i have an array with randomly placed numbers from 1 to 1000000 ,there is an integer duplicated in the array what is the easiest , perf. efficient way to find that value

If the array contains 1000001 entries, with one each of all integer values from 1 to 1000000, except for one value that is duplicated, then you can take the sum of all elements and subtract 500000500000

In databases, the 'standard' method is to sort the rows on the field you want to find the duplicates in.  Then duplicates will be in consecutive rows.  If you can sort the array, you can look for the occurrence of the same value in two consecutive positions.  I don't know C# but PHP has an array_sort function that I can use.  Sort functions are usually much more efficient than going thru the array and making comparisons to each and every element.  Then I can use 'foreach' to go thru the sorted array to find items that are duplicated.

If the array can contain some missing values as well as a duplicate value, then you might use a hash table or a bit vector to record numbers that you have seen and detect a duplicate when you find a number you have seen before

You could move the array to a dictionary and either stop when you get an error or even get a count of the number of duplicates eg

Dictionary dict = new Dictionary<int, int>();    
foreach (int i in array)
{
   if (dict.ContainsKey(i))
      dict[i]++;
    else
        dict.Add (i, 1)
 }

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Dictionary dict = new Dictionary<int, int>(); 
int dup = 0   
foreach (int i in array)
{
     if (dict.ContainsKey(i))
         dup = i;
         break;
}

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Second code block from my post is wrong. 2nd time lucky

Dictionary dict = new Dictionary<int, int>();
int dup = 0 
foreach (int i in array)
{
   if (dict.ContainsKey(i))
      dup = i;
      break;
    else
       dict.Add (i, 1)
 }

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Hello

You can find the duplicate values using the following algorithm.

int my_array[] = { 50, 51, 32, 50, 7, 3, 7 };
	int size = sizeof(my_array) / sizeof(my_array[0]);

	for (int i = 0; i < size; i++)
	{
		for (int j = i + 1; j < size; j++)
		{
			if (my_array[i] == my_array[j])
				printf(" %d ", my_array[i]);
		}
	}

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But also you can use 'Set' Associative containers which will ignore the duplicate values

I'd have suggested a dictionary - lot of the work already done for you - and check as you add to the dictionary if that key is in use.  BUT, Michael74 has already suggested that.
I note you request efficient.
You also talk of 1 to 1 million as the range of numbers being stored in the array but it is no clear how many slots there are in your array.  10, 100, 1000, 100,000,000 (extreme but not excluded from your wording as I understand it)
If the array has relatively few values then the 'double' looping is easy to code and understand and probably pretty much as efficient as the dictionary.  However with 100 million entries in the array (that still leaves a reasonable possibility of some numbers only appearing once) then that is very poor on performance.

Hi,

Here is an approach in C# that is more commonly used in an RDBMS. Its effective and quite fast. Simply using Grouping.

var Duplicates = Numbers
	.GroupBy(x => x)
	.Where(x => x.Count() > 1)
	.Select(x => x.Key)
	.ToArray();

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Any numbers found in the resulting array are duplicates. If you want to know how often each number of duplication then you can try the following query.

var Duplicates = Numbers
	.GroupBy(x => x)
	.Where(x => x.Count() > 1)
	.Select(x => new { Number = x.Key, Count = x.Count() })
	.ToArray();

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A different approach from the previous answers but very fast.

Hope this helps.

Bringing the figures to the table might clarify matters a bit. Using an example of 1 million randomly generated numbers such as the following yielded an elapsed time of 531.0978ms.

class Program
{
	static void Main(string[] args)
	{
		var rand = new Random((int) DateTime.Now.Ticks);
		var sw = new Stopwatch();

		var array = new int[1000000];

		for (int i = 0; i < array.Length; i++)
			array[i] = rand.Next(1000000);

		sw.Start();
			
		var duplicates = array
			.GroupBy(x => x)
			.Where(x => x.Count() > 1)
			.Select(x => x.Key)
			.ToArray();

		sw.Stop();

		Console.WriteLine("Elapsed Time: {0}", sw.Elapsed.TotalMilliseconds);
		Console.ReadKey();
	}
}

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Hope this helps clarify matters.

@DaTribe

Oh, it's on like Donkey Kong now...    ; )

My code is slower because of the printing to the console. Take that bit out, and you're looking at around 15 ms (on my i7-3770).

1/2 second is a very long time, IMO.

Okay I concede. :)

I'm still wondering if there is a reason the question author said "randomly placed numbers from 1 to 1000000" instead of "random numbers from 1 to 1000000"