dynamic two dimensional arrays

**Beowolf** · 10-30-2007

I can successful declare a dynamic one dimensional array (size is unknown at compilation) but I can't get my program to compile when I try to make it two dimensional. Here is what worked:

header.h

Code:

[snip]
int *myLocations;

main.cpp

Code:

[snip]
routeData myRoute;
myRoute.myLocations = new int[10];

Here is what didn't work:

header.h

Code:

int *myLocations(*);

main.coo

Code:

routeData myRoute;
myRoute.myLocations = new int[10][10];

The 10s are just examples of what may come from what the user specified the array size to be.

Or would it be best to just use a two dimensional vector, since those don't seem to need to have size constraints on them?

**cunnus88** · 10-30-2007

To declare a two dimensional array, you declare

Code:

int **myLocations

To allocate new memory for it, you have to

Code:

myRoute.myLocations = new int*[10]

Then you iterate through each of the myLocations pointers and allocate memory for that, too.

**Beowolf** · 10-30-2007

Then you iterate through each of the myLocations pointers and allocate memory for that, too.

Like this?

Code:

for(int i = 0; i < 10; i++)
    {
        *(myRoute.myLocations + 1) = new int[10];
    }

**brewbuck** · 10-30-2007

Originally Posted by Beowolf

Like this?

Code:

for(int i = 0; i < 10; i++)
    {
        *(myLocations + 1) = new int[10];
    }

No:

Code:

for(int i = 0; i < 10; i++)
    {
        *(myLocations + i) = new int[10];
    }

Or better:

Code:

for(int i = 0; i < 10; i++)
    {
       myLocations[i] = new int[10];
    }

**Daved** · 10-30-2007

Why not use vector<vector<int> >? You're already trying to get familiar with lists and sets, and a vector is a much better choice than a plain dynamic array.

**brewbuck** · 10-30-2007

Originally Posted by Daved

Why not use vector<vector<int> >? You're already trying to get familiar with lists and sets, and a vector is a much better choice than a plain dynamic array.

Also possible... But you'd still have to do some initialization:

Code:

// 10x10 array
vector<vector<int> > array_2d(10);
for(int i = 0; i < 10; i++)
{
    array_2d[i].reserve(10);
}

**Daved** · 10-30-2007

>> But you'd still have to do some initialization:
Certainly, but I would use this:

Code:

vector<vector<int> > array_2d(10, vector<int>(10));

Also note that if you did do it in a loop, you'd want resize rather than reserve.

**brewbuck** · 10-30-2007

Originally Posted by Daved

>> But you'd still have to do some initialization:
Certainly, but I would use this:

Code:

vector<vector<int> > array_2d(10, vector<int>(10));

Also note that if you did do it in a loop, you'd want resize rather than reserve.

Good catch.

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