CLI Calculator

main.rs

use std::io;
use std::io::Write;

// function to write the prompt
fn prompt(message: &str, result: &mut String) {

    // print the message and flush the buffer
    print!("{}", message);
    io::stdout().flush()
        .expect("Error flushing output");

    // read the input into a variable.  result is already a mutable pointer so we just pass it along
    io::stdin().read_line(result)
        .expect("Error reading input");

    // trim whitespace and assign result to a mutable pointer
    *result = String::from(result.trim())
}

fn prompt_number(message: &str, result: &mut f64) {
    // tmp variable to hold the input string
    let mut input = String::new();

    // passthrough to prompt
    prompt(message, &mut input);

    *result = input.parse::<f64>().expect("That's not a number!")
}

fn main() {
    loop {
        // define variables
        let mut operation = String::new();
        let mut num1: f64 = 0.0;
        let mut num2: f64 = 0.0;

        prompt("Add, Sub, Mul, Div? ", &mut operation);

        match operation.to_lowercase().as_ref() {
            "add" => {
                // prompt for inputs
                prompt_number("Number: ", &mut num1);
                prompt_number("Plus: ", &mut num2);

                // print the result
                println!("{} + {} = {}", num1, num2, num1 + num2)
            },
            "sub" => {
                // prompt for inputs
                prompt_number("Number: ", &mut num1);
                prompt_number("Minus: ", &mut num2);

                // print the result
                println!("{} - {} = {}", num1, num2, num1 - num2)
            },
            "mul" => {
                // prompt for inputs
                prompt_number("Number: ", &mut num1);
                prompt_number("Times: ", &mut num2);

                // print the result
                println!("{} * {} = {}", num1, num2, num1 * num2)
            },
            "div" => {
                // prompt for inputs
                prompt_number("Number: ", &mut num1);
                prompt_number("Divided by : ", &mut num2);

                // print the result
                println!("{} / {} = {}", num1, num2, num1 / num2)
            },
            "exit" => {
                break;
            }
            _ => {
                println!("Invalid Operation!");
            }
        }
    }
}