RNG in C++

This is a discussion on RNG in C++ within the C++ Programming forums, part of the General Programming Boards category; Well, the following is an exercise that I have to use RNG with it, but I don't have any experience ...

  1. #1
    Amy amirahasanen1's Avatar
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    Question RNG in C++

    Well, the following is an exercise that I have to use RNG with it, but I don't have any experience related to such function (RNG), so I can't understand how to relate between the program and the code at the end of the post which generates random numbers, I am kind of confused between the N & M used in the exercise, and those M & n used in the note.
    I can not understand exactly the code at the end of the post.
    Thank you,

    -Amy


    Exercise:
    -------
    Use a random number generator to simulate N throws of a pair of dice and to give the number of times their sums were 7 or 11. Allow the program to input N (e.g. N = 10, 100, 1000) and to repeat the simulation M runs for the same N (e.g. M = 10). For each N, find the average probability of 7 or 11 for the M runs (sum the results for the M runs and divide by M then divide by N).

    Note on using the C++ Random Number Generator (RNG):
    -----------------------------------------------------
    This program uses random numbers generated by a Random Number Generator (RNG). The RNG in C++ is a function rand( ) that returns a random integer from 0 to 32,767. To obtain random integers from 1 through n, use rand( ) % n + 1. To obtain a random sequence you need to first initialise the RNG using the time of the machine as a seed. This is done so that we do not get the same sequence every time we run the program.
    The following is an example of how to generate a random sequence of M integers with values (x) between 1 and n :

    Code:
    #include <time.h>
    int i , x , n , M
    srand ( (unsigned) time (NULL) );        //Initialize RNG
    for ( i = 1; i <= M; i++)              // Loop over the sequence
    {            
        x = rand( ) % n + 1;            // Generate a number from the sequence
        cout << x << endl;            // Print it
    }
    It ain't illegal until you get caught..

  2. #2
    Registered User major_small's Avatar
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    well in that code, a random number between 1 and n is generated M times...

    in the description, it looks like they want you to throw the dice N times. let's use 10 for example. then it says that it wants you to thow the dice that many times for M times. let's use 5 there. so, now you have N as 10 and M as 5. what it's saying is to create a program to simulate N(10) dice throws M(5) times, and find the probability that the sum will either be 7 or 11 on any given throw.

    basically, keep the M and N seperate from the M,n,i, and x from the program code. that was probably not a very good choice on your instructors part. the question has some difficult wording as well, but that's something you'll need to get used to in programming classes...

    FYI: it's not really a RNG, it's actually more of a PRNG, or a Psuedo Random Number Generator, because the numbers it generates are not truly random, but it'll be good enough for now...
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  3. #3
    Amy amirahasanen1's Avatar
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    Quote Originally Posted by major_small
    well in that code, a random number between 1 and n is generated M times...

    in the description, it looks like they want you to throw the dice N times. let's use 10 for example. then it says that it wants you to thow the dice that many times for M times. let's use 5 there. so, now you have N as 10 and M as 5. what it's saying is to create a program to simulate N(10) dice throws M(5) times, and find the probability that the sum will either be 7 or 11 on any given throw.

    basically, keep the M and N seperate from the M,n,i, and x from the program code. that was probably not a very good choice on your instructors part. the question has some difficult wording as well, but that's something you'll need to get used to in programming classes...

    FYI: it's not really a RNG, it's actually more of a PRNG, or a Psuedo Random Number Generator, because the numbers it generates are not truly random, but it'll be good enough for now...
    Aha, well, now I will try working on the program and post it once I am done with it to see if it is made the right way, thanks for clarifying.

    - Amy
    It ain't illegal until you get caught..

  4. #4
    Amy amirahasanen1's Avatar
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    So I think I am done with the program and it's working fine, that's the code, if there are any comments please add them here.
    Thank you,

    -Amy


    Code:
    #include <iostream>
    #include <time.h>
    
    using namespace std;
    
    int main()
    {
    	int iN, iM, x1, x2, n , N, M;
    	float counter = 0;
    	int sum;
    
    	n = 6;
    	N = 10;
    	M = 5;
    
    	for ( iM = 1; iM <= M; iM++ )
    	{
    		
    		srand ( (unsigned) time (NULL) );		//Initialize RNG
    		for ( iN = 1; iN <= N; iN++ ) // Loop over the sequence
    		{
    			x1 = rand( ) % n + 1;	// Generate a number from the sequence
    			cout<<x1<<" ";			// Print it
    			
    			x2 = rand( ) % n + 1;	// Generate a number from the sequence
    			cout<<x2<<endl;			// Print it
    
    			sum = x1 + x2;
    
    			if((sum==7) || (sum==11))
    				counter += 1;		
    		}
    
    		cout<<endl;
    	}
    
    	cout<<"Average Probability: "<<(counter/M)/N;
    
    	cin.get();
    	return 0;
    }
    It ain't illegal until you get caught..

  5. #5
    Registered User major_small's Avatar
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    looks good to me. I would only make a very few, very slight changes to the code you had:
    Code:
    #include <iostream>
    #include <ctime>        //slightly more standard
    
    using namespace std;
    
    int main()
    {
        int iN, iM, x1, x2, n , N, M;
        float counter = 0;
        int sum;
    
        n = 6;
        N = 10;
        M = 5;
    
        for ( iM = 1; iM <= M; iM++ )
        {
    
            srand ( (unsigned) time (0) );        //I usually try to avoid macros
            for ( iN = 1; iN <= N; iN++ )
            {
                x1 = rand( ) % n + 1;    
                cout<<rand( ) % n + 1<<" ";          
    
                x2 = rand( ) % n + 1;  
                cout<<x2<<endl;         
    
                sum = x1 + x2;
    
                if((sum==7) || (sum==11))
                    counter++;  //style
            }
    
            cout<<endl;
        }
    
        cout<<"Average Probability: "<<(((counter/M)/N)*100)<<'%';
        //using a percentage instead of a decimal
    
        cin.ignore(1); //style over substance
        return 0;
    }
    like I said in that last comment, it's all style over substance, and these changes are opinion rather than fact.

    good coding.
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  6. #6
    C++ Witch laserlight's Avatar
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    I think you could place the srand() call before the first for loop - you shouldnt need to call it multiple times.

    Also, using rand() % n can lead to a slight bias if the range of possible random integers is not congruent to n (i.e. RAND_MAX+1 is not 0 (mod n) )
    There are ways to fix this, and you might want to explore them for fun - or for marks if your teacher considers your solution as creative or something.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
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