Handling input values in Rust

Introduction

We use input handling to get information from the user while the program is running. This lets the program react to what the user types.

When you want to ask the user for their name.

When you need to get a number from the user to do math.

When you want the user to choose an option from a menu.

When you want to read a line of text from the user.

When you want to pause the program until the user presses Enter.

Syntax

Rust

use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).expect("Failed to read line");
    println!("You typed: {}", input.trim());
}

We create a mutable String to store what the user types.

read_line reads the input and adds it to the String.

Examples

This example asks for the user's name and greets them.

Rust

use std::io;

fn main() {
    let mut name = String::new();
    println!("Enter your name:");
    io::stdin().read_line(&mut name).expect("Failed to read line");
    println!("Hello, {}!", name.trim());
}

This example reads a number as text, then converts it to an integer.

Rust

use std::io;

fn main() {
    let mut number = String::new();
    println!("Enter a number:");
    io::stdin().read_line(&mut number).expect("Failed to read line");
    let number: i32 = number.trim().parse().expect("Please type a number!");
    println!("You typed the number: {}", number);
}

Sample Program

This program waits for the user to type something and then shows it back.

Rust

use std::io;

fn main() {
    let mut input = String::new();
    println!("Please type something and press Enter:");
    io::stdin().read_line(&mut input).expect("Failed to read line");
    println!("You typed: {}", input.trim());
}

Important Notes

Always use trim() to remove the newline character from the input.

Use expect() to handle errors simply during input reading.

To convert input to numbers, use parse() and handle possible errors.

Summary

Input handling lets your program talk with the user.

Use io::stdin().read_line() to get user input as text.

Remember to trim and convert input when needed.

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