Select a random number from stream, with O(1) space

// An efficient C# program to randomly 
// select a number from stream of numbers.
using System;

class GFG 
{
    // The resultant random number
    static int res = 0; 
    
    // Count of numbers visited
    // so far in stream
    static int count = 0; 
    
    // A method to randomly select 
    // a item from stream[0], 
    // stream[1], .. stream[i-1]
    static int selectRandom(int x)
    {
        // increment count of
        // numbers seen so far
        count++; 
        
        // If this is the first 
        // element from stream, 
        // return it
        if (count == 1)
            res = x;
        else
        {
            // Generate a random number 
            // from 0 to count - 1
            Random r = new Random();
            int i = r.Next(count);
            
            // Replace the prev random 
            // number with new number 
            // with 1/count probability
            if(i == count - 1)
                res = x;
        }
        return res;
    }
    
// Driver Code
public static void Main()
{
        int[] stream = {1, 2, 3, 4};
        int n = stream.Length;
        for(int i = 0; i < n; i++)
            Console.WriteLine("Random number from " + 
                              "first {0} numbers is {1}" ,
                          i + 1, selectRandom(stream[i]));
    }
}

// This code is contributed by mits

using System;

class Program
{
    static void Main()
    {
        int[] stream = { 1, 4, 5, 2, 7 };
        
        // Create an instance of the Random class
        Random rand = new Random();
        
        // Generate a random index between 0 and the length of the array
        int randomIndex = rand.Next(stream.Length);
        
        // Get the random element from the array using the generated index
        int randomNum = stream[randomIndex];
        
        // Print the random number
        Console.WriteLine("Random number is " + randomNum);
    }
}