random.c

/*
	random.c

	Compute random numbers within given ranges

	by: Steven Skiena
*/

/*
Copyright 2003 by Steven S. Skiena; all rights reserved. 

Permission is granted for use in non-commerical applications
provided this copyright notice remains intact and unchanged.

This program appears in my book:

"Programming Challenges: The Programming Contest Training Manual"
by Steven Skiena and Miguel Revilla, Springer-Verlag, New York 2003.

See our website www.programming-challenges.com for additional information.

This book can be ordered from Amazon.com at

http://www.amazon.com/exec/obidos/ASIN/0387001638/thealgorithmrepo/

*/

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

#include "random.h"

/**************************************************************************/

/*      These functions generate random numbers in the designated ranges */

void swap(int *a, int *b) {
    int x;

    x = *a;
    *a = *b;
    *b = x;
}

int random_int(int low, int high) {                 /* lower/upper bounds on numb*/
    int rand();
    int i, j, r;                                    /* random number*/

    i = RAND_MAX / (high - low + 1);
    i *= (high - low + 1);
    while ((j = rand()) >= i) {
        continue;
    }
    r = (j % (high - low + 1)) + low;
    if ((r < low) || (r > high)) {
        printf("Error: random integer %d out of range [%d,%d]\n",
                r, low, high);
    }
    return(r);
}

/*      Construct a random permutation of the $n$ elements of the
        given array.
*/

void random_permutation(int a[], int n) {
    int i;

    for (i=n; i>1; i--) {
        swap(&a[i - 1], &a[random_int(0, i - 1)]);
    }
}

double random_float(int low, int high) {       /*lower/upper bounds on numb*/
    int rand();
    double i, j;                               /* avoid arithmetic trouble */
    double r;                                  /* random number*/

    i = RAND_MAX / (high - low);
    i *= (high - low);
    while ((j = rand()) >= i) {
        continue;
    }
    r = (j / i) * (high - low) + low;

    if ((r < low) || (r > high)) {
        printf("ERROR: random real %f out of range [%d,%d]\n",
                r, low, high);
    }
    return(r);
}