Character type in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Rust char type used for?

The char type in Rust represents a single Unicode scalar value, which means it can hold any valid Unicode character, including letters, numbers, emojis, and symbols.

beginner

How many bytes does a Rust char use in memory?

A Rust char uses 4 bytes (32 bits) in memory to store a Unicode scalar value.

beginner

How do you write a character literal in Rust?

You write a character literal using single quotes, like 'a', '😊', or '\n' for newline.

beginner

Can a Rust char hold multiple characters or strings?

No, a Rust char holds exactly one Unicode scalar value, which is one character. For multiple characters, use a String or string slice &str.

intermediate

How can you convert a char to its numeric Unicode code point in Rust?

You can convert a <code>char</code> to a <code>u32</code> number using the <code>as</code> keyword, like <code>let code = 'A' as u32;</code>.

What is the size in bytes of a Rust char?

A 8 bytes

B 1 byte

C 4 bytes

D 2 bytes

A Rust char always uses 4 bytes to store a Unicode scalar value.

Which of these is a valid Rust character literal?

A "a"

B 'ab'

C "ab"

D 'a'

Rust character literals use single quotes and hold exactly one character, like 'a'.

What does the Rust char type represent?

A A single Unicode scalar value

B A byte of data

C A string of characters

D An integer number

The char type represents a single Unicode scalar value, which can be any valid Unicode character.

How do you convert a char to its Unicode code point number?

A Using <code>char as u32</code>

B Using <code>char.to_u32()</code>

C Using <code>char.to_string()</code>

D Using <code>char.to_int()</code>

You convert a char to a number with char as u32.

Can a Rust char hold an emoji character?

A No, only ASCII characters are allowed

B Yes, because emojis are Unicode scalar values

C Only if converted to a string first

D Only if stored as bytes

Rust char can hold any Unicode scalar value, including emojis.

Explain what the Rust char type is and how it differs from a string.

Think about the difference between a single letter and a word.

Describe how to write and use a character literal in Rust.

Remember how characters are shown in code.

Practice

1. Which of the following correctly declares a char variable in Rust?

easy

A. let letter: char = `a`;

B. let letter: char = 'a';

C. let letter: char = a;

D. let letter: char = "a";

Solution

1. Step 1: Understand Rust char syntax

   Rust uses single quotes for char literals, e.g., 'a'. Double quotes are for strings.

2. Step 2: Check each option

   let letter: char = 'a'; uses single quotes correctly. let letter: char = "a"; uses double quotes (string). let letter: char = a; misses quotes. let letter: char = `a`; uses backticks, which is invalid.

3. Final Answer:

   let letter: char = 'a'; -> Option B

4. Quick Check:

   char uses single quotes [OK]

Hint: Remember: single quotes for char, double quotes for string [OK]

Common Mistakes:

• Using double quotes instead of single quotes
• Omitting quotes around the character
• Using backticks instead of quotes

2. Which of the following is the correct way to assign the emoji 😀 to a char variable in Rust?

easy

A. let smiley: char = '😀';

B. let smiley: char = 😀;

C. let smiley: char = "😀";

D. let smiley: char = `😀`;

Solution

1. Step 1: Recall char literal rules

   Rust char literals must be in single quotes, even for emojis.

2. Step 2: Evaluate options

   let smiley: char = '😀'; uses single quotes correctly with the emoji. Double quotes ("😀") create a string. Bare 😀 lacks quotes. Backticks (`😀`) are invalid.

3. Final Answer:

   let smiley: char = '😀'; -> Option A

4. Quick Check:

   Emoji char uses single quotes [OK]

Hint: Use single quotes for any char, including emojis [OK]

Common Mistakes:

• Using double quotes for char
• Not quoting the emoji
• Using backticks instead of single quotes

3. What will be the output of this Rust code?

let c: char = 'Z';
println!("{}", c.is_alphabetic());

medium

A. true

B. false

C. Z

D. Compile error

Solution

1. Step 1: Understand is_alphabetic() method

   This method returns true if the char is a letter (A-Z or a-z or other alphabets).

2. Step 2: Check the character 'Z'

   'Z' is an uppercase letter, so is_alphabetic() returns true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   'Z'.is_alphabetic() = true [OK]

Hint: Check if char is letter with is_alphabetic() method [OK]

Common Mistakes:

• Confusing output with printing the char itself
• Thinking is_alphabetic() returns the char
• Expecting a compile error

4. Identify the error in this Rust code snippet:

let ch: char = 'ab';

medium

A. Missing semicolon at the end

B. Use double quotes for multiple characters

C. char type cannot store letters

D. Multiple characters in single quotes are not allowed

Solution

1. Step 1: Check char literal rules

   A char can only hold one Unicode character, so 'ab' is invalid.

2. Step 2: Analyze the error

   Using multiple characters inside single quotes causes a compile error.

3. Final Answer:

   Multiple characters in single quotes are not allowed -> Option D

4. Quick Check:

   char holds one char only [OK]

Hint: char must be one character in single quotes [OK]

Common Mistakes:

• Trying to store multiple characters in a char
• Using double quotes for char
• Ignoring compile errors

5. Given a string slice let s = "Rust 🦀";, which Rust code correctly extracts the crab emoji as a char?

hard

A. let crab = s.chars().nth(4).unwrap();

B. let crab = s[5];

C. let crab = s.chars().nth(5).unwrap();

D. let crab = s[4..5].chars().next().unwrap();

Solution

1. Step 1: Understand string and char indexing in Rust

   Rust strings are UTF-8 encoded. The string "Rust 🦀" has 6 chars: 'R'(0),'u'(1),'s'(2),'t'(3),' '(4),'🦀'(5). Crab emoji is the 6th character (index 5), counting chars, not bytes.

2. Step 2: Check each option

   let crab = s.chars().nth(5).unwrap(); correctly uses chars().nth(5) to get the crab emoji. let crab = s.chars().nth(4).unwrap(); gets the space (index 4). let crab = s[5]; uses byte index (u8, not char). let crab = s[4..5].chars().next().unwrap(); byte slice yields space.

3. Final Answer:

   let crab = s.chars().nth(5).unwrap(); -> Option C

4. Quick Check:

   Use chars().nth(5) for crab emoji [OK]

Hint: Use chars().nth(index) to get char by position [OK]

Common Mistakes:

• Indexing string bytes directly
• Using wrong index for emoji
• Not unwrapping Option from nth()