Dynamic 2D arrays?

[Structural Integrity]

How do I make a 2D array dynamic?
When I try this:

float *vertexA;
vertexA		= new float[vertexcounter][3];	//vertexes has 3 floats

I get the following error:
C:\WINDOWS\Desktop\Stage_Johan\PRJ\GL\GL10\object3D.cpp(440) : error C2440: '=' : cannot convert from 'float (*)[3]' to 'float *'

[Cyp]

Originally posted by Structural Integrity
How do I make a 2D array dynamic?
When I try this:

float *vertexA;
vertexA		= new float[vertexcounter][3];	//vertexes has 3 floats

I get the following error:
C:\WINDOWS\Desktop\Stage_Johan\PRJ\GL\GL10\object3D.cpp(440) : error C2440: '=' : cannot convert from 'float (*)[3]' to 'float *'

float **vertexA;
float *(vertexA[3]);
float (*vertexA)[3];

Probably one of the above, might be the one in the middle.

[Structural Integrity]

yeh! The third one doesn't give any errors (haven't tried the others tho )

But one more question: basically this is an array of pointers to arrays of floats (o, that was difficult)

 |array of pointers
\|/

pointer1 -> array of floats
pointer2 -> array of floats
pointer3 -> array of floats
...

Now, I know that I have to delete an dynamically made array, but do I have to delete each row in this array sperately?

Like:

 |array of pointers
\|/

pointer1 -> array of floats <-- Delete this seperately (delete [] vertexA[0])
pointer2 -> array of floats <-- Delete this seperately (delete [] vertexA[1])
pointer3 -> array of floats <-- Delete this seperately (delete [] vertexA[2])
... etc ...

delete [] vertexA; //delete remaining array of pointers

or can I delete it in one go:

delete [] vertexA;

????

[queball]

You can delete it in one go. It's not a pointer to an array of pointers, it a pointer to a large array:

vertexA[0][0]
vertexA[0][1]
vertexA[0][2]
vertexA[1][0]
vertexA[1][1]
vertexA[1][2]
......

[Gayle29uk]

Originally posted by queball
It's not a pointer to an array of pointers, it a pointer to a large array:

???

It's dynamic, surely it's an array of pointers?

[queball]

Originally posted by Gayle28uk

???

It's dynamic, surely it's an array of pointers?

It's not dynamic in two directions though, it's a dynamic array of float[3].

[Gayle29uk]

I've confused myself now

I thought arrays in C worked like this...

x[0][0] x[0][1] x[0][2] x[1][0] x[0][1] ...

is equivalent to...

float * x[0] x[1] x[2] x[3] x[4] x[5] ...

Obviously that falls down when you don't have contiguous memory addressing but that's why dynamic arrays in C were always a pain in the butt.

[edit]I just realised, you're talking C++. I wondered why malloc/calloc (the way I remember) weren't coming into this. No longer confused, I just don't know anything about C++[/edit]

[queball]

Well it's mostly the same in C,
int i, n;
float *array[3];

n = 57;
assert (array = malloc (sizeof (float) * n));

for (i = 0; i < n; i++)
{
array[i][0] = 1.0;
array[i][1] = 2.0;
array[i][2] = 3.0;
}

free (array);

Not an array of pointers.

[Gayle29uk]

I was thinking like this though...

float *array is allocated a block of memory using calloc, an element is to be added so array[counter]=calloc(3,sizeof (float)).

A static block is allocated to hold the maximum number of vertices (in SIs case) and the memory to hold the vertex data is allocated as required.

Hopefully that made sense.

[queball]

I don't like calloc with floats
Do you mean like this?

float **block = calloc (n, sizeof (float *));

for (i = 0; i < n; i++)
block[0] = malloc (3 * sizeof (float));

for (i = 0; i < n; i++)
{
array[i][0] = 1.0;
array[i][1] = 2.0;
array[i][2] = 3.0;
}

This sort of structure is possibly more "dynamic", you could have the same vertex pointed to twice or something. But it adds more indirection and isn't usually more useful or memory efficient especially with such small data structures. Though it's important for anyone doing C arrays to realise that float *p[3] is very different from float **p.

[Gayle29uk]

Pretty much. It's so long ago that I can't remember the specifics of pointer referencing and dereferencing but hopefully you see why I got confused trying to see what was going on in the original now I KNEW it didn't look familiar!