Output and Formatting

Learn how to print output in Rust using the println! macro and its rich formatting capabilities.

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Quick Overview

Rust uses macros for printing:

• println!() - Print with newline
• print!() - Print without newline
• Format strings use {} placeholders
• Rich formatting options built-in

Similar to Python’s f-strings or C’s printf!

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TypeScript/JavaScript Example

// Simple printing
console.log("Hello, world!");

// With variables
const name = "Alice";
const age = 30;

// Template literals
console.log(`Hello, ${name}!`);
console.log(`${name} is ${age} years old`);

// Multiple values
console.log("Name:", name, "Age:", age);

// Formatting numbers
const pi = 3.14159;
console.log(pi.toFixed(2)); // "3.14"

// Objects
const user = { name: "Bob", age: 25 };
console.log(user); // { name: 'Bob', age: 25 }
console.log(JSON.stringify(user)); // {"name":"Bob","age":25}

TypeScript uses template literals and method calls.

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Rust Equivalent

// Simple printing
println!("Hello, world!");

// With variables
let name = "Alice";
let age = 30;

// Format strings
println!("Hello, {}!", name);
println!("{} is {} years old", name, age);

// Inline capture (placeholder names a variable in scope)
println!("Hello, {name}!");

// Named arguments (placeholder maps to an explicit value)
println!("Hello, {who}!", who = name);

// Formatting numbers
let pi = 3.14159;
println!("{:.2}", pi); // "3.14"

// Debug formatting
let user = ("Bob", 25);
println!("{:?}", user); // ("Bob", 25)

// Pretty debug
println!("{:#?}", user);

Rust uses macros with expressive format strings.

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Detailed Explanation

Basic Printing

fn main() {
    // Print with newline
    println!("Hello!");
    println!("World!");

    // Print without newline
    print!("Hello, ");
    print!("World!\n");

    // Print to stderr
    eprintln!("Error message!");
}

Output:

Hello!
World!
Hello, World!
Error message!

Placeholders

fn main() {
    let name = "Alice";
    let age = 30;

    // Positional
    println!("{} is {} years old", name, age);

    // Indexed
    println!("{0} is {1} years old", name, age);
    println!("{1} is the age of {0}", name, age); // Reorder!

    // Inline capture (names a variable already in scope)
    println!("{name} is {age} years old");

    // Named arguments (the placeholder maps to a value you pass)
    println!("{who} is {years} years old", who = name, years = age);

    // Mixed positional and named
    println!("{0} is {years} years old", name, years = age);
}

Compare to TypeScript:

const name = "Alice";
const age = 30;

// Template literals
console.log(`${name} is ${age} years old`);

// Only sequential order possible

Display vs Debug

Display ({}) - Human-readable output:

let x = 42;
let s = "hello";
let b = true;

println!("{}", x);  // 42
println!("{}", s);  // hello
println!("{}", b);  // true

Debug ({:?}) - Programmer-friendly output:

let tuple = (1, "hello", true);
let vec = vec![1, 2, 3];

// println!("{}", tuple);  // Error: tuples don't implement Display
println!("{:?}", tuple);   // (1, "hello", true)
println!("{:?}", vec);     // [1, 2, 3]

Pretty Debug ({:#?}) - Formatted debug:

let data = vec![
    ("Alice", 30),
    ("Bob", 25),
    ("Charlie", 35),
];

println!("{:#?}", data);

Output:

[
    (
        "Alice",
        30,
    ),
    (
        "Bob",
        25,
    ),
    (
        "Charlie",
        35,
    ),
]

Number Formatting

let pi = 3.14159;

// Decimal places
println!("{:.2}", pi);      // 3.14
println!("{:.4}", pi);      // 3.1416

// Width and alignment
println!("{:10}", 42);      // "        42" (right-aligned)
println!("{:<10}", 42);     // "42        " (left-aligned)
println!("{:^10}", 42);     // "    42    " (centered)

// Zero-padding
println!("{:05}", 42);      // 00042

// Sign
println!("{:+}", 42);       // +42
println!("{:+}", -42);      // -42

// Hexadecimal
println!("{:x}", 255);      // ff
println!("{:X}", 255);      // FF
println!("{:#x}", 255);     // 0xff

// Binary
println!("{:b}", 42);       // 101010
println!("{:#b}", 42);      // 0b101010

// Octal
println!("{:o}", 42);       // 52
println!("{:#o}", 42);      // 0o52

Compare to TypeScript:

const pi = 3.14159;

console.log(pi.toFixed(2)); // "3.14"
console.log(pi.toPrecision(4)); // "3.142"

const n = 255;
console.log(n.toString(16)); // "ff"
console.log(n.toString(2)); // "11111111"

String Formatting

let name = "Alice";

// Truncate
println!("{:.5}", name);     // Alice (already < 5)
println!("{:.3}", name);     // Ali

// Width and fill
println!("{:10}", name);     // "Alice     "
println!("{:<10}", name);    // "Alice     " (left)
println!("{:>10}", name);    // "     Alice" (right)
println!("{:^10}", name);    // "  Alice   " (center)
println!("{:*<10}", name);   // "Alice*****" (custom fill)

Escape Sequences

println!("New line\n");
println!("Tab\there");
println!("Backslash: \\");
println!("Quote: \"");
println!("Single quote: \'");
println!("Unicode: \u{1F44D}");  //

Format! Macro

Create formatted strings without printing:

let name = "Alice";
let age = 30;

// Create a String
let s = format!("{} is {} years old", name, age);
println!("{}", s);  // Alice is 30 years old

// Useful for building strings
let filename = format!("report_{}.txt", 2024);

Compare to TypeScript:

const name = "Alice";
const age = 30;

const s = `${name} is ${age} years old`;
console.log(s);

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Key Differences from TypeScript/JavaScript

1. Macros vs Functions

TypeScript:

console.log("Hello"); // Function call

Rust:

println!("Hello"); // Macro invocation (note the !)

Why macros? They’re expanded at compile time and can check format strings!

2. Format String Syntax

TypeScript:

const name = "Alice";
console.log(`Hello, ${name}!`); // Template literal

Rust:

let name = "Alice";
println!("Hello, {}!", name); // Format string

3. Debug Printing

TypeScript:

const obj = { name: "Alice", age: 30 };
console.log(obj); // Pretty print in browser/Node.js
console.log(JSON.stringify(obj)); // Manual serialization

Rust:

let tuple = ("Alice", 30);
// println!("{}", tuple);  // Error
println!("{:?}", tuple);   // Debug format
println!("{:#?}", tuple);  // Pretty debug

4. Number Formatting

TypeScript:

const n = 42;
console.log(n.toString(16)); // "2a" (hex)
console.log(n.toString(2)); // "101010" (binary)

Rust:

let n = 42;
println!("{:x}", n); // 2a (hex)
println!("{:b}", n); // 101010 (binary)

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Common Pitfalls

Pitfall 1: Forgetting Braces

Problem:

let name = "Alice";
println!("Hello, name!"); // Prints literally "Hello, name!"

Solution:

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

Pitfall 2: Type Doesn’t Implement Display

Problem:

let tuple = (1, 2);
println!("{}", tuple); // Error: tuple doesn't implement Display

Solution:

let tuple = (1, 2);
println!("{:?}", tuple); // Use debug format

Pitfall 3: Wrong Number of Arguments

Problem:

println!("{} and {}", 42); // Error: 2 placeholders, 1 argument

Solution:

println!("{} and {}", 42, 100); // Match placeholders

Pitfall 4: Trying to Format References

Problem:

let s = String::from("hello");
println!("{}", &s); // Works, but...

Why it works: Rust automatically dereferences for Display.

Better:

let s = String::from("hello");
println!("{}", s); // Clearer

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Best Practices

1. Use Debug for Development

Implement Display for everything:

struct Point { x: i32, y: i32 }
// Lots of boilerplate to implement Display...

Use derive Debug:

#[derive(Debug)]
struct Point { x: i32, y: i32 }

let p = Point { x: 10, y: 20 };
println!("{:?}", p); // Point { x: 10, y: 20 }

2. Use Inline Captures for Clarity

Unclear:

println!("User {} (ID: {}) logged in at {}", name, id, time);

Clear:

println!("User {name} (ID: {id}) logged in at {time}");

Tip: Since Rust 2021, named placeholders capture identifiers from the surrounding scope directly. The older explicit form println!("{name}", name = name) is exactly what clippy::uninlined_format_args flags, with the suggestion to drop the redundant name = name argument.

3. Use format! for String Building

Concatenation:

let s = "Hello, ".to_string() + name + "!";

Use format!:

let s = format!("Hello, {}!", name);

4. Pretty Print Complex Data

Hard to read:

println!("{:?}", complex_data);

Pretty print:

println!("{:#?}", complex_data);

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Real-World Example

Logging with Formatting

TypeScript:

function log(level: string, message: string) {
  const timestamp = new Date().toISOString();
  console.log(`[${timestamp}] ${level}: ${message}`);
}

log("INFO", "Server started");
log("ERROR", "Connection failed");

Rust:

fn log(level: &str, message: &str) {
    let timestamp = chrono::Local::now().format("%Y-%m-%d %H:%M:%S");
    println!("[{}] {}: {}", timestamp, level, message);
}

fn main() {
    log("INFO", "Server started");
    log("ERROR", "Connection failed");
}

Displaying Table Data

fn print_table(users: &[(&str, u8, f64)]) {
    println!("{:<15} {:>5} {:>10}", "Name", "Age", "Score");
    println!("{:-<32}", ""); // Separator

    for (name, age, score) in users {
        println!("{:<15} {:>5} {:>10.2}", name, age, score);
    }
}

fn main() {
    let users = [
        ("Alice", 30, 95.5),
        ("Bob", 25, 87.3),
        ("Charlie", 35, 92.0),
    ];

    print_table(&users);
}

Output:

Name              Age      Score
--------------------------------
Alice              30      95.50
Bob                25      87.30
Charlie            35      92.00

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Format Specifiers Reference

Specifier	Description	Example	Output
{}	Display	println!("{}", 42)	42
{:?}	Debug	println!("{:?}", x)	(1, 2)
{:#?}	Pretty debug	println!("{:#?}", x)	Multi-line
{:b}	Binary	println!("{:b}", 42)	101010
{:x}	Lowercase hex	println!("{:x}", 42)	2a
{:X}	Uppercase hex	println!("{:X}", 42)	2A
{:o}	Octal	println!("{:o}", 42)	52
{:p}	Pointer	println!("{:p}", &x)	0x7fff…
{:e}	Lowercase exponential	println!("{:e}", pi)	3.14159e0
{:E}	Uppercase exponential	println!("{:E}", pi)	3.14159E0
{:.N}	Precision (N digits)	println!("{:.2}", pi)	3.14
{:W}	Width (W characters)	println!("{:5}", 42)	” 42”
{:<W}	Left align	println!("{:<5}", 42)	”42 “
{:>W}	Right align	println!("{:>5}", 42)	” 42”
{:^W}	Center align	println!("{:^5}", 42)	” 42 “
{:0W}	Zero pad	println!("{:05}", 42)	00042
{:+}	Always show sign	println!("{:+}", 42)	+42

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Further Reading

Official Documentation

• std::fmt module
• The Rust Book - Output
• println! macro

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Exercises

Exercise 1: Basic Printing

Print “Hello, Rust!” three times:

fn main() {
    // Your code here
}

Solution

fn main() {
    println!("Hello, Rust!");
    println!("Hello, Rust!");
    println!("Hello, Rust!");
}

Exercise 2: Variable Printing

Print a person’s name and age in format: “Alice is 30 years old”

fn main() {
    let name = "Alice";
    let age = 30;
    // Your code here
}

Solution

fn main() {
    let name = "Alice";
    let age = 30;
    println!("{} is {} years old", name, age);
}

Exercise 3: Number Formatting

Format π (3.14159) with 2 decimal places:

fn main() {
    let pi = 3.14159;
    // Your code here
}

Solution

fn main() {
    let pi = 3.14159;
    println!("{:.2}", pi); // 3.14
}

Exercise 4: Debug Printing

Print a vector using debug format:

fn main() {
    let numbers = vec![1, 2, 3, 4, 5];
    // Your code here
}

Solution

fn main() {
    let numbers = vec![1, 2, 3, 4, 5];
    println!("{:?}", numbers); // [1, 2, 3, 4, 5]
}

Exercise 5: Create Formatted String

Create a string “User: Alice, Score: 95.5” using format!:

fn main() {
    let name = "Alice";
    let score = 95.5;
    let s = /* your code */;
    println!("{}", s);
}

Solution

fn main() {
    let name = "Alice";
    let score = 95.5;
    let s = format!("User: {}, Score: {}", name, score);
    println!("{}", s);
}

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Summary

What you’ve learned:

• println! and print! macros
• Format placeholders ({}, {:?}, etc.)
• Number formatting (precision, width, padding)
• Debug vs Display formatting
• format! macro for string building
• Named and positional parameters

Key macros:

println!()   // Print with newline
print!()     // Print without newline
eprintln!()  // Print to stderr
format!()    // Create formatted String

Common patterns:

println!("{}", x);        // Display
println!("{:?}", x);      // Debug
println!("{:#?}", x);     // Pretty debug
println!("{:.2}", pi);    // Precision
println!("{:10}", n);     // Width

Formatting is expressive and type-safe in Rust!