help plz!!

[becool04]

write a C++ program that finds the x, which is the root between -100 to 100 in linear equation y = m * x + b. The values of m and b are read of the external data file. In this program must use loops and use if statement if nessecery.
The following is the source code i wrote which doesnt work...
#include <iostream.h>
#include <fstream.h>
#include <math.h>

int main ()
{
double m = 0.0;
double b = 0.0;
double x = 0.0;
int y = 0.;

ifstream infile ( "C:/WINDOWS/Temp/output.txt");

infile >> m;
infile >> b;
cout << "m = " << m << endl;
cout << "b = " << b << endl;


for ( x = -100.; x <= 100.; x = x + 0.001)
{
y = m * x + b;
if ( y = 0)

{
cout << x <<endl;
}


}
cout << x << endl;
return 0;
}//end of main

[Redline]

When you say it doesn't work, what do you mean? What is/are the errors you get?

[Redline]

Also, it looks like your program is only trying to find the x intercept? If that's true, it's unnecessary (and inaccurate) to use a loop. The formula to find the x intercept is

x = -b/m

where:
b = y intercept
m = slope

Explanation (if you want it)

The standard equation is y = mx + b. If you're looking for the x intercept, that is to say the exact point where a line with a given slope(m) and y intercept(b) cross the x axis, then the y value for that point would be zero. Plug that zero into the equation and solve for x to find the forumula for the x intercept:

y = m * x + b (y = 0)
0 = m * x + b (subtract b from both sides)
-b = m * x (divide both sides by m)
-b/m = x

So you could replace your for loop with the following code and achive the same results

x = -b/m;
cout << x <<endl;

That's a little easier and cleaner isn't it? The reason why you can't really do it the other way is this; when you increment x by .001 per cycle, the exact point at which y = 0 can and will get skipped over. The only time that would work is when x does not exceed 3 decimal places. I hope this was helpful

[Redline]

Here is my working interpretation of your code (it's also my first C++ program :rock: )

#include <iostream.h>
#include <fstream.h>

int main ()
{
	char temp[100];
	double m;
	double b;
	double x;

	ifstream  datafile ("test.txt");
	if (! datafile.is_open())
	{
		cout << "Error opening file"; exit (1);
	}

	datafile.getline(temp, 100);
	m = atof(temp);
	datafile.getline(temp, 100);
	b = atof(temp);
	cout << "m = " << m << endl;
	cout << "b = " << b << endl;

	x = -b/m;
	if (x >= -100 && x <= 100) {
		cout << "X Intercept: " << x << endl;
	}
	else {
		cout << "X Intercept is out bounds";
	}
	return 0;
}

[becool04]

we haven't gotten that far yet. so far, i need to use the ifstream datafile and the loop and if statement. Thank you for your program, very appreaciated.