CS433 Modeling and Simulation Lecture 15 Random Number Generator Dr. Anis Koubâa 24 May 2009 Al-Imam Mohammad Ibn Saud Islamic University College Computer and Information Sciences

Reading  Required  Chapter 4: Simulation and Modeling with Arena  Chapter 2: Discrete Event Simulation - A First Course,  Optional  Harry Perros, Computer Simulation Technique - The Definitive Introduction, 2007 Chapter 2 and Chapter 3

Goals of Today  Understand the fundamental concepts of Random Number Generators (RNGs)  Lean how to generate random variate for standard uniform distributions in the interval [0,1]  Learn how to generate random variate for any probability distribution

Outline  Why Random Number Generators   Desired Properties of RNGs  Lehmer’s Algorithm  Period and full-period RNGs  Modulus and multiplier selection (Lehmer)  Implementation - overflow

Why Random Number Generators   Random Numbers (RNs) are needed for doing simulations  Generate random arrivals (Poisson), random service times (Exponential)  Random numbers must be Independent (unpredictable) A1 A2 Random Inter-Arrival Time: Exp(A) D1 Random Service Time: Exp(  ) A3 NOW

The Concept  Goal: create random variates for random variables X of a certain distribution defined with its CDF  Approach: Inverse cumulative distribution function CDF F ( x )= u  Cumulative Distribution Function  Generate a random variable  Determine x such that u1u1 X1 Exp u2u2 X1 Poisson X2 Exp X2 Poisson

Problem Statement of RNGs Problem: We want to create a function: u = rand(); that  produces a floating point number u, where 0<u<1 AND  any value strictly greater than 0 and less than 1 is equally likely to occur (Uniform Distribution in ]0,1[ )  0.0 and 1.0 are excluded from possible values

Problem Statement of RNGs  This problem can be simply resolved by the following technique:  For a large integer m, let the set  m = {1, 2, … m-1}  Draw in an integer x   m randomly  Compute: u = x/m m should be very large  Our problem reduces to determining how to randomly select an integer in  m

Lehmer’s Algorithm  The objective of Lehmer’s Algorithm is to generate a sequence of random integers in  m : x 0, x 1, x 2, … x i, x i+1, …  Main idea: Generate the next x i+1 value based on the last value of random integer x i x i+1 = g(x i ) for some function g(.)

Lehmer’s Algorithm  In Lehmer’s Algorithm, g(.) is defined using two fixed parameters  Modulus m : a large, fixed prime integer  Multiplier a : a fixed integer in  m  Then, choose an initial seed  The function g(.) is defined as:  The mod function produces the remainder after dividing the first argument by the second  More precisely: where is the largest integer n such that n≤ x

Observations  The mod function ensures a value lower than m is always produced  If the generator produces the value 0, then all subsequent numbers in the sequence will be zero  (This is not desired)  Theorem if (m is prime and initial seed is non-zero) then the generator will never produce the value 0  In this case, the RNG produces values in  m = {1, 2, … m-1}

Observations  The above equation simulates drawing balls from an urn without replacement, where each value in  m represents a ball  The requirement of randomness is violated because successive draws are not independent  Practical Fact: The random values can be approximately considered as independent if the number of generated random variates (ball draws) is << m The Quality of the random number generator is dependent on good choices for a and m

The Period of a Sequence  Consider sequence produced by:  Once a value is repeated, all the sequence is then repeated itself  Sequence: where  p is the period: number of elements before the first repeat  clearly p ≤ m-1  It can be shown, that if we pick any initial seed x 0, we are guaranteed this initial seed will reappear

Full Period Sequences  [LP] Discrete-Event Simulation: A First Course by L. M. Leemis and S. K Park, Prentice Hall, 2006, page. 42 Theorem  If and the sequence is produced by the Lehmer generator where m is prime, then there is a positive integer p with such that:  are all different  and  In addition,

Full Period Sequences  Ideally, the generator cycles through all values in  m to maximize the number of possible values that are generated, and guarantee any number can be produced  The sequence containing all possible numbers is called a full-period sequence (p = m-1)  Non-full period sequences effectively partition  m into disjoint sets, each set has a particular period.

Modulus and Multiplier Selection Criteria  Selection Criteria 1: m to be “as large as possible”  m = 2 i - 1 where i is the machine precision (is the largest possible positive integer on a “two’s complement” machine)  Recall m must be prime  It happens that is prime (for a 32 bit machine)  Unfortunately, and are not prime ;-(  Selection Criteria 2: p gives full-period sequence (p = m-1)  For a given prime number m, select multiplier a that provide a full period  Algorithm to test if a is a full-period multiplier (m must be prime):

Modulus and Multiplier Selection Criteria  Criterias  m to be “as large as possible”  p gives full-period sequence (p = m-1) p = 1; x = a; // assume, initial seed is x0=1, thus x1=a while (x != 1) {// cycle through numbers until repeat p++; x = (a * x) % m;// careful: overflow possible } if (p == m-1)// a is a full period multiplier else// a is not a full period multiplier Algorithm for finding if p is full-period multiplier

Other Useful Properties Theorem 2.1.1[LP]: If the sequence x 0, x 1, x 2, … is produced by a Lehmer generator with multiplier a and modulus m, then Note this is not a good way to compute x i ! Theorem 2.1.4[LP, p. 45]: If a is any full-period multiplier relative to the prime modulus m, then each of the integers is also a full period multiplier relative to m if and only if the integer i has no prime factors in common with the prime factors of m -1 (i.e., i and m-1 are relatively prime, or co-prime)

Other Useful Properties // Given prime modulus m and any full period multiplier a, // generate all full period multipliers relative to m i = 1; x = a;// assume, initial seed is 1 while (x != 1) {// cycle through numbers until repeat if (gcd(i,m-1)==1) // x=a i mod m is full period multiplier i++; x = (a * x) % m;// careful: overflow possible } Generate all full-period multipliers

Implementation Issues  Assume we have a 32-bit machine, m=  Problem  Must compute  Obvious computation is to compute first, then do mod operation The multiplication might overflow, especially if m-1 is large!  First Solution: Floating point solution  Could do arithmetic in double precision floating point if multiplier is small enough  Double has 53-bits precision in IEEE floating point  May have trouble porting to other machines  Integer arithmetic faster than floating point

Implementation Issues: Mathematical Solutions  Problem: Compute without overflow  General Idea: Perform mod operation first, before multiplication.  Suppose that (not prime)   We have:  Thus, No overflow

 For the case, m is prime, so let  q = quotient; r = remainder  Let  It can be shown that (Page 59, Lemis/Park Textbook) and Implementation Issues: Mathematical Solutions

Random number variants Discrete random variables Continuous random variables Next

The Concept  Goal: create random variates for random variables X of a certain distribution defined with its CDF  Approach: Inverse cumulative distribution function CDF F ( x )= u  Cumulative Distribution Function  Generate a random variable  Determine x such that u1u1 X1 Exp u2u2 X1 Poisson X2 Exp X2 Poisson

Generating Discrete Random Variates Cumulative Distribution Function of X: F(x) = P(X≤x) PDF x F(x) xf(x) u = F(x) 0.5 = F(2) x u Inverse Distribution Function (idf) of X: F*(u) = min {x: u < F(x)} x F(x) x = F*(u) 2 = F(0.5) x u Random variate generation: 1. Select u, uniformly distributed (0,1) 2. Compute F*(u); result is random variate with distribution f() f(x=2)

Discrete Random Variate Generation  Uniform (a,b): equally likely to select an integer in interval [a,b]  Bernoulli(p)  Bernoulli(p):  Return 1 with probability p,  Return 0 with probability 1-p  Geometric(p):  Number of Bernoulli trials until first ‘0’)

Exponential Random Variates  Exponential distribution with mean 