Thread: Liner Regression bugs

> double size = 0;
Why would you want a double to store the number of elements in an array?

It's not like you can ever have 5.678 elements is it?

> while (!input.eof())
Read the FAQ on why using eof() to control a loop is bad.
Essentially, this loops ONE more time than you expect.

Originally Posted by bar5037

thanks... There are no bugs now... however.. nothing is being outputted and I cant quite seem to figure out why.

And that's not a bug? What is a bug, if "no output isn't"? To me, anything that doesn't conform to what you expect is a bug [unless of course you expect the impossible].

Aside from that, I can't see any good reason why you shouldn't get output. But perhaps your code gets stuck somewhere.

Code:

void linearRegress ( double x[], double y[], int size, double &m, double &b )
{
	int i = 0;
	while (i < size)
	{
		int m;
		int b;
		int sumX = 0;
		int sumY = 0;
		int sumXY = 0;
		int sumX2 = 0;
		int denom;

		for ( i; i < size; i++)
		{
			
			sumX += x[i];
			sumY += y[i];
			sumXY += x[i] + y[i];
			sumX2 += x[i] * x[i];

			denom = (size * sumX2) - (sumX * sumX);

			m = ((size * sumXY) - (sumX * sumY)) / denom;

			b = ((sumX2 * sumY) - (sumXY * sumX)) / denom;

		}
	}
}

The while-loop is completely useless here. The first time, it will be less than size, then it will enter the for-loop and after the for-loop, i is equal to size, to the while-loop exits. No meaning to that at all - the exact same thing would happen if you remove the while-loop part.

The local variables m and b are hiding the parameters into the function with the same name - so you will not see "any result" from the function.

I also believe that you only want to calculate denom, m and b when the for-loop is done.

But none of this explains why you don't get any output. I take it it's not a case of "the output is printed and then the screen disappears", and you just need a "getchar()" at the end of the main function?

--
Mats

Code:

void getData (double x[], double y[], int& size)
{
    ifstream input;
	
    input.open("linear.txt");
    size = 0;

    while (!input.eof()) 
    {
        input >> x[size] >> y[size];
        size++;
    }

    input.close();
}

That is the wrong way to read your data from the file. EOF is only set after an attempt to read data has failed. After reading the last line of data in the file, EOF is still false so your loop would execute one more time trying to get data. In most cases, this would result in the size parameter reporting there being one more x and y value than there really are and that last row of data would screw up your calculations. You also make no checks that the number of elements is less than or equal to the size of your arrays. This would be better:

Code:

void getData (double x[], double y[], int& size)
{
    ifstream input;
	
    input.open("linear.txt");
    size = 0;

    while( size < SIZE && input >> x[size] >> y[size] )
    {
        size++;
    }

    input.close();
}

The size < SIZE part makes sure we only store what the x and y arrays are capable of storing and the other modification makes sure we only store as much as what's in the file (up to SIZE of course).