Logical Operators Part 3

Part of the Fundamentals section of Coddy's C++ journey — lesson 26 of 74.

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

For example:

int x = 0;
int y = 5;
bool result = x != 0 && y / x > 2;

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

bool result = 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:

int a = 0;
int b = 2;
int c = 3;
int d = 5;
bool result = (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

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

Initialize these variables:

isSunny with the value true
windSpeed with the value 5.4
temperature with the value 23
solarPanelOutput with the value 9
isCloudy 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

Short-circuit evaluation means the computer stops checking conditions as soon as it knows the final answer.

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

int x = 0;
int y = 5;
bool result = x != 0 && y / x > 2; // y / x > 2 is not evaluated

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

bool result = (a > 0 && b < 2) || (c < -5 && d < 10);
// If a > 0 is false, then c < -5 and d < 10 won't be evaluated

This prevents errors (like division by zero) and optimizes performance by avoiding unnecessary evaluations.

Try it yourself

#include <iostream>

int main() {
    // Initialize variables

    
    // The complete logical expression
    bool result = 
    
    // Print results
    std::cout << "1. Is it sunny? " << std::boolalpha << isSunny << std::endl;
    std::cout << "2. Is wind speed safe? " << (windSpeed < 10) << std::endl;
    std::cout << "3. Do panels produce less? " << (solarPanelOutput < 15) << std::endl;
    std::cout << "4. Is temperature good OR no clouds? " << (temperature > 20 || !isCloudy) << std::endl;
    std::cout << "5. Final result: " << result << std::endl;

    return 0;
}