Handling input values in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust code reading input?

Consider this Rust program that reads a line from standard input and prints it back:

use std::io::{self, Write};

fn main() {
    let mut input = String::new();
    print!("Enter your name: ");
    io::stdout().flush().unwrap();
    io::stdin().read_line(&mut input).unwrap();
    println!("Hello, {}!", input.trim());
}

If the user types Alex and presses Enter, what will be printed?

Rust

use std::io::{self, Write};

fn main() {
    let mut input = String::new();
    print!("Enter your name: ");
    io::stdout().flush().unwrap();
    io::stdin().read_line(&mut input).unwrap();
    println!("Hello, {}!", input.trim());
}

A Enter your name: Hello, Alex!

B Hello, Alex!\nEnter your name:

C Enter your name: Hello, Alex\n!

D Hello, Alex!

💡 Hint

Remember that print! does not add a newline, so flushing is needed to show the prompt before input.

The program prints the prompt without a newline, flushes output to show it immediately, then reads input. After trimming the newline from input, it prints the greeting on the next line.

❓ Predict Output

intermediate

What error does this Rust code produce when reading input?

Look at this Rust code snippet:

use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(input).unwrap();
    println!("You typed: {}", input);
}

What error will this code cause when compiled?

Rust

use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(input).unwrap();
    println!("You typed: {}", input);
}

A error[E0599]: no method named `read_line` found for struct `String`

B error[E0308]: mismatched types: expected &mut String, found struct String

C No error, runs correctly

D error[E0425]: cannot find value `input` in this scope

💡 Hint

Check the argument type expected by read_line.

The method read_line expects a mutable reference &mut String, but the code passes input by value. This causes a type mismatch error.

🔧 Debug

advanced

Why does this Rust program panic when parsing input?

Examine this Rust code:

use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let num: i32 = input.trim().parse().unwrap();
    println!("Number plus 10 is {}", num + 10);
}

If the user types abc and presses Enter, what happens?

Rust

use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let num: i32 = input.trim().parse().unwrap();
    println!("Number plus 10 is {}", num + 10);
}

A Program panics with a parse error at unwrap()

B Program prints: Number plus 10 is 10

C Program prints: Number plus 10 is abc10

D Program waits for more input indefinitely

💡 Hint

What happens if parse() fails and you call unwrap()?

Parsing "abc" as an integer fails, so parse() returns an error. Calling unwrap() on an error causes the program to panic and crash.

📝 Syntax

advanced

Which Rust code correctly reads and parses an integer input?

Choose the Rust code snippet that correctly reads a line from input and parses it as an i32 without panicking.

A

let mut input = String::new();
io::stdin().read_line(&mut input).unwrap();
let num: i32 = input.parse().unwrap();

B

let mut input = String::new();
io::stdin().read_line(&mut input).unwrap();
let num: i32 = input.trim().parse().unwrap();

C

let mut input = String::new();
io::stdin().read_line(input).unwrap();
let num: i32 = input.trim().parse().unwrap();

D

let mut input = String::new();
io::stdin().read_line(&mut input).unwrap();
let num: i32 = match input.trim().parse() {
    Ok(n) => n,
    Err(_) => 0,
};

💡 Hint

Look for safe parsing that handles errors.

Option D uses match to handle parse errors safely, returning 0 if parsing fails. Others either panic or have syntax errors.

🚀 Application

expert

How many items are in the vector after this input processing?

This Rust program reads a line of space-separated numbers and collects them into a vector:

use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let numbers: Vec = input
        .trim()
        .split(' ')
        .filter_map(|s| s.parse().ok())
        .collect();
    println!("Count: {}", numbers.len());
}

If the user inputs: 10 20 abc 30 40x 50 and presses Enter, what will be printed?

Rust

use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).unwrap();
    let numbers: Vec<i32> = input
        .trim()
        .split(' ')
        .filter_map(|s| s.parse().ok())
        .collect();
    println!("Count: {}", numbers.len());
}

A Count: 5

B Count: 3

C Count: 4

D Count: 6

💡 Hint

Only valid integers are collected. Non-integers are skipped.

The input splits into: "10", "20", "abc", "30", "40x", "50". Only "10", "20", "30", and "50" parse as integers. So the vector length is 4.

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