Handling input values in Rust - Mini Project: Build & Apply

Handling input values

📖 Scenario: You are creating a simple Rust program that asks the user for their age and then tells them if they are old enough to vote.

🎯 Goal: Build a Rust program that reads an input value from the user, converts it to a number, and checks if the user is 18 or older.

📋 What You'll Learn

Use std::io::stdin() to read input

Convert the input string to an u32 number

Use an if statement to check the age

Print a message based on the age

💡 Why This Matters

🌍 Real World

Reading and handling user input is essential for interactive programs like command-line tools and games.

💼 Career

Many programming jobs require you to safely get and use input from users or other sources.

1

Create a variable to store user input

Create a mutable variable called input of type String and initialize it with String::new().

Rust

let mut input = String::new();
// Your code here

Hint

Use let mut input = String::new(); to create an empty string for input.

2

Read input from the user

Use std::io::stdin().read_line(&mut input).expect("Failed to read line") to read the user's input into the input variable.

Rust

fn main() {
    let mut input = String::new();
    std::io::stdin().read_line(&mut input).expect("Failed to read line");
    // Your code here
}

Hint

Use read_line to get input from the user and store it in input.

3

Convert input string to a number

Create a variable called age and set it to input.trim().parse::().expect("Please type a number!") to convert the input string to a number.

Rust

fn main() {
    let mut input = String::new();
    std::io::stdin().read_line(&mut input).expect("Failed to read line");
    let age = input.trim().parse::().expect("Please type a number!");
    // Your code here
}

Hint

Use trim() to remove whitespace and parse::() to convert to a number.

4

Check age and print message

Use an if statement to check if age is 18 or more. If yes, print "You are old enough to vote!". Otherwise, print "You are not old enough to vote.".

Rust

fn main() {
    let mut input = String::new();
    std::io::stdin().read_line(&mut input).expect("Failed to read line");
    let age = input.trim().parse::().expect("Please type a number!");
    if age >= 18 {
        println!("You are old enough to vote!");
    } else {
        println!("You are not old enough to vote.");
    }
    // Your code here
}

Hint

Use if age >= 18 to check the age and println! to show the message.

Practice

1. What does the Rust function io::stdin().read_line(&mut input) do in a program?

easy

A. It clears the input buffer.

B. It writes a line of text to the screen.

C. It reads a line of text from the user and stores it in the variable input.

D. It converts a string to a number.

Solution

1. Step 1: Understand the function purpose

   io::stdin().read_line(&mut input) reads user input from the keyboard as a string.

2. Step 2: Recognize the role of input

   The input is stored in the mutable variable input passed by reference.

3. Final Answer:

   It reads a line of text from the user and stores it in the variable input. -> Option C

4. Quick Check:

   read_line reads user input [OK]

Hint: read_line reads user input into a variable [OK]

Common Mistakes:

• Thinking read_line writes output
• Confusing input reading with clearing buffer
• Assuming it converts types automatically

2. Which of the following Rust code snippets correctly reads a line of input and trims whitespace?

easy

A. let input = io::stdin().read_line().trim();

B. let mut input = String::new(); io::stdin().read_line(&mut input).expect("Failed"); let input = input.trim();

C. io::stdin().read_line(input); let input = input.trim();

D. let mut input = String::new(); io::stdin().read_line(input); input.trim();

Solution

1. Step 1: Check correct usage of read_line

   It requires a mutable String reference: read_line(&mut input).

2. Step 2: Confirm trimming is done on the input string

   Calling input.trim() removes whitespace correctly.

3. Final Answer:

   let mut input = String::new(); io::stdin().read_line(&mut input).expect("Failed"); let input = input.trim(); -> Option B

4. Quick Check:

   read_line needs &mut String and trim() cleans input [OK]

Hint: Use &mut String and call trim() after read_line [OK]

Common Mistakes:

• Forgetting &mut before input
• Calling trim() on read_line() result
• Not creating a mutable String first

3. What will be the output of this Rust code if the user inputs 42?

use std::io;
fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).expect("Failed");
    let num: i32 = input.trim().parse().expect("Not a number");
    println!("Number plus 10 is {}", num + 10);
}

medium

A. Number plus 10 is 52

B. Number plus 10 is 42

C. Not a number

D. Failed

Solution

1. Step 1: Read and parse input

   The input string "42" is trimmed and parsed to integer 42 successfully.

2. Step 2: Add 10 and print

   42 + 10 equals 52, so the output prints "Number plus 10 is 52".

3. Final Answer:

   Number plus 10 is 52 -> Option A

4. Quick Check:

   42 + 10 = 52 [OK]

Hint: Trim and parse input before math operations [OK]

Common Mistakes:

• Not trimming input before parse
• Parsing input as wrong type
• Ignoring parse errors

4. Identify the error in this Rust code snippet that reads an integer from input:

let mut input = String::new();
io::stdin().read_line(&mut input).expect("Failed");
let num: i32 = input.parse().expect("Not a number");
println!("Number: {}", num);

medium

A. parse() called without trimming input

B. Missing &mut before input in read_line

C. input is not mutable

D. println! macro syntax error

Solution

1. Step 1: Check read_line usage

   read_line correctly uses &mut input and input is mutable.

2. Step 2: Identify parse error cause

   parse() is called on input without trimming, so newline remains causing parse failure.

3. Final Answer:

   parse() called without trimming input -> Option A

4. Quick Check:

   Always trim input before parse() [OK]

Hint: Always trim input string before parsing to number [OK]

Common Mistakes:

• Forgetting to trim input before parse
• Assuming read_line returns trimmed string
• Misusing &mut in read_line

5. You want to read multiple numbers from user input separated by spaces and store them in a vector of integers. Which Rust code correctly does this?

use std::io;
fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).expect("Failed");
    let numbers: Vec<i32> = input.trim().split_whitespace()
        .map(|x| x.parse().expect("Not a number"))
        .collect();
    println!("Numbers: {:?}", numbers);
}

hard

A. read_line() cannot read multiple numbers at once.

B. Fails because split_whitespace() does not split strings.

C. parse() should be called before split_whitespace().

D. Correctly reads and parses space-separated numbers into a vector.

Solution

1. Step 1: Read input line and trim

   Input is read as a string and trimmed to remove trailing whitespace.

2. Step 2: Split and parse each number

   split_whitespace() splits input by spaces, map parses each piece to i32, collect gathers into vector.

3. Final Answer:

   Correctly reads and parses space-separated numbers into a vector. -> Option D

4. Quick Check:

   split_whitespace + map(parse) + collect = vector [OK]

Hint: Use split_whitespace, map parse, then collect to vector [OK]

Common Mistakes:

• Parsing before splitting input
• Using wrong iterator methods
• Assuming read_line reads multiple inputs separately