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

<pre><code class="code-1 nolinks" id="code-20-40454166-1">Dictionary dict = new Dictionary&lt;int, int&gt;(); 
foreach (int i in array)
{
 if (dict.ContainsKey(i))
 dict[i]++;
 else
 dict.Add (i, 1)
 }
</code></pre>
<pre><code class="code-1 nolinks" id="code-20-40454166-2">Dictionary dict = new Dictionary&lt;int, int&gt;(); 
int dup = 0 
foreach (int i in array)
{
 if (dict.ContainsKey(i))
 dup = i;
 break;
}
</code></pre>
</div>

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

<pre><code class="code-10 nolinks" id="code-20-40454225-1">Dictionary dict = new Dictionary&lt;int, int&gt;();
int dup = 0 
foreach (int i in array)
{
 if (dict.ContainsKey(i))
 dup = i;
 break;
 else
 dict.Add (i, 1)
 }
</code></pre>
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<div>
Hello 
 
You can find the duplicate values using the following algorithm. 
 

<pre><code class="code-10 nolinks" id="code-20-40454282-1">int my_array[] = { 50, 51, 32, 50, 7, 3, 7 };
	int size = sizeof(my_array) / sizeof(my_array[0]);

	for (int i = 0; i &lt; size; i++)
	{
		for (int j = i + 1; j &lt; size; j++)
		{
			if (my_array[i] == my_array[j])
				printf(&quot; %d &quot;, my_array[i]);
		}
	}
</code></pre>
 
But also you can use 'Set' Associative containers which will ignore the duplicate values
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<div>
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. &nbsp;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. &nbsp;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. &nbsp;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.
</div>

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

<pre><code class="code-1 nolinks" id="code-20-40455212-1">var Duplicates = Numbers
	.GroupBy(x =&gt; x)
	.Where(x =&gt; x.Count() &gt; 1)
	.Select(x =&gt; x.Key)
	.ToArray();
</code></pre>
 
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. 
 

<pre><code class="code-1 nolinks" id="code-20-40455212-2">var Duplicates = Numbers
	.GroupBy(x =&gt; x)
	.Where(x =&gt; x.Count() &gt; 1)
	.Select(x =&gt; new { Number = x.Key, Count = x.Count() })
	.ToArray();
</code></pre>
 
A different approach from the previous answers but very fast. 
 
Hope this helps.
</div>

<div>
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. 
 

<pre><code class="code-10 nolinks" id="code-20-40455913-1">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 &lt; array.Length; i++)
			array[i] = rand.Next(1000000);

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

		sw.Stop();

		Console.WriteLine(&quot;Elapsed Time: {0}&quot;, sw.Elapsed.TotalMilliseconds);
		Console.ReadKey();
	}
}
</code></pre>
 
Hope this helps clarify matters.
</div>

<div>
@DaTribe 
 
Oh, it's on like Donkey Kong now... &nbsp; &nbsp;; ) 
 
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).
</div>

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

<div>
I'm still wondering if there is a reason the question author said &quot;randomly placed numbers from 1 to 1000000&quot; instead of &quot;random numbers from 1 to 1000000&quot;
</div>

<div>
@kaufman ,thank you for your great help and code of course !! it was really helpful 
@ozo ,thanks for your help 
@phoffric, thank you for your support
</div>

<div>
<div class="content wysiwyg-content">
Hi, 
 
I have been recently asked a question by someone and it drove me crazy as i could not give an efficient answer to a question. 
 
so the trick is; there is an array with numbers &nbsp;placed from 1 to 1 million randomly but one of the integers inside the array has been duplicated and i need to find this value in the most efficient way. what could this be? 
 
Obvious answer is to get the first value of the array and compare this with the rest and then move to the next one etc.. but this is not what is wanted, there needs to be a way to very efficiently retrieve this but how? 
 
could you help me on this puzzle like algorithm question? thanks
</div>
</div>

Hi,

I have been recently asked a question by someone and it drove me crazy as i could not give an efficient answer to a question.

so the trick is; there is an array with numbers  placed from 1 to 1 million randomly but one of the integers inside the array has been duplicated and i need to find this value in the most efficient way. what could this be?

Obvious answer is to get the first value of the array and compare this with the rest and then move to the next one etc.. but this is not what is wanted, there needs to be a way to very efficiently retrieve this but how?

could you help me on this puzzle like algorithm question? thanks

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

An algorithm is a self-contained step-by-step set of operations to be performed. Algorithms exist that perform calculation, data processing, and automated reasoning. Starting from an initial state and initial input (perhaps empty), the instructions describe a computation that, when executed, proceeds through a finite number of well-defined successive states, eventually producing "output" and terminating at a final ending state. The transition from one state to the next is not necessarily deterministic; some algorithms, known as randomized algorithms, incorporate random input.

Algorithms

C# is an object-oriented programming language created in conjunction with Microsoft’s .NET framework. Compilation is usually done into the Microsoft Intermediate Language (MSIL), which is then JIT-compiled to native code (and cached) during execution in the Common Language Runtime (CLR).

The .NET Framework is not specific to any one programming language; rather, it includes a library of functions that allows developers to rapidly build applications. Several supported languages include C#, VB.NET, C++ or ASP.NET.

.NET Programming

Language Integrated Query (LINQ) is a Microsoft .NET Framework component that adds native data querying capabilities to .NET languages, although ports exist for Java, PHP, JavaScript and ActionScript. LINQ extends the language by the addition of query expressions, which are akin to SQL statements, and can be used to conveniently extract and process data from arrays, enumerable classes, XML documents, relational databases, and third-party data sources.