Variables inside functions

[Kazar]

I'm rather new to C-programming and I have mostly some php background. I'm struggling with variables inside functions. I have a function that, among other things, controls the banking of an airplane that is turning. Here's the short version of it:

void HowToFly (TMesh airplane) {
		double blend;
		double frame,frameend,framebegin;
		TVec3 altitude = EntityPosition(airplane, 0);
		TVec3 meshrotation, meshrotation_old;

		// Turning animation
		meshrotation = EntityRotation (airplane,1);
		if ((meshrotation.Y > (meshrotation_old.Y+AppSpeed()*1.0)) && altitude.Y > 0.5) { // no banking when near the ground
			framebegin=80.0;    // turning to left keyframes
			frameend=81.0;
			blend=blend+(AppSpeed()*0.05);
		}
		else if (((meshrotation.Y+AppSpeed()*1.0) < meshrotation_old.Y) && altitude.Y > 0.5) { // no banking when near the ground
			framebegin=60.0;	// turning to right keyframes
			frameend=61.0;
			blend=blend+(AppSpeed()*0.05);
		}
		else {
			blend=blend-(AppSpeed()*0.025);
		}

		// level flight
		frame=AppTime()/10.0;
		frame=fmodf(frame,1.0-0.0)+0.0;		
		Animate(airplane,frame,1.0,0,true);	

		// blended animation
		frame=AppTime()/10.0;
		frame=fmodf(frame,frameend-framebegin)+framebegin;		
		Animate(airplane,frame,blend,0,true);	

		blend=max(blend,0.0);
		blend=min(blend,1.0);

		meshrotation_old = meshrotation;
}

 

Of course this worked as part of the main program with blend,frame,frameend,framebegin, etc. - variables initialized before the mainloop. I had to make a function out of it to control multiple plane's animation and this breaks it because all my variables are reset at the start of each loop.

 

I have ran out of all ideas to make it work (except global variables but that really isn't good solution for tens of planes, I think) and I could really use a nudge in to the right direction.

 

 

 

 

Oh, and sorry for the blatant crudeness of my code

[Rick]

If you want to stick with just C, you would create structs for each plane that has the variables per plane that is needed. I would use C++ and a class for this though. Make a plane class that has all the variables/functions you need.

 

class Plane
{
private:
  // variables for this plane go here
public:
  void HowToFly()
  {
  }
};

[Kazar]

Actually, I did read about structs (wrote a nice paper note about it to learn) but for some reason came to conclusion it wasn't what I needed. Seems I was wrong, though.

I will eventually move to C++ but it seems I have a lot to learn about C still

 

Thanks for the help, I think I'll use the C++ class, since it seems more straightforward.

[Canardia]

You could also make the variables static which you want to preserve their values between function calls.

[Kazar]

Oh, that's possible?

[Rick]

That won't work for multiple planes. They would all still be sharing the same variable, which I thought isn't what you were looking for? If you have 10 planes, you would need 10 different static variables in that method and know which one to use. Otherwise they would all share that 1 static. It's not really even an option in my view.

[Kazar]

Ah, I see. So it would be the same as using global variables for each one of them.

 

Writing class code atm.. with the help of a tutorial.

[Rick]

Yeah, in this case it would basically act the same. Statics just retain their value from where they were defined. So if you define one in a function, every time that function gets called it'll be that same exact variable with the same value it had the last time it was called. I almost never use static variables inside functions because it's almost always a bad design idea. There are other uses for static variables though inside classes that make them useful. Like in a Singleton pattern.

 

Let me know if you need any help with the C++ class. I love helping people learn C++

[Kazar]

Oh, sure Write the complete finished code (of my short example) for me so I can compare it to what I have done so far.

[Rick]

Here is a quick try with not having access to a compiler and my kid screaming.

 

This would be the general idea anyway.

 

class Plane
{
private:
double blend, frame, frameend, framebegin;
TVec3 meshrotation_old;
public:
Plane()
{
	blend = 0.0;
	frame = 0.0;
	frameend = 0.0;
	framebegin = 0.0;
	meshrotation_old = Vec3(0);
}
void HowToFly (TMesh airplane)
{
       	TVec3 altitude = EntityPosition(airplane, 0);
               TVec3 meshrotation;

               // Turning animation
               meshrotation = EntityRotation (airplane,1);
               if ((meshrotation.Y > (meshrotation_old.Y+AppSpeed()*1.0)) && altitude.Y > 0.5) { // no banking when near the ground
               	framebegin=80.0;    // turning to left keyframes
                       frameend=81.0;
                       blend=blend+(AppSpeed()*0.05);
               }
               else if (((meshrotation.Y+AppSpeed()*1.0) < meshrotation_old.Y) && altitude.Y > 0.5) { // no banking when near the ground
               	framebegin=60.0;        // turning to right keyframes
                       frameend=61.0;
                       blend=blend+(AppSpeed()*0.05);
               }
               else {
                       blend=blend-(AppSpeed()*0.025);
               }

               // level flight
               frame=AppTime()/10.0;
               frame=fmodf(frame,1.0-0.0)+0.0;         
               Animate(airplane,frame,1.0,0,true);     

               // blended animation
               frame=AppTime()/10.0;
               frame=fmodf(frame,frameend-framebegin)+framebegin;              
               Animate(airplane,frame,blend,0,true);   

               blend=max(blend,0.0);
               blend=min(blend,1.0);

               meshrotation_old = meshrotation;
}
};



// There are a number of better ways to create all of these, but this is the easiest to start learning
Plane plane1, plane2, plane3;

[Kazar]

Thanks a lot this problem had been a real stopper.