Return Array from C++

May 11th, 2004

Re: Return Array from C++

Quote originally posted by tlee ...

However, my current knowledge does not allow me to see how the pointer can solve this C++ limitation (return a pointer might be dangerous because it might point to the address of local variable, and this variable will go out of scope. Hence, this pointer will point to invalid value). I appreciate for your information.

Pass a pointer to the start as a parameter to the function, and then return this pointer when you're done.

C++ Syntax (Toggle Plain Text)
#include <iostream>
 
int *foo(int *array)
{
   int *start = array;
   while ( *array )
   {
	  std::cout << *array++ << ' ';
   }
   std::cout << std::endl;
   return start;
}
 
int *bar(int *array)
{
   int *start = array;
   while ( *array )
   {
	  *array++ += 1;
   }
   return start;
}
 
int main()
{
   int myarray[] = {1,2,3,4,5,0};
   foo(bar(foo(bar(foo(myarray)))));
   return 0;
}
 
/* my output
1 2 3 4 5
2 3 4 5 6
3 4 5 6 7
*/
#include <iostream>
 
int *foo(int *array)
{
   int *start = array;
   while ( *array )
   {
	  std::cout << *array++ << ' ';
   }
   std::cout << std::endl;
   return start;
}
 
int *bar(int *array)
{
   int *start = array;
   while ( *array )
   {
	  *array++ += 1;
   }
   return start;
}
 
int main()
{
   int myarray[] = {1,2,3,4,5,0};
   foo(bar(foo(bar(foo(myarray)))));
   return 0;
}
 
/* my output
1 2 3 4 5
2 3 4 5 6
3 4 5 6 7
*/

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May 11th, 2004

Re: Return Array from C++

I am sorry. I did not make my point clear. Is there any way to return the array from the function, this array is created within that function? For instance, I want to have one function to return an array like int[] getArray() (it is valid in Java, but not in C++), so is there any pointer trick to create one such function in C++?
Thanks in advance, by the way, thanks Dava

tlee

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Solved Threads: 0

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since May 2004

May 11th, 2004

Re: Return Array from C++

Hi all,
We can see that the result is not what we are expected. Can anyone tell me what need to fix in order to get the correct result using same function? Thanks in advance.

#include <iostream>
#define LENGHT 5

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

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

int main()
{
int* array = getArray();

cout << "array in main" << endl;
printArray(array);

return 0;
}

int* getArray()
{
int array[LENGHT] = {1, 2, 3, 4, 5};

cout << "array in getArray()" << endl;
printArray(array);

return array;
}

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

cout << endl;
cout << endl;
}

/*
Output
array in getArray()
1 2 3 4 5

array in main
-858993460 -858993460 -858993460 3 1245056

Press any key to continue

*/

tlee

Reputation Points: 21

Solved Threads: 0

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since May 2004

May 11th, 2004

Re: Return Array from C++

One way:

int* getArray()
{
   static int array[LENGHT] = {1, 2, 3, 4, 5};

   cout << "array in getArray()" << endl;
   printArray(array);

   return array;
}

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May 11th, 2004

Re: Return Array from C++

Hi Dave,
I appreciate for your help, but it does not work correctly in the following case. We declare static local variable, so this variable will stay until the program is terminated. Hence, although we want to have 2 different arrays, the first one is modified because 2 pointers in main() actually point in the same memory location. Would you show me another way to do it? By the way, in the first version, I mistyped LENGHT, sorry for that.

#include <iostream>
#define LENGTH 5

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

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

int main()
{
int* array = getArray();

cout << "array in main" << endl;
printArray(array);

int* array2 = getArray();

cout << "array in main" << endl;
printArray(array2);

cout << "array in main" << endl;
printArray(array);

return 0;
}

int* getArray()
{
int static array[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;
}

/*
Output:

Enter an integer: 1
Enter an integer: 2
Enter an integer: 3
Enter an integer: 4
Enter an integer: 5
array in getArray()
1 2 3 4 5

array in main
1 2 3 4 5

Enter an integer: 10
Enter an integer: 20
Enter an integer: 30
Enter an integer: 40
Enter an integer: 50
array in getArray()
10 20 30 40 50

array in main
10 20 30 40 50

array in main
10 20 30 40 50

Press any key to continue

*/

tlee

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Solved Threads: 0

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since May 2004

May 11th, 2004

Re: Return Array from C++

Quote originally posted by tlee ...

Hence, although we want to have 2 different arrays, the first one is modified because 2 pointers in main() actually point in the same memory location.

Ah. You want a function that creates a new array each time it is called.

#include <iostream>
 
#define LENGTH 5
 
using std::cout;
using std::endl;
using std::cin;
 
int* getArray();
void printArray(int* array);
 
int main()
{
   int* array = getArray();
   cout << "array in main" << endl;
   printArray(array);
   int* array2 = getArray();
   cout << "array2 in main" << endl;
   printArray(array2);
   cout << "array in main" << endl;
   printArray(array);
delete[] array2;
   delete[] array;
   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;
}
 
/* my output
Enter an integer: 1
Enter an integer: 2
Enter an integer: 3
Enter an integer: 4
Enter an integer: 5
array in getArray()
1 2 3 4 5 
array in main
1 2 3 4 5 
Enter an integer: 6
Enter an integer: 7
Enter an integer: 8
Enter an integer: 9
Enter an integer: 10
array in getArray()
6 7 8 9 10 
array2 in main
6 7 8 9 10 
array in main
1 2 3 4 5 
*/

Last edited by The Other Dave; May 11th, 2004 at 5:47 pm. Reason: Put back in (most of) the indention spaces that were removed when posting.

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May 11th, 2004

Re: Return Array from C++

Thanks Dave. However, would you mind if I have another question regarding to the same problem? While I was waiting for your solution, I developed one in below.
I know your code is much superior than mine. I am just confused that why the compiler issues the error when I try to delete the pointers. Could you give me the explanation please?

#include <iostream>
#define LENGTH 5

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

class Array
{
public:
Array()
{
array = new int(LENGTH);
}

~Array()
{
delete[] array;
}

void setElement(int index, int value)
{
if ( index < 0 || index >= LENGTH )
cout << "INVALID INDEX" << endl;

array[index] = value;
}

int getElement(int index) const
{
if ( index < 0 || index >= LENGTH )
cout << "INVALID INDEX" << endl;

return array[index];
}
private:
int* array;

};

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

int main()
{
Array* array = getArray();

cout << "array in main" << endl;
printArray(array);

Array* array2 = getArray();

cout << "array in main" << endl;
printArray(array2);

cout << "array in main" << endl;
printArray(array);

delete array;
delete array2;

return 0;
}

Array* getArray()
{
int value = 0;
Array* array = new Array();

for (int i = 0; i < LENGTH; i++)
{
cout << "Enter an integer: ";
cin >> value;
array->setElement(i, value);
}

cout << "array in getArray()" << endl;
printArray(array);

return array;
}

void printArray(Array* array)
{
for (int i = 0; i < LENGTH; i++)
cout << array->getElement(i) << " ";

cout << endl;
cout << endl;
}

/*
Output:

Enter an integer: 1
Enter an integer: 2
Enter an integer: 3
Enter an integer: 4
Enter an integer: 5
array in getArray()
1 2 3 4 5

array in main
1 2 3 4 5

Enter an integer: 10
Enter an integer: 20
Enter an integer: 30
Enter an integer: 40
Enter an integer: 50
array in getArray()
10 20 30 40 50

array in main
10 20 30 40 50

array in main
1 2 3 4 5

Press any key to continue
*/

tlee

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Solved Threads: 0

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May 11th, 2004

Re: Return Array from C++

C++ is not my native tongue, but I'll mention a couple of things and try to wade through the linter's output.

First, I believe you are trying to delete the objects twice. Once in main(), and then again in the destructor when the object goes out of scope.

[edit1]

C++ Syntax (Toggle Plain Text)
	 if ( index < 0 || index >= LENGTH )
		 cout << "INVALID INDEX" << endl;
	 array[index] = value;
	 if ( index < 0 || index >= LENGTH )
		 cout << "INVALID INDEX" << endl;
	 array[index] = value;

Here you may attempt to access out of array bounds.
[/edit1]

[edit2]

 Array()
{
	 array = new int(LENGTH);
}

I think you want the square brackets.

 Array()
{
	 array = new int[LENGTH];
}

[/edit2]

[edit3]
Lint:

Quote ...

[Info 1732] new in constructor for class 'Array' which has no assignment operator
[Info 1733] new in constructor for class 'Array' which has no copy constructor

I forget why, but it's right.
[/edit3]

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