Setting a return value from a function as the size of an array

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Hi,

I have a function that returns an integer. I store that value in a variable "count"

I want to define a new char array whose size is this return value.
e.g. char new_array[count][20]

The column width 20 is constant. This represents the max string length.

But when I do this and try to compile the program it gives me the error:

error C2057: expected constant expression

How can I do this?

Posted 14-Dec-09 20:05pm

RoopeshBhole

Updated 2-Feb-10 9:33am

Sean Ewington

v4

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4 solutions

Solution 1

The only way to do this is with pointers.

int * myArray = new int[count];

My syntax may be off, I've not done C++ for years, but the core concept is definitely right - the only way to create a dynamic array is with pointers. Your other option is to use dynamic containers like std::vector.

Posted 14-Dec-09 20:44pm

Christian Graus

Solution 4

You need to dynamically allocate memory for your array, for instance:

  int getSize(){ return 3;}

  typedef int Chunk[20];

  void main()
  {
    int i,j, size;   
    Chunk *p;

    size = getSize();
    p = new Chunk[size];
    for (i=0; i<size; i++)
    {
      for (j=0; j<20; j++)
      {
        p[i][j]=i*j;
      }
    }
    delete []p;
}

:)

Posted 14-Dec-09 22:06pm

CPallini

v2

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stebich · Accepted Answer · 2009-12-14T20:51:00

Hi,

you can't declare an array with a variable value, because the compiler has to know the array size at compilation time.

What you have to do is to use a pointer to create a dynamic array like this:

main()
{
   int count = 0;
   int *DynArray = NULL;

   count = GetArraySizeFunction();
   DynArray = new int[count];
   if( NULL != DynArray)
   {
      for( int i0 = 0; count > i0; ++i0)
      {
         DynArray[i0] = 0;
      }

      // do with the array whatever you want

      delete[] DynArray; //Rajesh: Replaced delete with delete[]
                         //Carlo: Replaced 'Replacedd' with 'Replaced'  :-\
                         //Armin: Replaced 'Replacced' with 'Replacedd' in Carlos comment ;-)
   }
}

Rajesh R Subramanian · Accepted Answer · 2009-12-14T21:12:00

Solution 3

If you're using C++, then you'd be far better using something like std::vector<int>.

Vectors, lists, queues, etc., provide both good performance and reliability, and they're easy to use!

Posted 14-Dec-09 21:12pm

Rajesh R Subramanian

Updated 14-Dec-09 21:13pm

v3