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:

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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?!

Quote:

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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?!

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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));

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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