3.1 Sequential Random Number Generation with trng

Let’s first examine how we can use the trng C++ library to create a sequential version of code (running on one process) that uses a loop to generate one value after another in a stream.

This is the main program that sets up the use of the random number generator and uses a uniform distribution of integers from 1 to 99 for the values in the stream. Study it carefully to see how we use trng to do this.

#include <stdio.h> // printf() #include <time.h> // time() // separate code for handling command line arguments is below #include <trng/yarn2.hpp> // trng YARN (yet another random number) generator class #include <trng/uniform_dist.hpp> // we'll use a uniform distribution // of the random numbers //////////////////////////////////////////////////////////// int main(int argc, char* argv[]) { // set up what conditions we will use: // - default size of repetitions in the loop // - whether to use a constant seed so we can repeat and // get same random values in stream // - fixed range of random numbers from 1 to 99 int repetitions = 8; int useConstantSeed = 0; // for same stream, set this to 1 on command line with -c // range for uniform distribution of random numbers int min = 1; int max = 99; // gather command line arguments getArguments(argc, argv, &repetitions, &useConstantSeed); // //////////// random generator setup ////////////////////////// // random numbers start from a seed value long unsigned int seedValue; // note for trng this is long unsigned // same constant seed will generate the same sequence of rndom numbers // use for testing to varify same sequence regardless of number of threads if (useConstantSeed) { seedValue = 888777666; } else { // variable seed based on computer clock time seedValue = (long unsigned int)time(NULL); // enables variation; use for simulations } // number generation needs two things: a generator and a distribution of the numbers // declare the generator object trng::yarn2 randGen; // declare the distribution to use (here it is uniform with values in the range min to max) trng::uniform_dist<> uniform(min, max); // Set the starting point of the generator by seeding it randGen.seed(seedValue); // //////////////// end PRNG setup //////////////////////////////////// // /////////////// get a portion of the stream ///////////////////////// int nextRandValue; // holds the next value as we go through the loop // loop to get each number in the PRNG stream and print it // Note here the ubiquitous for loop construction of incrementing by 1 int i; for (i=0; i < repetitions; i++) { // get next number in the stream from the distribution nextRandValue = uniform(randGen); // print tid(i):nextRandValue printf("t0 (%2d):%2d \n", i, nextRandValue); } }

There are several points to notice about this code:

• We declare both a generator object (randGen, line 44) and a type of distribution of the numbers (uniform, line 46). There are other types of distributions of numbers that are useful in many applications, but we will not cover those here.

• The syntax of the declaration of the distribution requires that we give it a minimum and maximum value for the range of numbers. The range includes the minimum, but not the maximum.

• A seed value is declared as a long unsigned int.

• Before requesting numbers from the stream, we first seed the generator (line 49).

• To get the next number inside the loop, we call the constructor for the object called uniform and send it the generator object (line 61). Since we declared nextRandValue an int, the use of uniform(randgen) will return an int. This is a nice feature of this C++ library.

If you run it without any changes, it is running with these command line arguments:

• -c means use a constant seed. From this if you repeat a run you should get the same stream of numbers. This shows that this generator is reproducible, which you will see is very handy when debugging your code.

• -n 8 indicates to generate 8 numbers (repeat the loop 8 times).

You should try it without the -c and run it a few more times to see that a new set of values is generated each time when we seed with a different number each time.

If you are interested in the code for handling these arguments for this sequential version, you could study the following code block. It is included with the code above when it is run, and not intended to be run on its own.

#include <stdio.h> #include <unistd.h> #include <stdlib.h> #include <ctype.h> void getArguments(int argc, char *argv[], int * N, int * useConstantSeed); int isNumber(char s[]); void exitWithError(char cmdFlag, char ** argv); void Usage(char *program); void getArguments(int argc, char *argv[], int * N, int * useConstantSeed) { int c; // result from getopt calls // The : after a character means a value is expected // No colon means it is simply a flag with no associated value while ((c = getopt (argc, argv, "n:hc")) != -1) { // getopt implicitly sets a value to a char * (string) called optarg // to what the user typed after -n switch (c) { // character string entered after the -N needs to be a number case 'n': if (isNumber(optarg)) { *N = atoi(optarg); } else { exitWithError(c, argv); } break; // If the -h is encountered, then we provide usage case 'h': Usage(argv[0]); exit(0); break; // If the -c is encountered, then we change the constant seed flag case 'c': *useConstantSeed = 1; break; case ':': printf("Missing arg for %c\n", optopt); Usage(argv[0]); exit(EXIT_FAILURE); break; case '?': if ( (optopt == 'v') || (optopt == 't') ) { Usage(argv[0]); exit(EXIT_FAILURE); } else if (isprint (optopt)) { fprintf (stderr, "Unknown option `-%c'.\n", optopt); Usage(argv[0]); exit(EXIT_FAILURE); } else { fprintf (stderr, "Unknown non-printable option character `\\x%x'.\n", optopt); Usage(argv[0]); exit(EXIT_FAILURE); } break; } } } // Check a string as a number containing all digits int isNumber(char s[]) { for (int i = 0; s[i]!= '\0'; i++) { if (isdigit(s[i]) == 0) return 0; } return 1; } // Called when isNumber() fails void exitWithError(char cmdFlag, char ** argv) { fprintf(stderr, "Option -%c needs a number value\n", cmdFlag); Usage(argv[0]); exit(EXIT_FAILURE); } void Usage(char *program) { fprintf(stderr, "This program demonstrates use of a loop to create a stream of random numbers in a sequential fashion."); fprintf(stderr, "\nUsage: %s [-h] [-n numReps][-c]\n", program); fprintf(stderr, " -h shows this message and exits.\n"); fprintf(stderr, " -n indicates the number of repetitions of the loop (default is 8).\n"); fprintf(stderr, " -c indicates that a fixed seed will be used, resulting in the same stream of numbers each time this is run.\n"); }

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