Im a newbie having problems with dynamic arrays, can anyone help?

Hello guys, I'm currently trying to implement a Biquad filter into my code but this involves creating a dynamic array that is constantly updated so that I can recall the current digital sample and the previous 2 digital samples at any point for many many samples.

I figured malloc() and free() were the only way to go with this and if anyone has a better suggestion then I'm all ears as I'm still very new to this whole C coding thing.

Anyway, the problem I encounter arises when I try to point to the previous samples stored in the dynamic array and get a "Floating Point Error: Domain" error.

Here is a overview of what ive got:

Code:

main()
{
	int current_sample = 1;
	float *p1;
	float *p2;
                
                p1 = malloc(3);
	p2 = malloc(3);

                wave[p1] = sine_osc * amplitude;
                filtered_wave[p2] = a0 * wave[p1] + a1 * wave[p1-1] + a2                * wave[p1-2] - b1 * filtered_wave[p2-1] - b2 * filtered_wave[p2-2]

                if (current_sample > 3)
	{
	                current_sample = 1;
		free(p1);
		free(p2);
	}
}

Unfortunately p1-1 doesnt seem to address the previous sample stored in the array like I hoped and using p1++ just causes errors.

What am I doing wrong?

Thanks

Rhys

PS Is there a trick to using tab when writing a message, I cant get it to work on this forum?!

Originally Posted by Dr.Spankenstein

PS Is there a trick to using tab when writing a message, I cant get it to work on this forum?!

Yes. Don't use tabs.

Have you thought about a circular queue?

Using dynamic allocation for 3 items seems silly. And your syntax doesn't make much sense.

Why are you allocating 3 bytes for a float anyway? If you want 3 floats, you should do (3 * sizeof(float)) or (3 * sizeof(*p1)), where p1 is the pointer you are setting to point to the return value of malloc().

But as was pointed out, if you only need 3 floats, then use an array or use separate variables.

If the number of floats you need is not user-input dependent, you can also just write:

Code:

float p[3];

instead of:

Code:

float *p;
p = malloc(3 * sizeof(*p));

Thanks guys with your help I've finally figured it out....
...and yeah the code above was scrappy but I was just taking chunks from what I've written and slapping em together to post at 1 in the morning!

Rhys