Programming Forums > Application Development > C++

jayme · Mar 29th, 2006, 11:45 PM

I've hit a brick wall when trying to get a function to return an int array. Here's what my code looks like:

//prototype
int GetInvent();

//definition
int Player::GetInvent()
{
    for(int i = 0; i < inventory; i++)
    {
        return inventorymax[i];
    }
}

I have tried these two methods, but no luck either:

(Toggle Plain Text)

//prototype
int[] GetInvent();

//definition
int[] Player::GetInvent()
{
    for(int i = 0; i < inventory; i++)
    {
        return inventorymax[i];
    }
}


// And

//prototype
int GetInvent()[];

//definition
int Player::GetInvent() []
{
    for(int i = 0; i < inventory; i++)
    {
        return inventorymax[i];
    }
}

I didn't think my second solution would work, but niether of them did so, any help?

King · Mar 30th, 2006, 12:18 AM

I see a couple problems with this (first solution).. first of all are you trying to return the whole array, or are you trying to return a position in your array? if your trying to return the whole array, you need to have the return time of vector<int>, instead if just int. and another thing that seemed a bit off was your for loop; you have it returning a position in the array... but it would only loop through once and hit the return an exit this fucntion. try this if your trying to return the whole array... i am assuming your array is a vector, if not you will get the general idea...

(Toggle Plain Text)

//definition
std::vector<int> Player::GetInvent()
{
  return inventorymax;
}

maybe be a bit more specific if thats not what your looking for

Soulstorm · Mar 30th, 2006, 3:47 AM

I really don't understand what you want to do the way you put it. Maybe you will need to provide some more code, so that we can see what is 'inventorymax'.

But basically, regardless wether returning a whole array is possible, you can set your functions as 'void' and modify the functions so that they can work with pointers. That way, when you call the functions, the arrays that you have given as arguments will be changed. No need to return anything that way.

EDIT: I think that I have found something that may help you:
check this out.

Seems you cannot return a whole array. Look at this in the page I gave you:

(Toggle Plain Text)

#include <iostream>

#define LENGTH 5

using std::cout;
using std::endl;
using std::cin;

int* getArray();
void printArray(int* array);

int main(){
	int *p;
	cout << p;
	return 0;
}

int* getArray()
{
	int *array = new int[LENGTH];
	for ( int i = 0; i < LENGTH; i++ )
	{
		cout << "Enter an integer: ";
		cin >> array[i];
	}
	cout << "array in getArray()" << endl;
	printArray(array);
	return array;
}

void printArray(int* array)
{
	for ( int i = 0; i < LENGTH; i++ )
		cout << *(array+i) << " ";
	cout << endl;
	cout << endl;
}

The function getArray() seems to return an array, but it actually returns the reference to that array, so that it can be assigned to the 'pointered' to the new array in main(). I believe this is a very good example of such a problem.

nnxion · Mar 30th, 2006, 4:27 AM

Within a class it is something like:

(Toggle Plain Text)

#include <iostream>

using namespace std;

class Player
{
	private:
		int inventorymax[10];
		int inventory;
	public:
		Player();
		void SetInventory(int inv);
		int* GetInvent();
		int GetInventItem();
};

Player::Player()
{
	inventorymax[0] = 3;
	inventorymax[1] = 2;
	inventorymax[2] = 1;
}

void Player::SetInventory(int inv)
{
	inventory = inv;
}

int* Player::GetInvent()
{
	return inventorymax;
}

int Player::GetInventItem()
{
	static int i = 0;
	//for(; i < inventory;)
	while(i < inventory)
	{
		return inventorymax[i++];
	}
	return 0;
}

int main()
{
	int inv = 3;
	Player player1;

	player1.SetInventory(inv);

	int *array = player1.GetInvent();

	cout << array[0] << array[1] << array[2] << endl;

	for(int i = 0; i < inv; i++)
		cout << player1.GetInventItem() << endl;

	return 0;
}

grumpy · Mar 30th, 2006, 5:09 AM

The closest things you can get to returning an array follow. I assume for sake of discussion you want to return an array of int.

1) Return a pointer to a statically allocated array;

(Toggle Plain Text)

int *Array()
{
    static int array[5];
    return array;
}

The problem with this is that the array is static. Which means you can't obtain two arrays from it (two calls will both return the same pointer). The length must also be known at compile time.

2) Return a pointer to a dynamically allocated array;

(Toggle Plain Text)

int *Array(int length)
{
     return new int[length];
}

This allows you to obtain multiple arrays (each call to the function yields a different array), but the caller has to remember to clean those arrays up to avoid a memory leak.

3) Return a structure that contains an array;

(Toggle Plain Text)

typedef struct {int x[5];} IntArray;

IntArray Array()
{
     IntArray x;
     return x;  
}

The problem with this is that the length of the array must be known at compile time, and accessing elements requires going through a struct (eg the third element will be accessed as array.x[2]).

4) Use a C++ class that encapsulates an array, and provides necessary operators (eg operator[]) to allow direct access of elements. This is an extended form of one of the previous methods. For example;

(Toggle Plain Text)

class IntArray
{
  public:
    IntArray(int size) : array(new int[thesize]), size(thesize) {};
    IntArray(const IntArray &x) : array(new int [x.size]), size(x.size) {};
    IntArray &operator=(const IntArray &x)
    {
         IntArray temp(x);
         int *itemp = temp.array;
         temp.array = array;
         array = itemp;
         int isize = temp.size;
         temp.size = size;
         size = isize;
         return *this; 
    };
    ~IntArray() {delete [] array;};
 
    int &operator[](int index)
    {
        if (index < 0 && index >= size) ComplainBitterly();
        return array[index];
    };

    int operator[](int index) const
    {
        if (index < 0 && index >= size) ComplainBitterly();
        return array[index];
    };

private:
     int *array;
     int size;
   
};

IntArray Array(int size)
{
    IntArray x(size);
    return x;   
}

The obvious issue is that it takes a bit of work.

5) Return a std::vector<int>. This is a special case of the previous method, except that you don't have to roll your own.

LudakoT · Mar 31st, 2006, 7:25 PM

You can't return a whole array think for a bit how arrays go in memory

(Toggle Plain Text)

Memory location    Value
1                         32
2                         34
3                         36
4                         38
5                         39

Now when you do

(Toggle Plain Text)

int a[10];
SomeFunction(a);

You don't send all 10 elements to the function, that way a lot of memory would be used, when you cout or printf only the name of the array you get the array's beginning address i.e. a[0]. What you can do is send a pointer to the function use that pointer to modify the array and then return the pointer. Of course you can always use void() function that way you don't have to return anything since the array will be modified after the function's execution.

nnxion · Apr 1st, 2006, 4:41 AM

Quote:

Originally Posted by LudakoT

You can't return a whole array think for a bit how arrays go in memory

But you can encapsulate your array in an object, and that way, you can return even more than a whole array.

Mar 30th, 2006, 12:18 AM	#2
King Professional Programmer Join Date: Jan 2006 Location: Ontario, Canada Posts: 380 Rep Power: 3	I see a couple problems with this (first solution).. first of all are you trying to return the whole array, or are you trying to return a position in your array? if your trying to return the whole array, you need to have the return time of vector<int>, instead if just int. and another thing that seemed a bit off was your for loop; you have it returning a position in the array... but it would only loop through once and hit the return an exit this fucntion. try this if your trying to return the whole array... i am assuming your array is a vector, if not you will get the general idea... (Toggle Plain Text) //definition std::vector<int> Player::GetInvent() { return inventorymax; } //definition std::vector<int> Player::GetInvent() { return inventorymax; } maybe be a bit more specific if thats not what your looking for __________________ I am Addicted to Linux!

Mar 30th, 2006, 3:47 AM	#3
Soulstorm Hobbyist Programmer Join Date: Jan 2006 Location: Menidi, Athens, Greece Posts: 243 Rep Power: 3	I really don't understand what you want to do the way you put it. Maybe you will need to provide some more code, so that we can see what is 'inventorymax'. But basically, regardless wether returning a whole array is possible, you can set your functions as 'void' and modify the functions so that they can work with pointers. That way, when you call the functions, the arrays that you have given as arguments will be changed. No need to return anything that way. EDIT: I think that I have found something that may help you: check this out. Seems you cannot return a whole array. Look at this in the page I gave you: (Toggle Plain Text) #include <iostream> #define LENGTH 5 using std::cout; using std::endl; using std::cin; int* getArray(); void printArray(int* array); int main(){ int p; cout << p; return 0; } int getArray() { int array = new int[LENGTH]; for ( int i = 0; i < LENGTH; i++ ) { cout << "Enter an integer: "; cin >> array[i]; } cout << "array in getArray()" << endl; printArray(array); return array; } void printArray(int array) { for ( int i = 0; i < LENGTH; i++ ) cout << (array+i) << " "; cout << endl; cout << endl; } #include <iostream> #define LENGTH 5 using std::cout; using std::endl; using std::cin; int getArray(); void printArray(int* array); int main(){ int p; cout << p; return 0; } int getArray() { int array = new int[LENGTH]; for ( int i = 0; i < LENGTH; i++ ) { cout << "Enter an integer: "; cin >> array[i]; } cout << "array in getArray()" << endl; printArray(array); return array; } void printArray(int array) { for ( int i = 0; i < LENGTH; i++ ) cout << (array+i) << " "; cout << endl; cout << endl; } The function getArray() seems to return an array, but it actually returns the reference* to that array, so that it can be assigned to the 'pointered' to the new array in main(). I believe this is a very good example of such a problem. __________________ Project::Soulstorm (personal homepage) Last edited by Soulstorm; Mar 30th, 2006 at 4:00 AM.

Mar 30th, 2006, 4:27 AM	#4
nnxion Programming Guru Join Date: Jun 2005 Location: elemental plane Posts: 1,429 Rep Power: 5	Within a class it is something like: (Toggle Plain Text) #include <iostream> using namespace std; class Player { private: int inventorymax[10]; int inventory; public: Player(); void SetInventory(int inv); int* GetInvent(); int GetInventItem(); }; Player::Player() { inventorymax[0] = 3; inventorymax[1] = 2; inventorymax[2] = 1; } void Player::SetInventory(int inv) { inventory = inv; } int* Player::GetInvent() { return inventorymax; } int Player::GetInventItem() { static int i = 0; //for(; i < inventory;) while(i < inventory) { return inventorymax[i++]; } return 0; } int main() { int inv = 3; Player player1; player1.SetInventory(inv); int array = player1.GetInvent(); cout << array[0] << array[1] << array[2] << endl; for(int i = 0; i < inv; i++) cout << player1.GetInventItem() << endl; return 0; } #include <iostream> using namespace std; class Player { private: int inventorymax[10]; int inventory; public: Player(); void SetInventory(int inv); int GetInvent(); int GetInventItem(); }; Player::Player() { inventorymax[0] = 3; inventorymax[1] = 2; inventorymax[2] = 1; } void Player::SetInventory(int inv) { inventory = inv; } int* Player::GetInvent() { return inventorymax; } int Player::GetInventItem() { static int i = 0; //for(; i < inventory;) while(i < inventory) { return inventorymax[i++]; } return 0; } int main() { int inv = 3; Player player1; player1.SetInventory(inv); int array = player1.GetInvent(); cout << array[0] << array[1] << array[2] << endl; for(int i = 0; i < inv; i++) cout << player1.GetInventItem() << endl; return 0; } __________________ "Employ your time in improving yourself by other men's writings, so that you shall gain easily what others have labored hard for." -- Socrates Last edited by nnxion; Mar 30th, 2006 at 4:41 AM.*

Mar 30th, 2006, 5:09 AM	#5
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,260 Rep Power: 5	The closest things you can get to returning an array follow. I assume for sake of discussion you want to return an array of int. 1) Return a pointer to a statically allocated array; (Toggle Plain Text) int Array() { static int array[5]; return array; } int Array() { static int array[5]; return array; } The problem with this is that the array is static. Which means you can't obtain two arrays from it (two calls will both return the same pointer). The length must also be known at compile time. 2) Return a pointer to a dynamically allocated array; (Toggle Plain Text) int Array(int length) { return new int[length]; } int Array(int length) { return new int[length]; } This allows you to obtain multiple arrays (each call to the function yields a different array), but the caller has to remember to clean those arrays up to avoid a memory leak. 3) Return a structure that contains an array; (Toggle Plain Text) typedef struct {int x[5];} IntArray; IntArray Array() { IntArray x; return x; } typedef struct {int x[5];} IntArray; IntArray Array() { IntArray x; return x; } The problem with this is that the length of the array must be known at compile time, and accessing elements requires going through a struct (eg the third element will be accessed as array.x[2]). 4) Use a C++ class that encapsulates an array, and provides necessary operators (eg operator[]) to allow direct access of elements. This is an extended form of one of the previous methods. For example; (Toggle Plain Text) class IntArray { public: IntArray(int size) : array(new int[thesize]), size(thesize) {}; IntArray(const IntArray &x) : array(new int [x.size]), size(x.size) {}; IntArray &operator=(const IntArray &x) { IntArray temp(x); int itemp = temp.array; temp.array = array; array = itemp; int isize = temp.size; temp.size = size; size = isize; return this; }; ~IntArray() {delete [] array;}; int &operator[](int index) { if (index < 0 && index >= size) ComplainBitterly(); return array[index]; }; int operator[](int index) const { if (index < 0 && index >= size) ComplainBitterly(); return array[index]; }; private: int array; int size; }; IntArray Array(int size) { IntArray x(size); return x; } class IntArray { public: IntArray(int size) : array(new int[thesize]), size(thesize) {}; IntArray(const IntArray &x) : array(new int [x.size]), size(x.size) {}; IntArray &operator=(const IntArray &x) { IntArray temp(x); int itemp = temp.array; temp.array = array; array = itemp; int isize = temp.size; temp.size = size; size = isize; return this; }; ~IntArray() {delete [] array;}; int &operator[](int index) { if (index < 0 && index >= size) ComplainBitterly(); return array[index]; }; int operator[](int index) const { if (index < 0 && index >= size) ComplainBitterly(); return array[index]; }; private: int array; int size; }; IntArray Array(int size) { IntArray x(size); return x; } The obvious issue is that it takes a bit of work. 5) Return a std::vector<int>. This is a special case of the previous method, except that you don't have to roll your own.

Mar 31st, 2006, 7:25 PM	#6
LudakoT Newbie Join Date: Mar 2006 Location: Macedonia Posts: 12 Rep Power: 0	You can't return a whole array think for a bit how arrays go in memory (Toggle Plain Text) Memory location Value 1 32 2 34 3 36 4 38 5 39 Memory location Value 1 32 2 34 3 36 4 38 5 39 Now when you do (Toggle Plain Text) int a[10]; SomeFunction(a); int a[10]; SomeFunction(a); You don't send all 10 elements to the function, that way a lot of memory would be used, when you cout or printf only the name of the array you get the array's beginning address i.e. a[0]. What you can do is send a pointer to the function use that pointer to modify the array and then return the pointer. Of course you can always use void() function that way you don't have to return anything since the array will be modified after the function's execution.

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Programming Forums > Application Development > C++

Function array