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Please helpwith how to return two dimensional array

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khalidxa

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Please helpwith how to return two dimensional array

Jul 31st, 2007

I have been trying to return a two dimensional array in a function but it is getting me no where, please tell me how to do it. the code is as follow :

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
 int *population();
 int *population();

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
int *testCase::population()
		{
 
for(int i=0;i<3;i++)
{
for(int j=0;j<2;j++){
 
matrix[i][j]=rand()%2;			 
 
							}
	}
 
	return matrix[0][0];
 
}
int *testCase::population()
		{
				
for(int i=0;i<3;i++)
{
for(int j=0;j<2;j++){

matrix[i][j]=rand()%2;			 

							}
	}
		
	return matrix[0][0];
  
}

when I run the program, it returns a memory address. I want it to return the value for the array.

Regards

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Re: Please helpwith how to return two dimensional array

Aug 1st, 2007

You must read this for pointers-http://www.eternallyconfuzzled.com

I think that you might be unable to access the value stored in the returned memory address , so you could point to that memory address to get the value for getting the things right

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Re: Please helpwith how to return two dimensional array

Aug 1st, 2007

At the site you must check out the Languages section of the index.

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Re: Please helpwith how to return two dimensional array

Aug 1st, 2007

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Originally Posted by khalidxa

... when I run the program, it returns a memory address. I want it to return the value for the array.

array names 'decay' into pointers (addresses); this is fundamental to their use in C and C++. to return an array by value, you could either wrap a structure around the array and return the structure ( ok for pod types, you know the size at compile time) or return std library containers (which support value semantics).

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 C++ Syntax (Toggle Plain Text) 
#include <cstddef>
#include <vector>
 
template< typename T, size_t M, size_t N > struct array2d
{
  T a[M][N] ; // T must have a default constructor
};
 
array2d<int,10,5> foo()
{
  array2d<int,10,5> array ;
  for( size_t i=0 ; i<10 ; ++i )
    for( size_t j=0 ; j<5 ; ++j )
      array.a[i][j] = i*j ;
  return array ;
}
 
std::vector< std::vector<int> > bar( size_t M, size_t N )
{
  std::vector< std::vector<int> > array( M, std::vector<int>(N) ) ;
  for( size_t i=0 ; i<M ; ++i )
    for( size_t j=0 ; j<N ; ++j )
      array[i][j] = i*j ;
  return array ;
};
#include <cstddef>
#include <vector>

template< typename T, size_t M, size_t N > struct array2d
{
  T a[M][N] ; // T must have a default constructor
};

array2d<int,10,5> foo()
{
  array2d<int,10,5> array ;
  for( size_t i=0 ; i<10 ; ++i )
    for( size_t j=0 ; j<5 ; ++j )
      array.a[i][j] = i*j ;
  return array ;
}

std::vector< std::vector<int> > bar( size_t M, size_t N )
{
  std::vector< std::vector<int> > array( M, std::vector<int>(N) ) ;
  for( size_t i=0 ; i<M ; ++i )
    for( size_t j=0 ; j<N ; ++j )
      array[i][j] = i*j ;
  return array ;
};

caveat: returning large objects by value can be inefficient

Last edited by vijayan121; Aug 1st, 2007 at 3:19 am.

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Re: Please helpwith how to return two dimensional array

Aug 1st, 2007

•

Originally Posted by khalidxa

I have been trying to return a two dimensional array in a function but it is getting me no where, please tell me how to do it. the code is as follow :
Help with Code Tags

C++ Syntax (Toggle Plain Text)
 int *population();
int *population();
Help with Code Tags

C++ Syntax (Toggle Plain Text)
int *testCase::population()
		{
 
 
for(int i=0;i<3;i++)
{
for(int j=0;j<2;j++){
 
matrix[i][j]=rand()%2;			 
 
 
							}
	}
 
 
	return matrix[0][0];
 
 
}
int *testCase::population() { for(int i=0;i<3;i++) { for(int j=0;j<2;j++){ matrix[i][j]=rand()%2; } } return matrix[0][0]; }
when I run the program, it returns a memory address. I want it to return the value for the array.

Then ditch the asterisk which causes your function to return a pointer value. If you want the value stored in the array, then just return a plain int.

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khalidxa

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Re: Please helpwith how to return two dimensional array

Aug 1st, 2007

The problem is, It only returns the value for matrix[0][0]. I want the whole array to be returned. I just don't know how to do it.

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Re: Please helpwith how to return two dimensional array

Aug 1st, 2007

Last edited by hinduengg; Aug 1st, 2007 at 9:39 am.

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Re: Please helpwith how to return two dimensional array

Aug 1st, 2007

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I want it to return the value for the array.

Why? The only thing I can think of is that you want a copy of the array instead of something that changes the array owned by the testCase class. If that's the case, you should still return the address of the array because that's how arrays work, but you still have to make a new array.

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 C++ Syntax (Toggle Plain Text) 
int **testCase::population() {
  // Make the first dimension
  int **copy = new int*[3];
 
  for ( int i = 0; i < 3; ++i ) {
    // Make the second dimension
    copy[i] = new int[2];
 
    for ( int j = 0; j < 2; ++j ) {
      copy[i][j] = matrix[i][j] = rand() % 2;
    }
  }
 
  return copy;
}
int **testCase::population() {
  // Make the first dimension
  int **copy = new int*[3];

  for ( int i = 0; i < 3; ++i ) {
    // Make the second dimension
    copy[i] = new int[2];

    for ( int j = 0; j < 2; ++j ) {
      copy[i][j] = matrix[i][j] = rand() % 2;
    }
  }

  return copy;
}

That returns the dynamic array by pointer, but it's still a copy of the original array. You have to remember to free the memory so that's not a good way to do it. A better way is a vector object.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
vector<vector<int> > testCase::population() {
  vector<vector<int> > copy = vector( 3, vector<int>( 2, int ) );
 
  for ( int i = 0; i < 3; ++i ) {
    for ( int j = 0; j < 2; ++j ) {
      copy[i][j] = matrix[i][j] = rand() % 2;
    }
  }
 
  return copy;
}
vector<vector<int> > testCase::population() {
  vector<vector<int> > copy = vector( 3, vector<int>( 2, int ) );

  for ( int i = 0; i < 3; ++i ) {
    for ( int j = 0; j < 2; ++j ) {
      copy[i][j] = matrix[i][j] = rand() % 2;
    }
  }

  return copy;
}

If you don't want a copy, why do you want to return the array by value?

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khalidxa

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Re: Please helpwith how to return two dimensional array

Aug 1st, 2007

Could you please tell me what does this statement does
matrix[i][j]=rand()%2;

check your loop working

Besides you are yourself returning matrix[0][0]; so please try with a[i][j] in the j's loop

matrix[i][j]=rand()%2; this will randomly allocate values to the array . I did try returning matrix[i][j] , but it did not work, I'm confused

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Re: Please helpwith how to return two dimensional array

#10

Aug 1st, 2007

The best solution, is the one which hamrick describes, avoiding arrays altogether and using an STL vector

The problem with 'returning an array' is that arrays are returned by pointer. Which means one of the following must happen

- The array is declared static (Similar to a global object)
- The array is dynamically created inside the function with new
- The array is passed into the function as a parameter

If the array is passed into the function as a parameter, there's probably no need to return it

If the array is dynamically created with new, then you have all sorts of problems ahead, with regards to keeping track of the memory and ensuring it gets properly delete[] 'd

If the array is declared static, you can return it by reference

 Help with Code Tags 
 CPP Syntax (Toggle Plain Text) 
int ( &my_func() )[4][5]
{
    static int arr[4][5];
    return arr;
}
int ( &my_func() )[4][5]
{
    static int arr[4][5];
    return arr;
}

Note, that this is essentially the same as having a global variable wrapped in a function

The alternative, that someone else suggested, is to wrap it in a struct, and pass the struct around. (which causes copies to be made)

However, it would be better if you described what exactly you are trying to achieve, and where it fits into your program. returning an array isn't a "normal" activity, so you could probably do something differently elsewhere, without needing a function returning an array.

Last edited by Bench; Aug 1st, 2007 at 11:46 am.

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