Can't get negative number result?

[thomasks]

I am trying to scale a PWM input and output a PWM signal to an DC gear motor.
The input comes from an RC receiver with a signal between 1000 and 2000.
I need to output a value from -255 to 255.
When I enter a value that should return a -255 I get a 4294967041 value. But the
top of the scale seems to work, and a value of 1500 returns a zero.
What am I missing?

long x;
long in_min;
long in_max;
long out_min;
long out_max;



int main()	
{
	long map(long x, long in_min, long in_max, long out_min, long out_max);
	
	x=1000;
	in_min=1000;
	in_max=2000;
	out_min=-255;
	out_max=255;
  	return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
	

}

[SamuelAdams]

I don't think the problem is in the code you posted, at least it gives me -255.

I'd guess it's something to do with the fact you do not initialize values and your getting random memory value in your calculation.

[48T7M4Gy]

The problem is long integer division. This overcomes it ...

# include <iostream>

int main()
{
    long x=2000;
    
    long in_min=1000;
    long in_max=2000;
    
    double range_of_input = in_max - in_min; // <--
    
    long out_min=-255;
    long out_max=255;
    
    long range_of_output = out_max - out_min;
    
    long output = out_min + (x - in_min)* range_of_output /range_of_input;
    
    std::cout << output;
    
}

x = 1000 -> output = -255
x = 1500 -> output = 0
x = 2000 -> output = 255

[thomasks]

Thanks guys. That works good. Appreciate the help.