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Rustprogramming~10 mins

Struct update syntax in Rust - Interactive Code Practice

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Practice - 5 Tasks
Answer the questions below
1fill in blank
easy

Complete the code to create a new struct instance using struct update syntax.

Rust
struct Point { x: i32, y: i32 }

let p1 = Point { x: 5, y: 10 };
let p2 = Point { x: [1], ..p1 };
Drag options to blanks, or click blank then click option'
Ax
Bp1
Cy
D7
Attempts:
3 left
💡 Hint
Common Mistakes
Using 'p1' as the value for 'x' instead of a number.
Trying to use 'y' or 'x' as a value directly.
2fill in blank
medium

Complete the code to create a new struct instance copying all fields except one.

Rust
struct Color { red: u8, green: u8, blue: u8 }

let c1 = Color { red: 255, green: 0, blue: 0 };
let c2 = Color { green: 128, ..[1] };
Drag options to blanks, or click blank then click option'
AColor
Bc2
Cc1
Dgreen
Attempts:
3 left
💡 Hint
Common Mistakes
Using the new variable c2 instead of the original c1.
Using the struct name Color instead of an instance.
3fill in blank
hard

Fix the error in the struct update syntax by completing the code.

Rust
struct Rectangle { width: u32, height: u32 }

let rect1 = Rectangle { width: 30, height: 50 };
let rect2 = Rectangle { width: 40, height: [1] };
Drag options to blanks, or click blank then click option'
Arect1.width
Brect1.height
C50
Dheight
Attempts:
3 left
💡 Hint
Common Mistakes
Using just 'height' without specifying the instance.
Using 'rect1.width' instead of 'rect1.height'.
4fill in blank
hard

Fill both blanks to create a new struct instance with updated fields using struct update syntax.

Rust
struct Circle { radius: f64, color: String }

let c1 = Circle { radius: 5.0, color: String::from("red") };
let c2 = Circle { radius: [1], color: [2], ..c1 };
Drag options to blanks, or click blank then click option'
A10.0
BString::from("blue")
CString::from("green")
D5.0
Attempts:
3 left
💡 Hint
Common Mistakes
Using the old radius or color values instead of new ones.
Not using String::from for the color string.
5fill in blank
hard

Fill all three blanks to create a new struct instance with updated fields and struct update syntax.

Rust
struct Employee { name: String, age: u32, department: String }

let emp1 = Employee { name: String::from("Alice"), age: 30, department: String::from("HR") };
let emp2 = Employee { name: [1], age: [2], department: [3], ..emp1 };
Drag options to blanks, or click blank then click option'
AString::from("Bob")
B25
CString::from("IT")
D30
Attempts:
3 left
💡 Hint
Common Mistakes
Using old values from emp1 instead of new ones.
Not using String::from for string fields.

Practice

(1/5)
1. What does the Rust struct update syntax ..old_struct do when creating a new struct?
easy
A. Copies all remaining fields from old_struct to the new struct
B. Deletes the old_struct after creating the new struct
C. Moves all fields from old_struct without copying
D. Creates a reference to old_struct instead of a new struct

Solution

  1. Step 1: Understand struct update syntax

    The syntax ..old_struct copies all fields from old_struct that are not explicitly set in the new struct.

  2. Step 2: Effect on new struct creation

    This lets you create a new struct with some fields changed, while reusing the rest from old_struct.

  3. Final Answer:

    Copies all remaining fields from old_struct to the new struct -> Option A

  4. Quick Check:

    Struct update syntax copies fields [OK]
Hint: Remember: ..old_struct copies leftover fields [OK]
Common Mistakes:
  • Thinking it moves fields instead of copying
  • Assuming it deletes the old struct
  • Confusing it with references
2. Which of the following is the correct way to use struct update syntax in Rust?
easy
A. let new_user = User { name: "Alice"; ..old_user };
B. let new_user = User { name: "Alice", ..old_user };
C. let new_user = User { name: "Alice" ..old_user };
D. let new_user = User { ..old_user, name: "Alice" };

Solution

  1. Step 1: Check correct syntax order

    In Rust, fields you want to change come first, then ..old_struct copies the rest.

  2. Step 2: Identify correct punctuation and placement

    let new_user = User { name: "Alice", ..old_user }; correctly places ..old_user after the changed field with commas separating fields.

  3. Final Answer:

    let new_user = User { name: "Alice", ..old_user }; -> Option B

  4. Quick Check:

    Changed fields first, then ..old_struct [OK]
Hint: Put changed fields before ..old_struct [OK]
Common Mistakes:
  • Placing ..old_struct before changed fields
  • Missing commas between fields
  • Using semicolons inside struct initialization
3. Given the code:
struct Point { x: i32, y: i32 }

let p1 = Point { x: 5, y: 10 };
let p2 = Point { x: 8, ..p1 };
println!("({}, {})", p2.x, p2.y);

What is the output?
medium
A. (8, 0)
B. (5, 10)
C. Compilation error
D. (8, 10)

Solution

  1. Step 1: Analyze struct initialization for p2

    p2 sets x to 8 explicitly, and copies y from p1 (which is 10) using struct update syntax.

  2. Step 2: Determine printed values

    Printing p2.x and p2.y outputs (8, 10).

  3. Final Answer:

    (8, 10) -> Option D

  4. Quick Check:

    Changed x=8, copied y=10 [OK]
Hint: Changed fields override, others copy from old struct [OK]
Common Mistakes:
  • Assuming y resets to 0
  • Thinking x stays 5
  • Expecting a compile error
4. What is wrong with this Rust code?
struct User { name: String, age: u32 }

let old_user = User { name: String::from("Bob"), age: 30 };
let new_user = User { age: 25, ..old_user };
println!("{} is {} years old", new_user.name, new_user.age);
medium
A. It causes a syntax error due to missing commas
B. It causes a move error because old_user is partially moved
C. It compiles and runs without errors
D. It prints wrong age value

Solution

  1. Step 1: Understand ownership with struct update

    Using ..old_user moves non-Copy fields like name (String) that are not explicitly overridden, while age is overridden with a new value.

  2. Step 2: Check usage of old_user

    old_user is not used after the update, so no ownership error occurs. The code compiles and runs, printing "Bob is 25 years old".

  3. Final Answer:

    It compiles and runs without errors -> Option C

  4. Quick Check:

    Non-Copy fields moved, but old_user not reused [OK]
Hint: Non-Copy fields move with ..old_struct, but OK if old not reused [OK]
Common Mistakes:
  • Thinking it causes a partial move error
  • Expecting automatic cloning of String
  • Assuming syntax error instead of checking ownership
5. You have a struct:
struct Config { debug: bool, version: u32, name: String }

You want to create a new Config with debug set to true, keep version the same, and change name to "NewApp". Which code correctly uses struct update syntax and avoids ownership errors?
hard
A. let new_config = Config { debug: true, name: String::from("NewApp"), ..old_config };
B. let new_config = Config { debug: true, ..old_config, name: String::from("NewApp") };
C. let new_config = Config { debug: true, name: "NewApp", ..old_config };
D. let new_config = Config { debug: true, name: old_config.name, ..old_config };

Solution

  1. Step 1: Place changed fields before struct update

    Changed fields debug and name must come before ..old_config.

  2. Step 2: Avoid ownership errors by creating new String

    Since name is a String, assign a new String with String::from("NewApp") to avoid moving old_config.name.

  3. Final Answer:

    let new_config = Config { debug: true, name: String::from("NewApp"), ..old_config }; -> Option A

  4. Quick Check:

    Changed fields first, clone String to avoid move [OK]
Hint: Clone or create new String before ..old_struct [OK]
Common Mistakes:
  • Placing ..old_config before changed fields
  • Using string literal instead of String::from
  • Moving old_config.name causing ownership error
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