Logical Operators Part 3

Part of the Fundamentals section of Coddy's Rust journey — lesson 23 of 75.

When checking multiple conditions, the computer stops checking as soon as it knows the final answer (This is called short-circuit evaluation).

For example:

let x: i32 = 0;
let y: i32 = 5;
let result: bool = x != 0 && y / x > 2;

Here x equals 0, therefore it will not evaluate y / x > 2. If we would reverse the order:

let result: bool = y / x > 2 && x != 0;

This will result in an error because y will be divided by 0 which is illegal in math.

This technique is used to optimize the evaluation of logical expressions. For example:

let a: i32 = 0;
let b: i32 = 2;
let c: i32 = 3;
let d: i32 = 5;
let result: bool = (a > 0 && b < 2) || (c < -5 && d < 10);

In this example b < 2 and d < 10 will not be evaluated because a > 0 and c < -5 are both false.

Challenge

Beginner

Let's create a program to decide if it's a good day for solar panel energy production

Initialize these variables:

is_sunny with the value true
wind_speed with the value 5.4
temperature with the value 23
solar_panel_output with the value 9
is_cloudy with the value false

Create one logical expression that checks ALL of these conditions:

It's sunny
The wind speed is less than 10
The solar panel output is less than 15
The temperature is above 20 OR there are no clouds

Cheat sheet

Rust uses short-circuit evaluation when checking multiple conditions - it stops evaluating as soon as the final result is determined.

With && (AND), if the first condition is false, the second won't be evaluated:

let x: i32 = 0;
let y: i32 = 5;
let result: bool = x != 0 && y / x > 2; // Safe - y/x won't be evaluated

With || (OR), if the first condition is true, the second won't be evaluated:

let a: i32 = 0;
let b: i32 = 2;
let c: i32 = 3;
let d: i32 = 5;
let result: bool = (a > 0 && b < 2) || (c < -5 && d < 10);
// b < 2 and d < 10 won't be evaluated

This optimization prevents unnecessary computations and can avoid runtime errors like division by zero.

Try it yourself

fn main() {
    // Initialize variables
    
    
    // The complete logical expression
    let result: bool = 
    
    // Don't delete the lines below
    println!("Checking conditions for solar energy production...");
    println!("1. Is it sunny? {}", is_sunny);
    println!("2. Is wind speed safe? {}", (wind_speed < 10.0));
    println!("3. Can panels produce more? {}", (solar_panel_output < 15));
    println!("4. Is temperature good OR no clouds? {}", (temperature > 20 || !is_cloudy));
    println!("\\nFinal result - Good day for solar energy production: {}", result);
}