Rs
Rust Foundations
Module 1Functions & Control FlowLesson 7 of 1022 min

Functions & Return Values

Write reusable pricing helpers: parameters, returns, and expression-bodied functions.

Anatomy

rust
fn mid_price(bid: f64, ask: f64) -> f64 {
    (bid + ask) / 2.0  // last expression = return value (no semicolon)
}
  • Parameters need types
  • Return type after ->
  • return x; works for early exits; final value usually omits return

Early return

rust
fn safe_div(a: f64, b: f64) -> f64 {
    if b == 0.0 {
        return 0.0;
    }
    a / b
}

Multiple helpers

rust
fn notional(qty: f64, px: f64) -> f64 { qty * px }
fn fee(notional: f64, bps: f64) -> f64 { notional * bps / 10_000.0 }

Exercise

Implement compound growth and a year-by-year reporter using a function.

Challenge

Write monthly_compound(p, annual_rate, years) using rate/12 and years*12 periods.

Tips
  • No semicolon on the returned expression
  • powi takes i32 for f64
  • Functions take ownership of arguments unless you borrow (next modules)

Try it live

Functions & Return ValuesRust 2021
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Output
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