thousands more characters. What happens to the length of the codes?

A. Longer codes are needed, so more bits per character ✓

B. The codes can stay the same length

n bits give 2^n unique codes. A bigger set needs more unique codes, so each code needs more bits.

Match the Pairs

character set → The agreed list of characters a system can use, each with its own unique binary code

01000001 = 'A' → One entry in a character set: a unique binary code that every device reads as the same character

A bigger character set needs longer codes, because more bits are needed to give each character a unique code → 7 bits = 128 codes, 8 bits = 256 codes

A character set covers letters, digits, punctuation and symbols, and each one gets its own code → 'a' = 01100001, '7' = 00110111, '!' = 00100001, '#' = 00100011

Phone A uses one character set and Phone B uses a different one. A text sent as 01000001 is always shown as 'A' on both phones. → Devices must share the same character set. If the sets differ, the same binary code can be shown as a different character, so the message is garbled.

A character set with 300 characters can give every character a unique code using only 8 bits. → 8 bits give only 256 different codes, which is fewer than 300. The set needs longer codes, such as 9 bits or more.

Key Term

character set

The agreed list of characters a system can use, with a unique binary code for each one, so every device reads the same code as the same character.

Fill in the Blanks

1. code = ord("A") print(code)

Hint: Type the built-in function that returns the number code for a character.

2. print(chr(65))

Hint: Type the built-in function that turns a number code back into its character.

3. print(bin(ord("A")))

Hint: Type the built-in function that shows a number as binary.

4. bits = 8 characters = 2 ** bits print(characters)

Hint: Type the variable that holds the code length, since more bits per code means a bigger set.

5. print(chr(65) == "A")

Hint: Type the operator that checks whether two values are equal, showing both sides read the same code as the same character.

Spot the Error

Question 1

A bits = 7

B size = 2 * bits

C print(size)

Line B — The number of codes n bits can store is 2 to the power n, so the line must be size = 2 ** bits. With 7 bits that gives 128 codes, but 2 * bits would give only 14.

Question 2

A letter = 'A'

B code = chr(letter)

C print(code)

Line B — ord() turns a character into its number code. chr() does the opposite and turns a number into a character. The line should be code = ord(letter).

Question 3

A code = ord('A')

B next_code = code + 1

C print(ord(next_code))

Line C — next_code is already a number, so ord() cannot be used on it. To show the character that has that code, use chr(next_code), which prints B.

Question 4

A bits = '8'

B chars = 2 ** bits

C print(chars)

Line A — The number of bits must be a number, not a string. With bits = '8', the calculation 2 ** bits causes an error. It should be bits = 8.

Question 5

A char = A

B code = ord(char)

C print(code)

Line A — A character must be written inside quotes. Without quotes Python treats A as a variable name that does not exist. It should be char = 'A'.

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