Returning two-dimensional array in C++

This is a discussion on Returning two-dimensional array in C++ within the C++ Programming forums, part of the General Programming Boards category; Hi! I want to make a get() function for a two-dimensional array in my class. Like this: Code: class MyClass ...

  1. #1
    Registered User Drogin's Avatar
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    Returning two-dimensional array in C++

    Hi!
    I want to make a get() function for a two-dimensional array in my class.
    Like this:
    Code:
    class MyClass {
      public:
      getArray();  // <-----How do I define and decleare this function?
    
      private:
      int array[5][5];
    };

    So, I would like to use the function like this:

    Code:
    int main() {
    MyClass foo;
    
    // *Decleare the array somehow* <-------how would I do this?
    array = foo.getArray();
    return 0;
    }

  2. #2
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    I'm guessing you want to use something like:
    Code:
    array[1][3] = 40;
    I may not know how to achieve that, but you could instead. workaround is simply using int* as the return type/declaration type. Then add getWidth() and getHeight() functions.

    Code:
    array[1 * getHeight() + 3]
    Assuming you have differing dimensions:

    Code:
    int arr[H][W];
    arr[y][x] = v;
    then the native index:
    Code:
    arr[ y * H + W ] = v;

  3. #3
    C++まいる!Cをこわせ! Elysia's Avatar
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    std::vector< std::vector<type> > foo()
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  4. #4
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    Actually, there was another post just now, with a similar problem, so with no life of mine i made a sample for you:

    Code:
    typedef int array2d [5][5];
    
    array2d arr;
    
    array2d* get_arr()
    {
    	return &arr; 
    }
    
    int main()
    {
    	int i;
    	int *t;
    
    	t = (int*)(arr);
    	for (i = 0; i < 25; i++) {
    		t[i] = i;
    	}
    	printf("%d\n", arr[3][2]);
    
    	array2d *a1;
    	a1 = get_arr();
    
    	printf("%d\n", (*a1)[3][2]);
    
    	return 0;
    }
    thank this guy King Mir:
    Array -> 2D array

  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
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    Use a vector. It's far easier.
    No global variables either.
    This is C++, not C...
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  6. #6
    The larch
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    A vector of vectors may have too much overhead to be a replacement for small multidimensional stack array. (I believe there are cases where it is justified not to want dynamic allocations.)

    By the way, do you want to return a copy of the array?

    In that case, it might be meaningful to use an out parameter (the contents are copied to a suitable array provided by the caller).

    If you don't want a copy, then you might try returning a pointer to pointer (int**).

    -----
    Best advice depends on what exactly you are doing this for...
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  7. #7
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    Quote Originally Posted by anon View Post
    A vector of vectors may have too much overhead to be a replacement for small multidimensional stack array. (I believe there are cases where it is justified not to want dynamic allocations.)

    By the way, do you want to return a copy of the array?

    In that case, it might be meaningful to use an out parameter (the contents are copied to a suitable array provided by the caller).

    If you don't want a copy, then you might try returning a pointer to pointer (int**).

    -----
    Best advice depends on what exactly you are doing this for...
    Returning a pointer to pointer won't work, since you loose the dimension of the array. You will need to indicate at least the last dimension of the array.

    gcc (and g++) doesn't seem to allow functions returning arrays.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  8. #8
    Registered User Drogin's Avatar
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    I can give you a very simple Java-verison of what I want.
    I will comment it well, so non-java people can read it.

    So, I would like a C++ verison of this java-code:
    Code:
    class MyClass {
        private int[][] iPointer = new int[5][5]; // This makes a new array of int, and let iPointer point to it
    
        // This is the get-function. It returns a pointer to the array
        public int[][] getArray() {
            return iPointer;
        }
    }
    
    
    class MainClass {
        public static void main(String[] args) { // This is the java int main()
            MyClass myclass = new MyClass(); // Run the above class.
            int[][] anotherPointer; // Decleare a pointer that should be pointing to a 2-dimensional int array
            anotherPointer = myclass.getArray(); // Set the pointer to point to the array returned
    
    
            // Then, I want to be able to do this, on the array returned.
            // (This pointer points to the iPointer member in myclass)
            anotherPointer[0][0]=5;
            anotherPointer[0][1]=3;
        }
    }

  9. #9
    C++まいる!Cをこわせ! Elysia's Avatar
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    If you want Java approach, it's easy if you use std::vector. You can return std::vector, so essentially, you can return an array that way.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  10. #10
    Registered User Drogin's Avatar
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    Well, I know.
    But there should be a way to do this without vectors too...?
    Let's say I would use this code-block inside a game-loop, using vectors. That would hurt. If you catch my drift

  11. #11
    C++まいる!Cをこわせ! Elysia's Avatar
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    Then use references.
    Create the vector inside the class and let getArray return a reference to it.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  12. #12
    Registered User Drogin's Avatar
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    So there is no way to do it without vectors?
    (Hm, maybe I should've posted in the C-forum instead, given the devlopement of this thread. Oh well..)

  13. #13
    C++まいる!Cをこわせ! Elysia's Avatar
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    Why? Why are you so anti-vector?
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  14. #14
    Kernel hacker
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    You should be able to return a pointer to an array in a class. Let me work on it for few minutes.

    Not very pretty:
    Code:
    #include <iostream>
    
    typedef int myarray[5][5];
    
    class myclass
    {
        myarray arr;
    
    public:
        myclass() {
    	for(int i = 0; i < 5; i++)
    	{
    	    for(int j = 0; j < 5; j++)
    		arr[j][i] = i * j;
    	}
        }
        friend std::ostream &operator<<(std::ostream &os, const myclass &c);
        myarray * getArray()
    	{
    	    return &arr;
    	}
    
    
    };
    
    std::ostream &operator<<(std::ostream &os, const myclass &c) 
    {
        for(int i = 0; i < 5; i++)
        {
    	for(int j = 0; j < 5; j++)
    	    os << c.arr[j][i] << " ";
    	
    	os << std::endl;
        }
        
        return os;
    }
    
    
    
    int main()
    {
        myclass c;
        int (*p)[5][5];
       
        std::cout << c;
      
        p = c.getArray();
        for(int i = 0; i < 5; i++)
        {
    	for(int j = 0; j < 5; j++)
    	    std::cout << (*p)[j][i] << " ";
    	std::cout << std::endl;
        }
        return 0;
    
    }
    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  15. #15
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    Quote Originally Posted by Drogin View Post
    I want to make a get() function for a two-dimensional array in my class.
    Why? Something smells bad here...

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