How Strings Work Differently Across Languages in DSA Rust - Debugging & Practice

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust string slicing code?

Consider the following Rust code that slices a string. What will be printed?

DSA Rust

fn main() {
    let s = String::from("hello");
    let slice = &s[0..2];
    println!("{}", slice);
}

A "he"

B "hel"

C "hello"

D Runtime panic

💡 Hint

Remember Rust string slices use byte indices and must be on valid UTF-8 boundaries.

The slice &s[0..2] takes the first two bytes of the string "hello", which correspond to the characters 'h' and 'e'. So it prints "he".

❓ Predict Output

intermediate

What happens when slicing a UTF-8 string incorrectly in Rust?

What error or output results from this Rust code?

DSA Rust

fn main() {
    let s = String::from("Здравствуйте");
    let slice = &s[0..1];
    println!("{}", slice);
}

A Runtime panic due to invalid UTF-8 slice

B Compilation error

C Prints an empty string

D Prints the first character "З"

💡 Hint

UTF-8 characters can be multiple bytes; slicing must respect character boundaries.

The character "З" is 2 bytes in UTF-8. Slicing only the first byte causes a runtime panic because it breaks UTF-8 encoding rules.

🔧 Debug

advanced

Why does this Rust code fail to compile when concatenating strings?

Identify the error in this Rust code and what causes it.

DSA Rust

fn main() {
    let s1 = String::from("Hello, ");
    let s2 = "world!";
    let s3 = s1 + s2;
    println!("{}", s3);
    println!("{}", s1);
}

A Runtime panic due to invalid string concatenation

B Compilation error because s2 is not a String

C Prints "Hello, world!" and then "Hello, "

D Compilation error because s1 is moved and cannot be used after +

💡 Hint

The + operator moves the left String operand in Rust.

The + operator takes ownership of s1, so s1 cannot be used after concatenation. The second println! causes a compile error.

❓ Predict Output

advanced

What is the output of this Rust code using String::from_utf8_lossy?

What does this Rust code print?

DSA Rust

fn main() {
    let bytes = vec![240, 159, 146, 150, 255];
    let s = String::from_utf8_lossy(&bytes);
    println!("{}", s);
}

A "�����"

B Compilation error

C "💖�"

D Runtime panic

💡 Hint

from_utf8_lossy replaces invalid bytes with replacement character �.

The bytes represent a valid UTF-8 heart emoji followed by an invalid byte 255, replaced by �.

🧠 Conceptual

expert

Why does Rust differentiate between String and &str types?

Choose the best explanation for why Rust has both String and &str types.

A &str owns the data; String is a pointer to &str

B String owns its data and is mutable; &str is a borrowed view and usually immutable

C String is for ASCII only; &str is for UTF-8 strings

D String is a fixed-size array; &str is a dynamic array

💡 Hint

Think about ownership and mutability in Rust.

String owns its heap-allocated data and can be changed; &str is a borrowed reference to string data and is usually immutable.

Practice

1. In Rust, which of the following best describes how String differs from string types in some other languages like Python or JavaScript?

easy

A. String in Rust is immutable and fixed-length like Python strings.

B. String in Rust is a growable, heap-allocated UTF-8 encoded string.

C. String in Rust stores characters as UTF-16 code units internally.

D. String in Rust is a simple alias for a byte array without encoding.

Solution

1. Step 1: Understand Rust's String type

   Rust's String is designed to be growable and stored on the heap, unlike fixed-length strings in some languages.

2. Step 2: Compare with other languages

   Languages like Python have immutable strings, and JavaScript strings are UTF-16 encoded internally, but Rust uses UTF-8 encoding for String.

3. Final Answer:

   String in Rust is a growable, heap-allocated UTF-8 encoded string. -> Option B

4. Quick Check:

   Rust String = growable UTF-8 heap string [OK]

Hint: Remember Rust strings are UTF-8 and growable, unlike fixed immutable strings [OK]

Common Mistakes:

• Thinking Rust strings are immutable like Python strings
• Assuming Rust strings use UTF-16 encoding
• Confusing String with byte arrays without encoding

2. Which of the following is the correct way to create a mutable String in Rust?

easy

A. let mut s = String::from("hello");

B. let s = "hello";

C. let s = String::new("hello");

D. let s = "hello".to_string();

Solution

1. Step 1: Identify mutable string creation syntax

   In Rust, to create a mutable String, you declare it with mut and use String::from or to_string().

2. Step 2: Check each option

   let s = "hello"; creates an immutable string slice (&str), not a String. let mut s = String::from("hello"); correctly creates a mutable String. let s = String::new("hello"); is invalid syntax. let s = "hello".to_string(); creates a String but lacks mut, making the binding immutable.

3. Final Answer:

   let mut s = String::from("hello"); -> Option A

4. Quick Check:

   Mutable String = let mut s = String::from(...) [OK]

Hint: Use let mut s = String::from(...) for mutable strings [OK]

Common Mistakes:

• Using string literals without mut for mutability
• Trying to use String::new("hello") which is invalid
• Confusing string slices with String type

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

let s = String::from("hello");
let h = &s[0..1];
println!("{}", h);

medium

A. Runtime panic due to invalid UTF-8 slicing

B. he

C. h

D. hello

Solution

1. Step 1: Understand Rust string slicing rules

   Rust strings are UTF-8 encoded, so slicing must be on valid UTF-8 boundaries. Index 0..1 slices the first byte only.

2. Step 2: Check if slicing 0..1 is valid UTF-8

   For "hello", ASCII characters are 1 byte each, so 0..1 is valid and equals "h".

3. Step 3: Confirm output

   println!("{}", h) where h = &s[0..1] = "h" outputs h.

4. Final Answer:

   h -> Option C

5. Quick Check:

   Slice 0..1 of "hello" = "h" [OK]

Hint: Remember ASCII chars are 1 byte, so 0..1 slice is valid [OK]

Common Mistakes:

• Assuming slicing 0..1 causes panic always
• Confusing byte index with character index
• Expecting multiple characters from 0..1 slice

4. Identify the error in this Rust code snippet:

let s = String::from("Здравствуйте");
let slice = &s[0..1];
println!("{}", slice);

medium

A. No error, prints first character

B. Prints garbage characters

C. Compile-time error: invalid slice syntax

D. Runtime panic: slicing not on UTF-8 boundary

Solution

1. Step 1: Analyze the string content and slicing

   The string "Здравствуйте" contains Cyrillic characters, each encoded in multiple bytes in UTF-8.

2. Step 2: Check slicing 0..1 validity

   Slicing 0..1 takes only the first byte, which is not a valid UTF-8 character boundary for Cyrillic letters, causing a runtime panic.

3. Final Answer:

   Runtime panic: slicing not on UTF-8 boundary -> Option D

4. Quick Check:

   UTF-8 multi-byte chars require valid slice boundaries [OK]

Hint: Multi-byte UTF-8 chars cannot be sliced by single bytes [OK]

Common Mistakes:

• Assuming slicing always works like ASCII
• Ignoring UTF-8 multi-byte character boundaries
• Expecting compile-time error instead of runtime panic

5. Given a Rust String containing Unicode text, which approach correctly iterates over each Unicode character safely?

hard

A. Using for c in s.chars() to iterate over Unicode scalar values.

B. Using for i in 0..s.len() and slicing &s[i..i+1] for each byte.

C. Using for c in s.bytes() to get each character.

D. Using for c in s.split(' ') to get characters.

Solution

1. Step 1: Understand Rust string iteration methods

   s.chars() iterates over Unicode scalar values (characters) safely, respecting UTF-8 boundaries.

2. Step 2: Evaluate other options

   Using for i in 0..s.len() and slicing &s[i..i+1] slices bytes which can cause invalid UTF-8. for c in s.bytes() iterates bytes, not characters. for c in s.split(' ') splits by spaces, not characters.

3. Final Answer:

   Using for c in s.chars() to iterate over Unicode scalar values. -> Option A

4. Quick Check:

   Use s.chars() for safe Unicode iteration [OK]

Hint: Use s.chars() to safely iterate Unicode characters [OK]

Common Mistakes:

• Iterating bytes instead of characters
• Slicing string by byte indices unsafely
• Using split which divides by delimiter, not characters