Construct your own random number generator


In Las Vegas random numbers move billions of Dollars in and out of gamblers' pockets. For the mildly inquisitive here is a test of three simple generators.

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// evaluation of three simple random number generators
// Written in Turbo C, modified for Pelles C
// get Pelles C for free:
// this C package comes with a great IDE

#include <stdio.h>   // printf(), getchar(), puts()

// three different custom random number generators
int rand1(int lim);
int rand2(int lim);
int rand3(int lim);

int main(void)
        int  k, rnd1, rnd2, rnd3, sum1, sum2, sum3;

        sum1 = sum2 = sum3 = 0;
        puts("Random numbers between 1 and 44:\n");
        printf("%15s%10s %10s %10s","Custom functions","rnd1()","rnd2()","rnd3()");
        for (k = 0; k < 40; k++) {
                rnd1 = rand1(44);   // method one
                rnd2 = rand2(44);   // method two
                rnd3 = rand3(44);   // method three
                printf("\n%25d %10d %10d",rnd1,rnd2,rnd3);
                sum1 += rnd1;  
                sum2 += rnd2;
                sum3 += rnd3;
        printf("\n\n%15s%10d %10d %10d   (ideal 22)\n",
        "   Average:",sum1/k,sum2/k,sum3/k);
        getchar();  // wait
        return 0;

// returns random integer from 1 to lim
int rand1(int lim)
        static long a = 100001;

        a = (a * 125) % 2796203;
        return ((a % lim) + 1);

// returns random integer from 1 to lim (Gerhard's generator)
int rand2(int lim)
        static long a = 1;  // could be made the seed value

        a = (a * 32719 + 3) % 32749;
        return ((a % lim) + 1);

// returns random integer from 1 to lim (Bill's generator)
int rand3(int lim)
        static long a = 3;

        a = (((a * 214013L + 2531011L) >> 16) & 32767);
        return ((a % lim) + 1);
bumsfeld 413 Nearly a Posting Virtuoso

I like simpel stuff!

ultirian 0 Newbie Poster

How would you implement this using the time as the seed?

nanodano 0 Junior Poster in Training

To use the time as the seed is very simple.

Make sure you include the <ctime> header.

#include <ctime>

Then, inside ctime there is a function you can call like this:


When you pass this method zero, it returns the time elapsed since January 1, 1970. This way you are guaranteed a differerent number each run time(if you run it less than 1 time per second).

Ok, so back to "how to make the time the seed" question. In these code examples replace the arbitrary number assigned to a with:

static long a = long(time(0));

Next, perform your random number generation using whichever method.

kirti4 0 Newbie Poster

why in 1st method u have used value 100001 to intialize the varible it a range of something.

irum.nageen.3 -6 Newbie Poster

It is one of the simplest logics, got it from a blog. in this logic you can limit the random numbers with that given modulus(%) operator inside the for loop,its just a copoy and paste from that blog, but any way check it out:


// random numbers generation in C++ using builtin functions
#include <iostream>

using namespace std;

#include <iomanip>

using std::setw;

#include <cstdlib>   // contains function prototype for rand

int main()
    // loop 20 times
    for ( int counter = 1; counter <= 20; counter++ ) {

        // pick random number from 1 to 6 and output it
        cout << setw( 10 ) << ( 1 + rand() % 6 );

        // if counter divisible by 5, begin new line of output
        if ( counter % 5 == 0 )
            cout << endl;


    return 0;  // indicates successful termination

} // end main
- See more at:
rubberman 1,355 Nearly a Posting Virtuoso Featured Poster

Two problems. One is that this thread is 8 years old! Two is that writing good random number generators is not a trivial undertaking, and thinking it is just shows your ignorance of the subject. Read this:

From the document, 2 relevant quotes:

Anyone who considers arithmetical methods of producing random digits is, of course, in a state of sin. -- John von Neumann (1951)

Anyone who has not seen the above quotation in at least 100 places is probably not very old. -- D. V. Pryor (1993)

nitin1 commented: those quotes are simple awesome!! :D +3
deceptikon commented: Yes. +12
JorgeM commented: very true.. nice.. +12
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