Println

In Rust we print values to the console with the println macro. This is the easiest way to output data—but faster approaches are available.

In Rust, each println invocation will have overhead as it locks the underlying implementation. If we group together output lines, we can improve performance.

Println example

In this program we see the basic syntax for the println macro. It is important to include the exclamation mark at the end.

fn main() {
    // Version 1: Print some content to the console.
    let count = 300;
    let value = "friend";
    println!("Hello {value}: {count}");

    // Version 2: Can use arguments instead.
    // Arguments can be more complex expressions.
    println!("Hello {}: {}", value, count + 1);
}
Hello friend: 300
Hello friend: 301

Println performance

Each call to println (or print) has some overhead. If we merge together the print calls, we can improve overall program speed.

use std::time::Instant;

fn main() {
    // Version 1: Use println 100 times.
    let t0 = Instant::now();
    for _ in 0..100 {
        println!("Line");
    }
    println!("{} ns", t0.elapsed().as_nanos());

    // Version 2: Create a string with 100 lines, and use print once.
    let t1 = Instant::now();
    let mut v = String::new();
    for _ in 0..100 {
        v.push_str("Line");
        v.push('\n');
    }
    print!("{}", v);
    println!("{} ns", t1.elapsed().as_nanos());
}
...Line
Line
Line
149916 ns
...Line
Line
Line
62792 ns

Format

It is sometimes desirable to use format strings but not print them to the console. We can use the "format!" macro for this purpose.

fn main() {
    let color = "orange";

    // Version 1: use println with formatting.
    println!("The cat is {}", color);

    // Version 2: use format with println.
    let value = format!("The cat is {}", color);
    println!("{value}");
}
The cat is orange
The cat is orange

Literals

The println macro supports string literals with no substitutions. So we can print simple output with just one argument. And with no arguments, we get an empty line.

fn main() {
    // We can use string literals directly.
    // If no argument is passed, an empty line is printed.
    println!("Hi");
    println!();
    println!("Friends");
}
Hi

Friends

Stdout

Sometimes we want to write data to the console that is not handled through println. This can be done by accessing stdout.

use std::io;
use std::io::*;

fn main() {
    // A byte vector.
    let mut data = vec![];
    data.extend_from_slice("Hello\n".as_bytes());

    // Print all bytes to stdout.
    io::stdout().write_all(&data).expect("Invalid write");
}
Hello

Eprintln

Console programs have standard output and error output. If we have an error message we want to write, it may be better to write it to the error output.

use std::fs::File;

fn main() {
    if let Ok(_file) = File::open("/does/not/exist") {
        // Handle file.
    } else {
        // Write to error output.
        eprintln!("File not found");
    }
}
File not found

Writeln

This macro is like println but we can target a specific buffer. We must pass a mutable reference to a buffer (like a vector).

use std::io::*;

fn main() {
    let name = "Friend";
    let mut buffer: Vec<u8> = vec![];
    if writeln!(&mut buffer, "Hello {name}").is_err() {
        panic!("Write failed!");
    }

    // Output the buffer as string.
    print!("{}", String::from_utf8_lossy(&buffer));
}
Hello Friend

Compile-time error

Println is a macro, which means it is evaluated at compile-time and code is generated. This allows Rust to verify all the arguments are correct.

fn main() {
    // Must have correct number of arguments to println.
    let test = 1;
    println!("{} {}", test);
}
error: 2 positional arguments in format string, but there is 1 argument

Debug trait

In Rust we cannot pass a struct directly to println unless it has the Debug trait implemented. We can do this with the derive Debug syntax on the struct.

#[derive(Debug)]
struct Item {
    width: usize,
    height: usize,
}

fn main() {
    let item = Item {
        width: 10,
        height: 12,
    };
    // Print the struct with debug trait.
    println!("{:?}", item);
}
Item { width: 10, height: 12 }

Summary

Rust provides robust support for console writing in its standard library. We must become familiar with macro syntax. And merging together writes can improve performance.