Help with arrays

I can't figure out what I am doing wrong with this program. When I print the array it outputs a bunch of random large negative numbers and then the proper numbers. Please help me!

Code:

#include <iostream>  //for cin, cout
#include <conio.h>     //for _getch()

using namespace std;

const int MAX_SIZE = 20;          // maximum size of array



void ReadList( int list[],     // array of nonnegative integers
               int& length);   // number of nonnegative integers
                               //  in the array
//----------------------------------------------------------------------------
//Purpose: To read nonnegative integers into an array
//Precondition: None
//Postcondition: length has been assigned the number of nonnegative integers read 
//                 and stored in list  length >0
//               list[0] through list[length - 1] have been filled with nonnegative
//               integers entered from the keyboard
//------------------------------------------------------------------------------


void PrintList(const int list[],  // array of nonnegative integers
               int length);       // number of nonnegative integers
                                  //    in the array
//---------------------------------------------------------------------------
//Purpose: To print the nonnegative integers stored in the array list
//Precondition: length is the number of nonnegative integers in list.
//                list[0] through list[length-1] are assigned nonnegative integers
//Postcondition: nonnegative integers in list[0] through list[length-1] have been
//                    printed
//-----------------------------------------------------------------------------

int main()
{
    int list[MAX_SIZE];         // array of nonnegative integers
    int length;                 // number of nonnegative integers in array
    
    //Call the functions
    ReadList(list, length);
    PrintList(list, length);
          
       
      
   


    _getch();  

    return 0;
}


//-----------------------------------------------------------------------------

void ReadList(int list[],    // array of nonnegative integers
              int& length)  // number of nonnegative integers
                            //  in the array

//Purpose: To read nonnegative integers into an array

{
    int number;        
    int index = 0;
    cout << "Enter nonnegative integers each separated by a blank space,\n"
         << "and mark the end of the list with a negative number: ";
     cin >> number;    //read the first integer entered

       //check that the number is nonnegative and
       // the size of the array is not exceeded
    while ((number >=0) && (index < MAX_SIZE))
      {
         list[index] = number;   //store the integer in the array
         index++;                 // increment the index
         cin >> number;             // read the next integer          
    }

    length = index;  // length is the number of nonnegative integers
                     // in the list
}


//----------------------------------------------------------------------------


void PrintList(const int list[],
               int length)
{
   int index;

    cout << "\nThe list contains " << length
         << " nonnegative integer(s) as follows: \n";

   for (index = MAX_SIZE; index >= 0; index--)
   {
       cout << list[index] << endl;
   }

       

}

//----------------------------------------------------------------------------

 

What input do you give, what output do you get, what output do you expect, and what values do you think index will take within the following loop:

Code:

for (index = MAX_SIZE; index >= 0; index--)

 

I input a string of numbers ending them with a negative number. My output gives me those numbers, (which I expected and need), but after those the remainder of the space in the array is filled up with a large negative number. I think the number is -8123460 or something like that.

The output looks like this:
Input: 1 2 -3

Output:
1
2
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460
-899837460

The large negatives seem to fill in the space unused by the inputted values, because if I enter 20 values (MAX_SIZE) there are none of those negative numbers.

 

I'm puzzled by the fact that you loop from MAX_SIZE to zero in PrintList, on two counts:
1. why are you looping from MAX_SIZE instead of length
2. you appear to be getting output in the correct direction, i.e. list[0] first. If you loop from the end to the beginning then you *should* get
-899837460
-899837460
...
-899837460
2
1


The reason for the junk values is that you're displaying uninitialised data. If you've entered 1,2 as input, then list[0]=1 and list[1]=2, but list[2] through list[MAX_SIZE-1] just contain whatever was on the stack. LESSON here: always initialise data!

 

Also another problem with looping from MAX_SIZE is that list[MAX_SIZE] is actually undefined. MAX_SIZE is the number of elements in the array, and the first is at zero, so if MAX_SIZE is 5 then list[0], list[1], list[2], list[3], list[4] are defined, but list[5] isn't. So attempting to use list[MAX_SIZE] will give you undefined behaviour.

 

Ok, so i changed the loop from MAX_SIZE to length and it worked. Thank you very much for the help