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introductionToProgramming/common/StdRandom.java
Daniel Bulant 4bf15294e7
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2025-09-23 12:00:00 +02:00

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package common;
/******************************************************************************
* 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 pseudo-random numbers in [0, 1).
*
* - This library allows you to set and get the pseudo-random number seed.
*
* - See http://www.honeylocust.com/RngPack/ for an industrial
* strength random number generator in Java.
*
******************************************************************************/
import java.util.Random;
/**
* The {@code StdRandom} class provides static methods for generating
* random number from various discrete and continuous distributions,
* including uniform, Bernoulli, geometric, Gaussian, exponential, Pareto,
* Poisson, and Cauchy. It also provides method for shuffling an
* array or subarray and generating random permutations.
*
* <p><b>Conventions.</b>
* By convention, all intervals are half open. For example,
* <code>uniformDouble(-1.0, 1.0)</code> returns a random number between
* <code>-1.0</code> (inclusive) and <code>1.0</code> (exclusive).
* Similarly, <code>shuffle(a, lo, hi)</code> shuffles the <code>hi - lo</code>
* elements in the array <code>a[]</code>, starting at index <code>lo</code>
* (inclusive) and ending at index <code>hi</code> (exclusive).
*
* <p><b>Performance.</b>
* The methods all take constant expected time, except those that involve arrays.
* The <em>shuffle</em> method takes time linear in the subarray to be shuffled;
* the <em>discrete</em> methods take time linear in the length of the argument
* array.
*
* <p><b>Additional information.</b>
* For additional documentation,
* see <a href="https://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 pseudo-random 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 pseudo-random 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)
* @deprecated Replaced by {@link #uniformDouble()}.
*/
@Deprecated
public static double uniform() {
return uniformDouble();
}
/**
* Returns a random real number uniformly in [0, 1).
*
* @return a random real number uniformly in [0, 1)
*/
public static double uniformDouble() {
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}
* @deprecated Replaced by {@link #uniformInt(int n)}.
*/
@Deprecated
public static int uniform(int n) {
return uniformInt(n);
}
/**
* 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 uniformInt(int n) {
if (n <= 0) throw new IllegalArgumentException("argument must be positive: " + n);
return random.nextInt(n);
}
/**
* Returns a random long integer uniformly in [0, n).
*
* @param n number of possible {@code long} integers
* @return a random long integer uniformly between 0 (inclusive) and {@code n} (exclusive)
* @throws IllegalArgumentException if {@code n <= 0}
* @deprecated Replaced by {@link #uniformLong(long n)}.
*/
@Deprecated
public static long uniform(long n) {
return uniformLong(n);
}
/**
* Returns a random long integer uniformly in [0, n).
*
* @param n number of possible {@code long} integers
* @return a random long integer uniformly between 0 (inclusive) and {@code n} (exclusive)
* @throws IllegalArgumentException if {@code n <= 0}
*/
public static long uniformLong(long n) {
if (n <= 0L) throw new IllegalArgumentException("argument must be positive: " + n);
// https://docs.oracle.com/javase/8/docs/api/java/util/Random.html#longs-long-long-long-
long r = random.nextLong();
long m = n - 1;
// power of two
if ((n & m) == 0L) {
return r & m;
}
// reject over-represented candidates
long u = r >>> 1;
while (u + m - (r = u % n) < 0L) {
u = random.nextLong() >>> 1;
}
return r;
}
///////////////////////////////////////////////////////////////////////////
// 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 #uniformDouble()}.
*/
@Deprecated
public static double random() {
return uniformDouble();
}
/**
* 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}
* @deprecated Replaced by {@link #uniformInt(int a, int b)}.
*/
@Deprecated
public static int uniform(int a, int b) {
return uniformInt(a, b);
}
/**
* 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 uniformInt(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}
* @deprecated Replaced by {@link #uniformDouble(double a, double b)}.
*/
@Deprecated
public static double uniform(double a, double b) {
return uniformDouble(a, b);
}
/**
* 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 uniformDouble(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 1 - p}
* @throws IllegalArgumentException unless {@code 0} &le; {@code p} &le; {@code 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: " + p);
return uniformDouble() < 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 = uniformDouble(-1.0, 1.0);
y = uniformDouble(-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 &mu;
* and standard deviation &sigma;.
*
* @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>.
* The integer represents the number of independent trials
* before the first success.
*
* @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)) {
throw new IllegalArgumentException("probability p must be greater than 0: " + p);
}
if (!(p <= 1.0)) {
throw new IllegalArgumentException("probability p must not be larger than 1: " + p);
}
// using algorithm given by Knuth
return (int) Math.ceil(Math.log(uniformDouble()) / Math.log(1.0 - p));
}
/**
* Returns a random integer from a Poisson distribution with mean &lambda;.
*
* @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: " + lambda);
if (Double.isInfinite(lambda))
throw new IllegalArgumentException("lambda must not be infinite: " + lambda);
// 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 *= uniformDouble();
} 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 &alpha;.
*
* @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: " + alpha);
return Math.pow(1 - uniformDouble(), -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 * (uniformDouble() - 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 must not be null");
double EPSILON = 1.0E-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 non-negative: " + 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 = uniformDouble();
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 must not be null");
long sum = 0;
for (int i = 0; i < frequencies.length; i++) {
if (frequencies[i] < 0)
throw new IllegalArgumentException("array entry " + i + " must be non-negative: " + 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 probability proportional to frequency
double r = uniformInt((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 &lambda;.
*
* @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 exponential(double lambda) {
if (!(lambda > 0.0))
throw new IllegalArgumentException("lambda must be positive: " + lambda);
return -Math.log(1 - uniformDouble()) / lambda;
}
/**
* Returns a random real number from an exponential distribution
* with rate &lambda;.
*
* @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}
* @deprecated Replaced by {@link #exponential(double)}.
*/
@Deprecated
public static double exp(double lambda) {
return exponential(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) {
validateNotNull(a);
int n = a.length;
for (int i = 0; i < n; i++) {
int r = i + uniformInt(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) {
validateNotNull(a);
int n = a.length;
for (int i = 0; i < n; i++) {
int r = i + uniformInt(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) {
validateNotNull(a);
int n = a.length;
for (int i = 0; i < n; i++) {
int r = i + uniformInt(n-i); // between i and n-1
int 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(char[] a) {
validateNotNull(a);
int n = a.length;
for (int i = 0; i < n; i++) {
int r = i + uniformInt(n-i); // between i and n-1
char 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 (exclusive)
* @throws IllegalArgumentException if {@code a} is {@code null}
* @throws IllegalArgumentException unless {@code (0 <= lo) && (lo < hi) && (hi <= a.length)}
*
*/
public static void shuffle(Object[] a, int lo, int hi) {
validateNotNull(a);
validateSubarrayIndices(lo, hi, a.length);
for (int i = lo; i < hi; i++) {
int r = i + uniformInt(hi-i); // between i and hi-1
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 (exclusive)
* @throws IllegalArgumentException if {@code a} is {@code null}
* @throws IllegalArgumentException unless {@code (0 <= lo) && (lo < hi) && (hi <= a.length)}
*/
public static void shuffle(double[] a, int lo, int hi) {
validateNotNull(a);
validateSubarrayIndices(lo, hi, a.length);
for (int i = lo; i < hi; i++) {
int r = i + uniformInt(hi-i); // between i and hi-1
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 (exclusive)
* @throws IllegalArgumentException if {@code a} is {@code null}
* @throws IllegalArgumentException unless {@code (0 <= lo) && (lo < hi) && (hi <= a.length)}
*/
public static void shuffle(int[] a, int lo, int hi) {
validateNotNull(a);
validateSubarrayIndices(lo, hi, a.length);
for (int i = lo; i < hi; i++) {
int r = i + uniformInt(hi-i); // between i and hi-1
int temp = a[i];
a[i] = a[r];
a[r] = temp;
}
}
/**
* Returns a uniformly random permutation of <em>n</em> elements.
*
* @param n number of elements
* @throws IllegalArgumentException if {@code n} is negative
* @return an array of length {@code n} that is a uniformly random permutation
* of {@code 0}, {@code 1}, ..., {@code n-1}
*/
public static int[] permutation(int n) {
if (n < 0) throw new IllegalArgumentException("n must be non-negative: " + n);
int[] perm = new int[n];
for (int i = 0; i < n; i++)
perm[i] = i;
shuffle(perm);
return perm;
}
/**
* Returns a uniformly random permutation of <em>k</em> of <em>n</em> elements.
*
* @param n number of elements
* @param k number of elements to select
* @throws IllegalArgumentException if {@code n} is negative
* @throws IllegalArgumentException unless {@code 0 <= k <= n}
* @return an array of length {@code k} that is a uniformly random permutation
* of {@code k} of the elements from {@code 0}, {@code 1}, ..., {@code n-1}
*/
public static int[] permutation(int n, int k) {
if (n < 0) throw new IllegalArgumentException("n must be non-negative: " + n);
if (k < 0 || k > n) throw new IllegalArgumentException("k must be between 0 and n: " + k);
int[] perm = new int[k];
for (int i = 0; i < k; i++) {
int r = uniformInt(i+1); // between 0 and i
perm[i] = perm[r];
perm[r] = i;
}
for (int i = k; i < n; i++) {
int r = uniformInt(i+1); // between 0 and i
if (r < k) perm[r] = i;
}
return perm;
}
// throw an IllegalArgumentException if x is null
// (x can be of type Object[], double[], int[], ...)
private static void validateNotNull(Object x) {
if (x == null) {
throw new IllegalArgumentException("argument must not be null");
}
}
// throw an exception unless 0 <= lo <= hi <= length
private static void validateSubarrayIndices(int lo, int hi, int length) {
if (lo < 0 || hi > length || lo > hi) {
throw new IllegalArgumentException("subarray indices out of bounds: [" + lo + ", " + hi + ")");
}
}
/**
* Unit tests the methods in this class.
*
* @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 ", uniformInt(100));
StdOut.printf("%8.5f ", uniformDouble(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));
StdOut.printf("%11d ", uniformLong(100000000000L));
StdRandom.shuffle(a);
for (String s : a)
StdOut.print(s);
StdOut.println();
}
}
}
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