### Calculation problem

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.

 ``12345678910111213141516171819202122232425`` `````` #include #include 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"); }``````
Do not compare floating point values directly (==, !=). 0.0000000001 is still not 0. So cast `a` to int.
I have tried to cast a to int

 ``123456789101112131415161718192021222324252627`` ``````#include #include 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.
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.

 ``1234567`` ``````#include int main() { int input_day = 43; int week_day = input_day % 7; std::cout << week_day << std::end; }``````
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 ``123456789101112`` ``````#include #include 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```
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.
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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).

 ``12345678910`` ``````#include 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; }``````
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