Farenheight and Celsius (Negative Ints?)

[NerdTastic]

I'm having trouble figuring out what's not right with my program. The code is:

#include <iostream.h>

int x=-80;
int y=0; //cannot equal zero due to the for loop
int i=0;


void main()
{
	cout << "x" << " " << "i" << " " << "y" << endl;

	do
	{
		i=x;				//x must be constant for incrementation
		cout << x << " " << i << " " << y << endl; //debug
		i=((5/9)(i-32));//formula for finding a celsius temperature from a farenheight one
		x++;	//increment x
		cout << x << " " << i << " " << y <<endl; //debug
		cout << endl; //debug
	}while(y!=x);

	cout << "The values of Celsius and Farenheight are equal at: " << y << " degrees." << endl;
}

It is supposed to find the value at which celsius and farenheight are equal, which I know is -40 but cannot hardcode into the program. I thought this would work, but every time I run it, my debug output statements show that i is being set to 0 after the equation to find the celsius temperature has concluded. When I took out the 5/9 part, it began changing values. Why is this? I've tried making the variables all floats, but it doesn't change anything.

[o1vk4iN6]

You can't use math syntax here, (x)(y) does not mean (x) * (y). Also 5/9 is 0, you need to use floating point precision if you want decimals.

This problem can be approached using:

F(x) = x
C(x) = (5/9) * (x - 32)

F(x) = C(x)

x = 5/9 * (x - 32)

x / (x - 32) = 5/9

1 + (32 / (x - 32)) = 5/9

32 / (x - 32) = 5 / 9 - 1

x - 32 = -72

x = -40

Which makes your program that uses trial and error a bit inefficient and not very accurate.

[NerdTastic]

What do you mean by floating point precision? Do you mean I must use float variables?

[o1vk4iN6]

Yes you must use float or double variables, as well for constants, if you have an int divide by an int, that's an int.

float i = 5 / 9; // i = 0

float i = 5.f / 9; // i = 0.5555556f

at least one of the two variables in an operation involving variables or constants must be a float.

The 'f' means that it is single precision and should be used when you want floats.

float i = 5.f;

double j = 5.;

[Disch]

If you don't want to use floating points, just do the multiplication first:

int boilingF = 212;

int boilingC = (boilingF - 32) * (5 / 9);  // bad, is zero
boilingC = (boilingF - 32) * 5 / 9;  // OK, multiplication is first, so not zero. 

[NerdTastic]

Okay, I understand. Thank you!