Module 1Getting StartedLesson 2 of 1020 min

Printing & Formatting

Master println!, format!, debug {:?} prints, and numeric formatting you’ll use for prices and rates.

Display vs Debug

{} — human-friendly Display (strings, numbers)
{:?} — Debug view (great for arrays, structs later)
{:#?} — pretty Debug

rust

fn main() {
    let px = 100.2567;
    println!("price = {}", px);
    println!("price 2dp = {:.2}", px);
    println!("scientific = {:.3e}", 0.000123);
}

Output

price = 100.2567
price 2dp = 100.26
scientific = 1.230e-4

Named and positional args

rust

fn main() {
    println!("{ticker} mid={mid:.4}", ticker = "AAPL", mid = 190.125);
    println!("{0} / {1} / {0}", "bid", "ask"); // positional
}

Output

AAPL mid=190.1250
bid / ask / bid

format! builds a String

rust

let label = format!("BUY {} @ {:.2}", 100, 150.5);
// label is owned String — useful for logs and order tags

Why this matters in finance

You’ll constantly print:

Prices with fixed decimals
Quantities as integers
Rates as percents (0.05 → 5.0%)
Debug dumps of intermediate Greeks during development

Guided example: order ticket

rust

fn main() {
    let side = "BUY";
    let qty = 250;
    let limit = 412.375;
    println!("--- ORDER ---");
    println!("side     {}", side);
    println!("qty      {}", qty);
    println!("limit    {:.3}", limit);
    println!("notional {:.2}", qty as f64 * limit);
}

Output

--- ORDER ---
side     BUY
qty      250
limit    412.375
notional 103093.75

Exercise

Format a small mark-to-market blurb with 2 decimal places for money and 1 for percent.

Challenge

Add a third price (yesterday’s close) and print day-change in dollars and percent.

Tips

{:.2} means 2 digits after the decimal
as f64 converts integers for multiplication with floats
format! is like println! but returns a String

Try it live

Printing & FormattingRust 2021

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Output

Press Run to compile and execute.