StdRandom.java
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/******************************************************************************
* Compilation: javac StdRandom.java
* Execution: java StdRandom
* Dependencies: StdOut.java
*
* A library of static methods to generate pseudo-random numbers from
* different distributions (bernoulli, uniform, gaussian, discrete,
* and exponential). Also includes a method for shuffling an array.
*
*
* % java StdRandom 5
* seed = 1316600602069
* 59 16.81826 true 8.83954 0
* 32 91.32098 true 9.11026 0
* 35 10.11874 true 8.95396 3
* 92 32.88401 true 8.87089 0
* 72 92.55791 true 9.46241 0
*
* % java StdRandom 5
* seed = 1316600616575
* 96 60.17070 true 8.72821 0
* 79 32.01607 true 8.58159 0
* 81 59.49065 true 9.10423 1
* 96 51.65818 true 9.02102 0
* 99 17.55771 true 8.99762 0
*
* % java StdRandom 5 1316600616575
* seed = 1316600616575
* 96 60.17070 true 8.72821 0
* 79 32.01607 true 8.58159 0
* 81 59.49065 true 9.10423 1
* 96 51.65818 true 9.02102 0
* 99 17.55771 true 8.99762 0
*
*
* Remark
* ------
* - Relies on randomness of nextDouble() method in java.util.Random
* to generate pseudorandom numbers in [0, 1).
*
* - This library allows you to set and get the pseudorandom number seed.
*
* - See http://www.honeylocust.com/RngPack/ for an industrial
* strength random number generator in Java.
*
******************************************************************************/
package edu.princeton.cs.algs4;
import java.util.Random;
/**
* The {@code StdRandom} class provides static methods for generating
* random number from various discrete and continuous distributions,
* including Bernoulli, uniform, Gaussian, exponential, pareto,
* Poisson, and Cauchy. It also provides method for shuffling an
* array or subarray.
* <p>
* For additional documentation,
* see <a href="http://introcs.cs.princeton.edu/22library">Section 2.2</a> of
* <i>Computer Science: An Interdisciplinary Approach</i>
* by Robert Sedgewick and Kevin Wayne.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public final class StdRandom {
private static Random random; // pseudo-random number generator
private static long seed; // pseudo-random number generator seed
// static initializer
static {
// this is how the seed was set in Java 1.4
seed = System.currentTimeMillis();
random = new Random(seed);
}
// don't instantiate
private StdRandom() { }
/**
* Sets the seed of the pseudorandom number generator.
* This method enables you to produce the same sequence of "random"
* number for each execution of the program.
* Ordinarily, you should call this method at most once per program.
*
* @param s the seed
*/
public static void setSeed(long s) {
seed = s;
random = new Random(seed);
}
/**
* Returns the seed of the pseudorandom number generator.
*
* @return the seed
*/
public static long getSeed() {
return seed;
}
/**
* Returns a random real number uniformly in [0, 1).
*
* @return a random real number uniformly in [0, 1)
*/
public static double uniform() {
return random.nextDouble();
}
/**
* Returns a random integer uniformly in [0, n).
*
* @param n number of possible integers
* @return a random integer uniformly between 0 (inclusive) and {@code n} (exclusive)
* @throws IllegalArgumentException if {@code n <= 0}
*/
public static int uniform(int n) {
if (n <= 0) throw new IllegalArgumentException("argument must be positive");
return random.nextInt(n);
}
///////////////////////////////////////////////////////////////////////////
// STATIC METHODS BELOW RELY ON JAVA.UTIL.RANDOM ONLY INDIRECTLY VIA
// THE STATIC METHODS ABOVE.
///////////////////////////////////////////////////////////////////////////
/**
* Returns a random real number uniformly in [0, 1).
*
* @return a random real number uniformly in [0, 1)
* @deprecated Replaced by {@link #uniform()}.
*/
@Deprecated
public static double random() {
return uniform();
}
/**
* Returns a random integer uniformly in [a, b).
*
* @param a the left endpoint
* @param b the right endpoint
* @return a random integer uniformly in [a, b)
* @throws IllegalArgumentException if {@code b <= a}
* @throws IllegalArgumentException if {@code b - a >= Integer.MAX_VALUE}
*/
public static int uniform(int a, int b) {
if ((b <= a) || ((long) b - a >= Integer.MAX_VALUE)) {
throw new IllegalArgumentException("invalid range: [" + a + ", " + b + "]");
}
return a + uniform(b - a);
}
/**
* Returns a random real number uniformly in [a, b).
*
* @param a the left endpoint
* @param b the right endpoint
* @return a random real number uniformly in [a, b)
* @throws IllegalArgumentException unless {@code a < b}
*/
public static double uniform(double a, double b) {
if (!(a < b)) {
throw new IllegalArgumentException("invalid range: [" + a + ", " + b + "]");
}
return a + uniform() * (b-a);
}
/**
* Returns a random boolean from a Bernoulli distribution with success
* probability <em>p</em>.
*
* @param p the probability of returning {@code true}
* @return {@code true} with probability {@code p} and
* {@code false} with probability {@code p}
* @throws IllegalArgumentException unless {@code p >= 0.0} and {@code p <= 1.0}
*/
public static boolean bernoulli(double p) {
if (!(p >= 0.0 && p <= 1.0))
throw new IllegalArgumentException("probability p must be between 0.0 and 1.0");
return uniform() < p;
}
/**
* Returns a random boolean from a Bernoulli distribution with success
* probability 1/2.
*
* @return {@code true} with probability 1/2 and
* {@code false} with probability 1/2
*/
public static boolean bernoulli() {
return bernoulli(0.5);
}
/**
* Returns a random real number from a standard Gaussian distribution.
*
* @return a random real number from a standard Gaussian distribution
* (mean 0 and standard deviation 1).
*/
public static double gaussian() {
// use the polar form of the Box-Muller transform
double r, x, y;
do {
x = uniform(-1.0, 1.0);
y = uniform(-1.0, 1.0);
r = x*x + y*y;
} while (r >= 1 || r == 0);
return x * Math.sqrt(-2 * Math.log(r) / r);
// Remark: y * Math.sqrt(-2 * Math.log(r) / r)
// is an independent random gaussian
}
/**
* Returns a random real number from a Gaussian distribution with mean μ
* and standard deviation σ.
*
* @param mu the mean
* @param sigma the standard deviation
* @return a real number distributed according to the Gaussian distribution
* with mean {@code mu} and standard deviation {@code sigma}
*/
public static double gaussian(double mu, double sigma) {
return mu + sigma * gaussian();
}
/**
* Returns a random integer from a geometric distribution with success
* probability <em>p</em>.
*
* @param p the parameter of the geometric distribution
* @return a random integer from a geometric distribution with success
* probability {@code p}; or {@code Integer.MAX_VALUE} if
* {@code p} is (nearly) equal to {@code 1.0}.
* @throws IllegalArgumentException unless {@code p >= 0.0} and {@code p <= 1.0}
*/
public static int geometric(double p) {
if (!(p >= 0.0 && p <= 1.0)) {
throw new IllegalArgumentException("probability p must be between 0.0 and 1.0");
}
// using algorithm given by Knuth
return (int) Math.ceil(Math.log(uniform()) / Math.log(1.0 - p));
}
/**
* Returns a random integer from a Poisson distribution with mean λ.
*
* @param lambda the mean of the Poisson distribution
* @return a random integer from a Poisson distribution with mean {@code lambda}
* @throws IllegalArgumentException unless {@code lambda > 0.0} and not infinite
*/
public static int poisson(double lambda) {
if (!(lambda > 0.0))
throw new IllegalArgumentException("lambda must be positive");
if (Double.isInfinite(lambda))
throw new IllegalArgumentException("lambda must not be infinite");
// using algorithm given by Knuth
// see http://en.wikipedia.org/wiki/Poisson_distribution
int k = 0;
double p = 1.0;
double expLambda = Math.exp(-lambda);
do {
k++;
p *= uniform();
} while (p >= expLambda);
return k-1;
}
/**
* Returns a random real number from the standard Pareto distribution.
*
* @return a random real number from the standard Pareto distribution
*/
public static double pareto() {
return pareto(1.0);
}
/**
* Returns a random real number from a Pareto distribution with
* shape parameter α.
*
* @param alpha shape parameter
* @return a random real number from a Pareto distribution with shape
* parameter {@code alpha}
* @throws IllegalArgumentException unless {@code alpha > 0.0}
*/
public static double pareto(double alpha) {
if (!(alpha > 0.0))
throw new IllegalArgumentException("alpha must be positive");
return Math.pow(1 - uniform(), -1.0/alpha) - 1.0;
}
/**
* Returns a random real number from the Cauchy distribution.
*
* @return a random real number from the Cauchy distribution.
*/
public static double cauchy() {
return Math.tan(Math.PI * (uniform() - 0.5));
}
/**
* Returns a random integer from the specified discrete distribution.
*
* @param probabilities the probability of occurrence of each integer
* @return a random integer from a discrete distribution:
* {@code i} with probability {@code probabilities[i]}
* @throws IllegalArgumentException if {@code probabilities} is {@code null}
* @throws IllegalArgumentException if sum of array entries is not (very nearly) equal to {@code 1.0}
* @throws IllegalArgumentException unless {@code probabilities[i] >= 0.0} for each index {@code i}
*/
public static int discrete(double[] probabilities) {
if (probabilities == null) throw new IllegalArgumentException("argument array is null");
double EPSILON = 1E-14;
double sum = 0.0;
for (int i = 0; i < probabilities.length; i++) {
if (!(probabilities[i] >= 0.0))
throw new IllegalArgumentException("array entry " + i + " must be nonnegative: " + probabilities[i]);
sum += probabilities[i];
}
if (sum > 1.0 + EPSILON || sum < 1.0 - EPSILON)
throw new IllegalArgumentException("sum of array entries does not approximately equal 1.0: " + sum);
// the for loop may not return a value when both r is (nearly) 1.0 and when the
// cumulative sum is less than 1.0 (as a result of floating-point roundoff error)
while (true) {
double r = uniform();
sum = 0.0;
for (int i = 0; i < probabilities.length; i++) {
sum = sum + probabilities[i];
if (sum > r) return i;
}
}
}
/**
* Returns a random integer from the specified discrete distribution.
*
* @param frequencies the frequency of occurrence of each integer
* @return a random integer from a discrete distribution:
* {@code i} with probability proportional to {@code frequencies[i]}
* @throws IllegalArgumentException if {@code frequencies} is {@code null}
* @throws IllegalArgumentException if all array entries are {@code 0}
* @throws IllegalArgumentException if {@code frequencies[i]} is negative for any index {@code i}
* @throws IllegalArgumentException if sum of frequencies exceeds {@code Integer.MAX_VALUE} (2<sup>31</sup> - 1)
*/
public static int discrete(int[] frequencies) {
if (frequencies == null) throw new IllegalArgumentException("argument array is null");
long sum = 0;
for (int i = 0; i < frequencies.length; i++) {
if (frequencies[i] < 0)
throw new IllegalArgumentException("array entry " + i + " must be nonnegative: " + frequencies[i]);
sum += frequencies[i];
}
if (sum == 0)
throw new IllegalArgumentException("at least one array entry must be positive");
if (sum >= Integer.MAX_VALUE)
throw new IllegalArgumentException("sum of frequencies overflows an int");
// pick index i with probabilitity proportional to frequency
double r = uniform((int) sum);
sum = 0;
for (int i = 0; i < frequencies.length; i++) {
sum += frequencies[i];
if (sum > r) return i;
}
// can't reach here
assert false;
return -1;
}
/**
* Returns a random real number from an exponential distribution
* with rate λ.
*
* @param lambda the rate of the exponential distribution
* @return a random real number from an exponential distribution with
* rate {@code lambda}
* @throws IllegalArgumentException unless {@code lambda > 0.0}
*/
public static double exp(double lambda) {
if (!(lambda > 0.0))
throw new IllegalArgumentException("lambda must be positive");
return -Math.log(1 - uniform()) / lambda;
}
/**
* Rearranges the elements of the specified array in uniformly random order.
*
* @param a the array to shuffle
* @throws IllegalArgumentException if {@code a} is {@code null}
*/
public static void shuffle(Object[] a) {
if (a == null) throw new IllegalArgumentException("argument array is null");
int n = a.length;
for (int i = 0; i < n; i++) {
int r = i + uniform(n-i); // between i and n-1
Object temp = a[i];
a[i] = a[r];
a[r] = temp;
}
}
/**
* Rearranges the elements of the specified array in uniformly random order.
*
* @param a the array to shuffle
* @throws IllegalArgumentException if {@code a} is {@code null}
*/
public static void shuffle(double[] a) {
if (a == null) throw new IllegalArgumentException("argument array is null");
int n = a.length;
for (int i = 0; i < n; i++) {
int r = i + uniform(n-i); // between i and n-1
double temp = a[i];
a[i] = a[r];
a[r] = temp;
}
}
/**
* Rearranges the elements of the specified array in uniformly random order.
*
* @param a the array to shuffle
* @throws IllegalArgumentException if {@code a} is {@code null}
*/
public static void shuffle(int[] a) {
if (a == null) throw new IllegalArgumentException("argument array is null");
int n = a.length;
for (int i = 0; i < n; i++) {
int r = i + uniform(n-i); // between i and n-1
int temp = a[i];
a[i] = a[r];
a[r] = temp;
}
}
/**
* Rearranges the elements of the specified subarray in uniformly random order.
*
* @param a the array to shuffle
* @param lo the left endpoint (inclusive)
* @param hi the right endpoint (inclusive)
* @throws IllegalArgumentException if {@code a} is {@code null}
* @throws IndexOutOfBoundsException unless {@code (0 <= lo) && (lo <= hi) && (hi < a.length)}
*
*/
public static void shuffle(Object[] a, int lo, int hi) {
if (a == null) throw new IllegalArgumentException("argument array is null");
if (lo < 0 || lo > hi || hi >= a.length) {
throw new IndexOutOfBoundsException("invalid subarray range: [" + lo + ", " + hi + "]");
}
for (int i = lo; i <= hi; i++) {
int r = i + uniform(hi-i+1); // between i and hi
Object temp = a[i];
a[i] = a[r];
a[r] = temp;
}
}
/**
* Rearranges the elements of the specified subarray in uniformly random order.
*
* @param a the array to shuffle
* @param lo the left endpoint (inclusive)
* @param hi the right endpoint (inclusive)
* @throws IllegalArgumentException if {@code a} is {@code null}
* @throws IndexOutOfBoundsException unless {@code (0 <= lo) && (lo <= hi) && (hi < a.length)}
*/
public static void shuffle(double[] a, int lo, int hi) {
if (a == null) throw new IllegalArgumentException("argument array is null");
if (lo < 0 || lo > hi || hi >= a.length) {
throw new IndexOutOfBoundsException("invalid subarray range: [" + lo + ", " + hi + "]");
}
for (int i = lo; i <= hi; i++) {
int r = i + uniform(hi-i+1); // between i and hi
double temp = a[i];
a[i] = a[r];
a[r] = temp;
}
}
/**
* Rearranges the elements of the specified subarray in uniformly random order.
*
* @param a the array to shuffle
* @param lo the left endpoint (inclusive)
* @param hi the right endpoint (inclusive)
* @throws IllegalArgumentException if {@code a} is {@code null}
* @throws IndexOutOfBoundsException unless {@code (0 <= lo) && (lo <= hi) && (hi < a.length)}
*/
public static void shuffle(int[] a, int lo, int hi) {
if (a == null) throw new IllegalArgumentException("argument array is null");
if (lo < 0 || lo > hi || hi >= a.length) {
throw new IndexOutOfBoundsException("invalid subarray range: [" + lo + ", " + hi + "]");
}
for (int i = lo; i <= hi; i++) {
int r = i + uniform(hi-i+1); // between i and hi
int temp = a[i];
a[i] = a[r];
a[r] = temp;
}
}
/**
* Unit test.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
int n = Integer.parseInt(args[0]);
if (args.length == 2) StdRandom.setSeed(Long.parseLong(args[1]));
double[] probabilities = { 0.5, 0.3, 0.1, 0.1 };
int[] frequencies = { 5, 3, 1, 1 };
String[] a = "A B C D E F G".split(" ");
StdOut.println("seed = " + StdRandom.getSeed());
for (int i = 0; i < n; i++) {
StdOut.printf("%2d ", uniform(100));
StdOut.printf("%8.5f ", uniform(10.0, 99.0));
StdOut.printf("%5b ", bernoulli(0.5));
StdOut.printf("%7.5f ", gaussian(9.0, 0.2));
StdOut.printf("%1d ", discrete(probabilities));
StdOut.printf("%1d ", discrete(frequencies));
StdRandom.shuffle(a);
for (String s : a)
StdOut.print(s);
StdOut.println();
}
}
}
/******************************************************************************
* Copyright 2002-2016, Robert Sedgewick and Kevin Wayne.
*
* This file is part of algs4.jar, which accompanies the textbook
*
* Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
* Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
* http://algs4.cs.princeton.edu
*
*
* algs4.jar is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* algs4.jar is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with algs4.jar. If not, see http://www.gnu.org/licenses.
******************************************************************************/