resizeable 2-dimen array

[simply thunder]

i'm trying to make a matrix class and i want to use 2 dimensional arrays for them. i know you can use vectors(easy way) but is there a way to re-size the array?? with out accidentally over writing something else?

i know it has something to do with new and delete but i'm not familiar with them

i can do it with one dimensional arrays but not 2

[firedraco]

Well, you could create another array with the "new" size, then copy all the data over, then set that array as the "real" array.

[Umz]

Theres no way to change the size of the current array but firedraco's way will work

[Mythios]

I may be wrong in saying this - but is this possible at all?

//Include Class Header and this files header here...
MatrixClass* CreateMatrix(int &arraySizeOne, int &arraySizeTwo)
{
	std::cin >> arraySizeOne, arraySizeTwo;

	MatrixClass* myCreatedMatrix = new MatrixClass[arraySizeOne][arraySizeTwo];

	return myCreatedMatrix;
}
//New File for delete function
void DeleteMatrix(MatrixClass* myCreatedmatrix, int arraySizeOne, int arraySizeTwo)
{
		delete[][] myCreatedMatrix;	
}

Probably not correct syntax - but it's kind of possible like this isn't it?

For the top bit using the class like method - in another file or somewhere else on the page. You would have another pointer thats pointing to the real class and make it edit that?

[Mythios]

So in another file it might be something like...

#include "class.h"
#include "create_matrix.h"

void startup()
{
	Matrix *pMatrix;
	int sizeOne, sizeTwo;
	pMatrix = CreateMatrix(sizeOne, sizeTwo);
}

Let me know if you manage to get what you want working :)

[Zaita]

If you wanna go back to the old C style, you can use realloc() :)

[Mythios]

Good old realloc() - that is another way of doing it which i still think is pretty good.

[simply thunder]

i still cant do it

declaration

float* mat;

this works:

 r = numRow();
 c = numCol();
 mat = new float[r][c]; 

this does but not what i need

 r = numRow();
 c = numCol();
 mat = new float[r*c]; 

also i'm getting an
invalid types float[int] for array subscript
error in this:

 mat[r][c] = num;

[Mythios]

Hmm, now i'm a little confused - mat = new float[r][c]; is a 2d array which i gather is what you want. But your other one is a single array its just 1 long line. As lets say your r = 10 and c = 10 - then all thats doing is making a long single array with an x-axis value of 100. Doing it the way you have at the top using "new float[r][c]" would be making a y-axis value of 10 and an x-axis value of 10 as well.

And as for mat[r][c] = num; what exactly do you want to happen here?

[Bazzy]

to understand better how to use new float[r][c] and new float[r*c] read this: http://www.cplusplus.com/doc/tutorial/arrays.html#multidimensional_arrays

[simply thunder]

I know how multidimensional arrays work, i was just saying that works, but not what i want.

and i do want an 2 dimensional array. when i do mat[r][c] = num; i mean to change that specific spot([r][c]) in the array to be = to "num" sent in.

i also looked here for info:
http://www.cplusplus.com/doc/tutorial/dynamic.html

[firedraco]

mat[r][c] = num;

Should work fine...(assuming you aren't accessing outside of the bounds of the array). What is the value of "num"?

[simply thunder]

num is a float and a paramater in member function changeEntery(int r, int c, float num)

[firedraco]

Hmmm...then it should work fine...are you getting an error or something.

[simply thunder]

i am getting an error
'c' cannot appear in a constant-expression
for:
mat = new float[r][c];

and
invalid types 'float[int]' for array subscript
for:

 if (r <= numRow() && c <= numCol())
    mat[r][c] = num; 

and
invalid types 'float[int]' for array subscript
for:
cout << mat[i][j];

[Mythios]

what does you for loop look like for your - displaying of "cout << mat[i][j]; ?

[Zaita]

Dynamic 2d array.

int **array = new int*[rows];
for (int i = 0; i < rows; ++i) 
 array[i] = new int[cols];

array[row][col] = num;

cout << "Value = " << array[row][col] << endl;

// Free memory
for (int i = 0; i < rows; ++i) {
 delete [] array[i];
}
 delete [] array;

[Mythios]

Beautiful - that has even helped me heh

[simply thunder]

ohh i get. smart. thanks

[Zaita]

There are other ways to do it. But they either involve overloading operators, or using a vector of vectors.

The way I've illustrated is for people who don't mind working with pointers and are confident in their memory management abilities.