Handling a NullReferenceException in C#

Question

I'm getting a NullReferenceException on the line:

items[size] = name; //This line is in the halfway through the code in the insert function.

I understand why I am getting it, but how do I handle it?

    using System;
    using System.IO;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;

    namespace IP01
    {
    public class Program
    {
    public class MinHeap
    {
        static string[] items;
        int size = 0;

        static void Main(string[] args)
        {
            string[] array = new string[5] { "f", "a", "c", "d", "e" };

            Console.Write("Original Array: ");
            for (int i = 0; i < array.Length; i++)
                Console.Write(" " + array[i]);
            Console.WriteLine();

            MinHeap heap = new MinHeap();
            heap.createHeap(array);
            heap.sort();

            Console.Write("Sorted Array: ");
            for (int i = 0; i < items.Length; i++)
                Console.Write(" " + items[i]);
            Console.WriteLine();
        }



        public void createHeap(string[] array)
        {
            if (array.Length > 0)
            {
                for (int i = 0; i < array.Length; i++)
                    insert(array[i]);
            }
        }

        public void rebuild(int rootIndex, int n)
        {
            int max = 2 * rootIndex + 1;
            bool larger;
            int less = string.Compare(items[max], items[max + 1]);

            if (less < 0)
                larger = true;
            else
                larger = false;

            bool larger2;
            int less2 = string.Compare(items[rootIndex], items[max]);

            if (less2 < 0)
                larger2 = true;
            else
                larger2 = false;

            if ((2 * rootIndex + 1) < n)
            {

                if ((2 * rootIndex + 2) < n)
                {
                    max = larger ? max + 1 : max;
                }

                if (larger2)
                {
                    string temp;

                    temp = items[rootIndex];
                    items[rootIndex] = items[max];
                    items[max] = temp;

                    rebuild(max, n);
                }
            }
        }

        public void Heap()
        {
            size = 0;
        }

        public bool isEmpty()
        {
            return size == 0;
        }

        public void insert(string name)
        {
            int child;
            int parent;

            items[size] = name;
            child = size;
            parent = (child - 1) / 2;

            bool larger3;
            int less3 = string.Compare(items[parent], items[child]);

            if (less3 < 0)
                larger3 = true;
            else
                larger3 = false;

            while (parent >= 0 && larger3)
            {
                string temp;
                temp = items[child];
                items[child] = items[parent];
                items[parent] = temp;

                child = parent;

                parent = (child - 1) / 2;
            }

            size++;
        }

        public void remove()
        {
            if (size == 0)
            {
                Console.Write("The heap is empty. No object removed.\n");
            }
            else
            {
                items[0] = items[size - 1];
                size--;

                rebuild(0, size);
            }
        }

        public void sort()
        {
            for (int i = size - 1; i >= 0; i--)
            {
                rebuild(i, size);
            }

            int last = size - 1;

            for (int i = 1; i <= size; i++)
            {
                string temp;

                temp = items[0];
                items[0] = items[last];
                items[last] = temp;

                last--;

                rebuild(0, last);
            }
        }
    }
}

}

You should not dump the entirety of your code and expect us to go through the entire thing to pinpoint the problem. That being said, I don't see anywhere in your code where you are actually initializing `items`. — Abion47, Jan 21 '17 at 06:40
Please re-read [MCVE] guidance on posting code to make your future questions better. — Alexei Levenkov, Jan 21 '17 at 07:09

kritikaTalwar · Accepted Answer · 2017-01-21T06:46:34.180

2

You have not initialized your array. try this. May be this could help you.

 items=new string[]{""};
 items[size] = name;

edited Jan 21 '17 at 06:46

answered Jan 21 '17 at 06:39

kritikaTalwar

1,730
1
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25

That fixed the error. Thanks! However, now, the same line says the IndexOutOfRangeException is not handled. – Leslie Jan 21 '17 at 06:59
you have used quotaion marks while initializing or not? – kritikaTalwar Jan 21 '17 at 07:02
Yes. I used exactly what you wrote. – Leslie Jan 21 '17 at 07:04
what is the size of array? Actually you have to use "" the number of times the size of array. example if size is 2 then items=new string[]{" "," "}; – kritikaTalwar Jan 21 '17 at 07:05
The length depends on how many strings are currently inserted into the min heap. I call the insert function in the createHeap() function. – Leslie Jan 21 '17 at 07:09
I am using the made-up "array" array in the main to populate the global "items" array once it is sorted – Leslie Jan 21 '17 at 07:11
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/133693/discussion-between-kritikatalwar-and-leslie). – kritikaTalwar Jan 21 '17 at 07:14

Handling a NullReferenceException in C#

1 Answers1