Bubble Sort C# giving an extra value

Question

I've tried to make a bubble sort algorithm but when I output the values sorted in order it gives me an extra value at the beginning and this value isn't in my array but I can't work out how to fix this.

For example when I run the program and my array is: 8, 3, 68, 74, 67, 82, 82, 18, 48, 53

The sorted values show: 60, 3, 8, 18, 48, 53, 67, 68, 74,82, 82

public static void Main(string[] args)
{
    int n = 10; //10 values in array
    Random r = new Random();

    int[] a; //array
    int temp;
    int i;
    a = new int[n + 1];
    a[0] = 1; //starts at 0

    for (i = 0; i <= n; i++) // set the array up
        a[i] = r.Next(1, 100); // + random numbers 

    for (i = 1; i <= n; i++)
        Console.WriteLine(a[i] + " "); // outputs the numbers of array

    Console.WriteLine();
    Console.ReadLine();

    for (i = 1; i <= n; i++)
    {
        for (int k = 1; k < a.Length - 1; k++) // n - 1 passes
        {
            if (a[k] > a[k + 1])
            {
                temp = a[k + 1]; //stores temporarily
                a[k + 1] = a[k];
                a[k] = temp;
            }
        }
    }

    Console.WriteLine("Array is sorted: ");
    foreach (int number in a) Console.Write(number + " ");
    Console.Read();
}

Answer 1

Here's what's happening:

1. You're initializing your array with 11 elements when you do this:

    int n = 10; int[] a = new int[n + 1]; // n + 1 is 11, so this creates an 11-element array

2. Then when you populate the array, you loop from 0 to 10 , populating all 11 indexes:

    for (i = 0; i <= n; i++) a[i] = r.Next(1, 100);`

3. When you sort the array, however, you ignore the first element (at index 0 ) because your loops always start with 1 , for example:

    for (i = 1; i <= n; i++) { for (int k = 1; k < a.Length - 1; k++) { // Sort the elements here } }

4. But then at the very end, you output ALL the elements, so you get to see the first number that you were skipping during the sort:

    foreach (int number in a) Console.Write(number + " ");

---

To fix this, normally when looping through an array we start at index 0 and we loop while our index variable is less than the array length:

for (i = 0; i < a.Length; i++)

This will always ensure you iterate over each item in the array.

Answer 2

I would consider a more Linq-supported approach:

using System;
using System.Collections.Generic;
using System.Linq;

namespace BubbleSort
{
    class Program
    {
        static readonly Random rnd = new Random();
        static void Swap(IList<int> list, int indexA, int indexB)
        {
            int tmp = list[indexA];
            list[indexA] = list[indexB];
            list[indexB] = tmp;
        }
        static void Main(string[] args)
        {
            var array = Enumerable.Range(1, 10).Select(r => rnd.Next(1,100)).ToArray();

            Console.WriteLine("Unsorted array:" + String.Join(" ", array.Select(x => x.ToString())));

            // array.Sort(); // probably not allowed
            for (int i = 0; i < array.Count() - 1; ++i)
            {
                for (int j = 0; j < array.Count() - 1; ++j)
                {
                    if (array[j] > array[j + 1]) Swap(array, j, j + 1);
                }
            }

            Console.WriteLine("Sorted array:" + String.Join(" ", array.Select(x => x.ToString())));
        }
    }
}

Note that your Bubble sort algo is in it's most inefficient form, always repeating n-1 times. Check Wikipedia for some tips on optimizing. Eg

var swapped = true;
while (swapped)
{
    swapped = false;
    for (int i = 0; i < array.Count() - 1; ++i)
    {
        if (array[i] > array[i + 1])
        {
            Swap(array, i, i + 1);
            swapped = true;
        }
    }
}

Answer 3

Make all your loops:

for(int SOMETHING = 0; ...

Not

for(int SOMETHING = 1; ...

OR

Make your final attempt at printing out the array like the earlier attempt (use an index starting from 1 not a foreach)

---

Arrays in c# start from 0. Per my comments; you were filling and sorting your arrays starting from index 1, but the final time that you print your array out you use a foreach which will include the 0th (first) element, which you didnt print, or sort, in the earlier parts of the algorithm

Here's a fixed up version of your code:

public static void Main(string[] args)
{
    int n = 10; //10 values in array
    Random r = new Random();

    int[] a = new int[n]; //array

    for (int i = 0; i < a.Length; i++) // set the array up
        a[i] = r.Next(1, 100); // + random numbers 

    foreach(int x in a)
        Console.WriteLine(x + " "); // outputs the numbers of array

    Console.WriteLine();
    Console.ReadLine();

    for (int i = 0; i < a.Length - 1; i++) 
    {
        for (int j = 0; j < a.Length - 1; j++) 
        {
            if (a[j] > a[j + 1])
            {
                int temp = a[j + 1]; //stores temporarily
                a[j + 1] = a[j];
                a[j] = temp;
            }
        }
    }

    Console.WriteLine("Array is sorted: ");
    foreach (int number in a) 
      Console.Write(number + " ");
    Console.Read();
}