String Basics and Memory Representation in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new empty String in Rust.

DSA Rust

let mut s = String::[1]();

Drag options to blanks, or click blank then click option'

Anew

Blen

Cpush

Dfrom

Attempts:

3 left

💡 Hint

Common Mistakes

Using from without an argument causes an error.

Trying to use push to create a String.

The String::new() function creates a new empty String.

2fill in blank

medium

Complete the code to convert a string slice to a String.

DSA Rust

let s = "hello".[1]();

Drag options to blanks, or click blank then click option'

Apush_str

Bclone

Cto_string

Dlen

Attempts:

3 left

💡 Hint

Common Mistakes

Using clone on a &str does not create a String.

Using push_str requires a mutable String.

The to_string() method converts a string slice (&str) to a String.

3fill in blank

hard

Fix the error in the code to append a character to a String.

DSA Rust

let mut s = String::new();
s.[1]('a');

Drag options to blanks, or click blank then click option'

Apush

Bappend

Cadd

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using append which expects a &str, not a char.

Using add which is not a method on String.

The push() method appends a single character to a String.

4fill in blank

hard

Fill both blanks to create a String from a &str and then get its length.

DSA Rust

let s = String::[1]("hello");
let len = s.[2]();

Drag options to blanks, or click blank then click option'

Afrom

Bto_string

Clen

Dcapacity

Attempts:

3 left

💡 Hint

Common Mistakes

Using to_string as a static method instead of on a &str.

Using capacity instead of len for length.

String::from("hello") creates a String from a string slice.
len() returns the number of bytes in the String.

5fill in blank

hard

Fill both blanks to create a String, append a &str, and then print it.

DSA Rust

let mut s = String::[1]("Hello");
s.[2](" World");
println!("{}", s);

Drag options to blanks, or click blank then click option'

Afrom

Bpush_str

Dto_string

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of push_str for &str.

Trying to call to_string on String before printing.

String::from("Hello") creates the String.
push_str(" World") appends the string slice.
Printing s directly shows the full string.

Practice

1. What does the String type in Rust represent?

easy

A. A growable, heap-allocated string

B. A fixed-size array of characters

C. A pointer to a string literal

D. An immutable string slice

Solution

1. Step 1: Understand Rust string types

   Rust has two main string types: String and string slices (&str). String is growable and stored on the heap.

2. Step 2: Compare options with definitions

   A growable, heap-allocated string correctly describes String as growable and heap-allocated. Others describe slices or fixed arrays.

3. Final Answer:

   A growable, heap-allocated string -> Option A

4. Quick Check:

   String = growable heap string [OK]

Hint: Remember: String is growable and heap-allocated [OK]

Common Mistakes:

• Confusing String with &str (string slice)
• Thinking String is fixed size
• Assuming String is immutable

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

easy

A. let s = String::new();

B. let s = String();

C. let s = new String();

D. let s = String[];

Solution

1. Step 1: Recall Rust String creation syntax

   In Rust, String::new() creates a new empty String.

2. Step 2: Check each option syntax

   let s = String::new(); uses correct Rust syntax. The other options use invalid Rust syntax for creating a String.

3. Final Answer:

   let s = String::new(); -> Option A

4. Quick Check:

   String::new() creates empty String [OK]

Hint: Use String::new() to create empty String [OK]

Common Mistakes:

• Using parentheses like a function call without ::new()
• Trying to use new keyword like other languages
• Using array brackets for String

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

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

medium

A. Runtime error

B. hel

C. hello

D. he

Solution

1. Step 1: Understand string slicing in Rust

   The slice &s[0..2] takes characters from index 0 up to but not including 2.

2. Step 2: Extract substring from "hello"

   Characters at index 0 and 1 are 'h' and 'e', so the slice is "he".

3. Final Answer:

   he -> Option D

4. Quick Check:

   Slice 0..2 of "hello" = "he" [OK]

Hint: Slice 0..n means characters from 0 up to n-1 [OK]

Common Mistakes:

• Assuming slice includes index 2
• Expecting full string output
• Thinking slicing causes runtime error

4. Identify the error in this Rust code snippet:

let s = String::from("hello");
let slice = &s[0..4];
println!("{}", slice);
let ch = &s[4];

medium

A. Slice range 0..4 is invalid

B. Cannot index String with single usize

C. Missing semicolon after println!

D. String::from is not valid

Solution

1. Step 1: Check indexing rules for String

   Rust does not allow indexing a String with a single usize like s[4] because strings are UTF-8 encoded and indexing by byte is unsafe.

2. Step 2: Verify other parts

   Slice &s[0..4] is valid. Semicolons and String::from usage are correct.

3. Final Answer:

   Cannot index String with single usize -> Option B

4. Quick Check:

   Single index on String causes error [OK]

Hint: Use slices, not single index, to access String parts [OK]

Common Mistakes:

• Trying to access String by single index
• Confusing slice range validity
• Ignoring Rust UTF-8 string rules

5. Given a UTF-8 encoded Rust String containing emoji and letters, which method safely returns the number of characters (not bytes)?

hard

A. s.len()

B. s.as_bytes().len()

C. s.chars().count()

D. s.bytes().count()

Solution

1. Step 1: Understand difference between bytes and characters

   UTF-8 strings store characters as 1 or more bytes. len() returns byte count, not character count.

2. Step 2: Identify method counting characters

   s.chars().count() iterates over Unicode scalar values (characters) and counts them correctly.

3. Final Answer:

   s.chars().count() -> Option C

4. Quick Check:

   chars().count() counts characters, not bytes [OK]

Hint: Use chars().count() for character count, not len() [OK]

Common Mistakes:

• Using len() which counts bytes, not characters
• Confusing bytes() with chars()
• Assuming as_bytes().len() counts characters