Calculation problem

[cidex]

Hello, can someone please explain what is wrong with my code/calculation? With my calculations on my calculator the values should be right, but it doesn't output right.

  #include <iostream>
  #include <cmath>
int main(){

	int k;
	std::cout << "Enter number 8-365: " << std::endl;
	std::cin >> k;

        double a, b, c, d;
	d = k;
	a = d / 7;
	b = floor(a);
	c = a - b;
	a = int(c * 7);

	if (a == 0) { std::cout << "Sunday" << std::endl; }
	else if (a == 1) { std::cout << "Monday" << std::endl; }
	else if (a == 2) { std::cout << "Tuesday" << std::endl; }
	else if (a == 3) { std::cout << "Wednesday" << std::endl; }
	else if (a == 4) { std::cout << "Thursday" << std::endl; }
	else if (a == 5) { std::cout << "Friday" << std::endl; }
	else if (a == 6) { std::cout << "Saturday" << std::endl; }

        system("pause");
}

[coder777]

Do not compare floating point values directly (==, !=). 0.0000000001 is still not 0. So cast a to int.

[cidex]

I have tried to cast a to int

#include <iostream>
#include <cmath>

int main(){

	int k;
	std::cout << "Enter number 8-365: " << std::endl;
	std::cin >> k;

        double a, b, c, d;
	d = k;
	a = d / 7;
	b = floor(a);
	c = a - b;
	a = int(c * 7);
        int g = a;

	if (g== 0) { std::cout << "Sunday" << std::endl; }
	else if (g == 1) { std::cout << "Monday" << std::endl; }
	else if (g== 2) { std::cout << "Tuesday" << std::endl; }
	else if (g== 3) { std::cout << "Wednesday" << std::endl; }
	else if (g== 4) { std::cout << "Thursday" << std::endl; }
	else if (g== 5) { std::cout << "Friday" << std::endl; }
	else if (g== 6) { std::cout << "Saturday" << std::endl; }

        system("pause");
}

But now it gives me always Sunday output.

[Ganado]

What is the point of all that math? What are you trying to do (in English and not in code)?
I have a feeling you might want to use the modulus operator %.

Please use more meaningful variable names.

#include <iostream>
int main()
{
    int input_day = 43;
    int week_day = input_day % 7;
    std::cout << week_day << std::end;
}

[lastchance]

1 2 3 4 5 6 7 8 9 10 11 12

#include <iostream> #include <string> using namespace std; int main() { string days[] = { "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday" }; int day, Jan1st = 1; cout << "Input a day in 2018: "; cin >> day; cout << "It's a " << days[ ( day - 1 + Jan1st ) % 7 ]; }

Input a day in 2018: 16 It's a Tuesday

[cidex]

Sorry for the lack of variable names. But the modulus operator % worked, thanks.
I was trying to calculate the day by inputting day of the year (1-365), I excluded 1-7. The 1. of January is Monday.
So the theory was, you take a number divide by 7 and subtract the first number, so leaving only number after decimal point. Then multiply by 7 and you get number from 0 to 6 and it reflects the day name.

[Ganado]

Yep, modulus is definitely the way to go here. lastchance's code is very nice. But just for your knowledge, you can avoid working with decimal (floating point) numbers by doing a - b * floor(a / b), which is equivalent to a - b * (a / b) if a and b are integers (integer division is floor division for n >= 0).

#include <iostream>
int main()
{
    // you should use %
    // this is to just show what it's mathematically equivalent to, for positive numbers
    int a = 43;
    int b = 7;
    std::cout << a % b << std::endl;
    std::cout << a - b * (a / b) << std::endl;
}