Formatting output in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust code using format! macro?

Consider the following Rust code snippet that formats a floating-point number:

let pi = 3.14159;
let formatted = format!("{:.2}", pi);
println!("{}", formatted);

What will be printed?

Rust

let pi = 3.14159;
let formatted = format!("{:.2}", pi);
println!("{}", formatted);

A 3.14

B 3.14159

C 3.1

D 3

💡 Hint

Look at the format specifier {:.2}. It controls the number of decimal places.

The format specifier {:.2} rounds the floating-point number to 2 decimal places, so 3.14159 becomes 3.14.

❓ Predict Output

intermediate

What is the output of this Rust code using println! with padding?

What will this Rust code print?

let num = 42;
println!("{:>5}", num);

Rust

let num = 42;
println!("{:>5}", num);

A 43

B 42

C 41

D 84

💡 Hint

The {:>5} means right-align in a field of width 5.

The number 42 is printed right-aligned in a space of 5 characters, so it has 3 spaces before it.

🔧 Debug

advanced

Why does this Rust code cause a compile error?

Examine this Rust code:

let name = "Alice";
println!("Hello, {:10}", name);

It causes a compile error. Why?

Rust

let name = "Alice";
println!("Hello, {:10}", name);

A The format specifier {:10} is valid and code compiles without error.

B The format specifier {:10} is missing alignment character, causing a compile error.

C The format specifier {:10} is invalid for &str, causing a compile error.

D The format specifier {:10} is valid, but the code is missing a semicolon.

💡 Hint

Check if the format specifier works with strings in Rust.

The specifier {:10} means left-align in a field of width 10 (default for strings), which is valid for strings. The code compiles and prints "Hello, Alice ".

❓ Predict Output

advanced

What is the output of this Rust code using named arguments in format string?

What will this Rust code print?

let width = 6;
let height = 4;
println!("Rectangle: {w} x {h}", w=width, h=height);

Rust

let width = 6;
let height = 4;
println!("Rectangle: {w} x {h}", w=width, h=height);

A Rectangle: 4 x 6

B Rectangle: {w} x {h}

C Rectangle: 6 x 4

D Rectangle: width x height

💡 Hint

Look at how named arguments are passed to println!.

The named arguments w and h are replaced by the values of width and height, so it prints "Rectangle: 6 x 4".

🧠 Conceptual

expert

How does Rust's formatting traits affect output customization?

Rust uses traits like Display and Debug to control output formatting. Which statement best describes their difference?

A <code>Display</code> and <code>Debug</code> are interchangeable and produce the same output.

B <code>Display</code> formats output as JSON; <code>Debug</code> formats as XML.

C <code>Display</code> is only for numbers; <code>Debug</code> is only for strings.

D <code>Display</code> is for user-facing output; <code>Debug</code> is for developer debugging output.

💡 Hint

Think about when you want pretty output for users vs detailed info for debugging.

Display is implemented for user-friendly output, while Debug is for detailed, programmer-focused output useful during debugging.