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I recently started programming in C++, I have quite some experience in JAVA programming but I am facing a rather unclear situations whilst trying to initialize multi dimensional arrays in c++.

The code I would use in java would be something like:

int x = 5;
int y = 10;

int array [][] = new int[x][y];

which works fine, I could assign any value to x and y using a scanner or option pane. However (and please bear with me, I am quite new to c++) in c++ I am required to use constants which prevent me from using for example:

int x;
cin >> x;
int y;
cin >> y;
int array [][] = new int[x][y];

I am trying to make a random map generator, eventually i will assign 3d object to positions within the array and design an algorithm to sort all of the objects. However I want the user to be able to specify the size of the grid, specify x and y, the rows and columns.

Any help would be greatly appreciated!

Many thanks in advance.

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  • This was somewhat answered over here. I believe your problem is the preceding brackets to the equal sign. You're making a pointer of pointers. Commented Mar 31, 2014 at 21:47
  • The C++ equivalent of a Java array is a C++ vector. Don't muck around with C-style arrays and manual memory management. Commented Mar 31, 2014 at 22:38

3 Answers 3

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2

This record

int array [][] = new int[x][y];

is invalid in C++. If you want to allocate an array in the heap using operator new and the right dimension is not a constant expression then you should write

int x;
cin >> x;
int y;
cin >> y;

int **array = new int *[x];

for ( int i = 0; i < x; i++ ) array[i] = new int[y];

If the right dimension is set by a constant expression then you can write

int x;
cin >> x;
const int y = SomeValue;

int ( *array )[y] = new int [x][y];

Take into account that you could use standard container std::vector instead of a manually allocated array.

For example

std::vector<std::vector<int>> v( x, std::vector<int>( y ) );
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3 Comments

Thank you, this sounds very logical, will try this. Many thanks :)!
Can be shortened to std::vector<std::vector<int>> v( x, y );
@ Matt McNabb No it can not. There is no conversion constructor that implicitly converts an integer to an object of type std::vector<int>
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There are multiple ways to do this. To create a permanent array on the heap:

int** data = new int*[x];
for(int i = 0; i < x;i++)
{
    data[i] = new int[y];
}

To create an array of fixed size on the stack:

int data[5][5];
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0

The dimensions of the array must be known at compile time.

For me this simple code fails to compile due to lack of knowledge of the array size.

int i = 5;

int arr [] = new int[i];

arr[2] = 2;

If you want to use arrays you have two choices, use constants that are known at compile time, or create the array using malloc to reserve memory.

If you are able to use the STL use the vector class.

int i = 5;

vector<int> vec(i);

vec[2] = 2;

Or you could use pointers as shown in 'Vlad from Moscow' or 'user3482801' answers.

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3 Comments

-1 for the malloc mention. This is C++, use new.
@JorenHeit, I don't say it's elegant, I would rather go for the vector, but malloc is part of the cstdlib. But I see your point.
Sure, vector would be best for dynamic arrays. But if you want to give advice on how to do dynamic allocation yourself in C++ (not C!), malloc is the forbidden word.

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