Question

How to find the sum of all the multiples of 3 or 5 below 1000 in C++

Answer 1

#include <iostream>

int Sum(int n) {
	return n * (n + 1) / 2;
}

int main() {
	int below1000 = 999;

	int sum = 3 * Sum(static_cast<int>(below1000 / 3)) + 5 * Sum(static_cast<int>(below1000 / 5)) - (3 * 5) * Sum(static_cast<int>(below1000 / 15));

	std::cout << "sum = " << sum;
}




/*
run:
    
sum = 233168
    
*/

 

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Answer 2

#include <iostream>

int main() {
	int sum = 0;

	for (int i = 3; i < 1000; i++) {
		if (i % 3 == 0 || i % 5 == 0) {
			sum += i;
		}
	}

	std::cout << "sum = " << sum;
}




/*
run:
    
sum = 233168
    
*/

 

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