Lesson 02 of 20

5 mins read

Formatting, Comments & Macros (println!)

In Plain English

Rust macros like println! and format! use curly braces ({}) as placeholders for positional parameters, and {:?} for Debug printing. Semicolons denote statements in function bodies.

The Mental Model

"The {} placeholder in println! is like a customized name badge holder where you slot in the visitor’s card at print time."

Deep Dive: How It Works

Display ({}) vs Debug ({:?}): Display formats for end-user readability, while Debug prints internal structural state.

Semicolons: Semicolons turn expressions into statements. A line ending with a semicolon returns the unit type ().

Comments: Use // for line comments and /// for doc comments formatted in Markdown.

Syntax Blueprint

println!("Value: {}", val); | println!("Debug: {:?}", obj);

Use {} for Display types and {:?} for types implementing the Debug trait.

Core Rules to Remember

Positional Arguments: println!("{0} + {1} = {2}", a, b, c) formats by explicit index.

Named Arguments: println!("{user} is {status}", user="Alex", status="Active").

Debug Trait: {:?} prints structures, tuples, and arrays without custom string parsers.

Common Beginner Traps & How to Fix Them

Mistake:Trying to print an array or tuple with {}.

Why it happens: Complex compound structures do not implement the Display trait by default.

How to fix: Use the debug formatter {:?} or {:#?} (pretty-print) instead.

Pro Tip: Use dbg!(expression) during development—it prints file, line number, and value, then returns the value directly!

Live Interactive Example

Hit Run Code to see it live

Formatting and Debug Output

Rust 1.78 / rustc

Output Console

Click "Run Code" to view the rendered output.

How it works: Demonstrates standard {} string formatting and {:?} tuple debug printing.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Print formatted system telemetry

Inside fn main(), declare let host = "node-01"; and let memory_mb = 512;
Print "Host: node-01 | Memory: 512 MB" using println!.

Sandbox Output

Click "Run & Check" to test your solution.

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