edu.princeton.cs.algs4

## Class StdRandom

• Object
• edu.princeton.cs.algs4.StdRandom

• public final class StdRandom
extends Object
The 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.

For additional documentation, see Section 2.2 of Computer Science: An Interdisciplinary Approach by Robert Sedgewick and Kevin Wayne.

Author:
Robert Sedgewick, Kevin Wayne
• ### Method Summary

Methods
Modifier and Type Method and Description
static boolean bernoulli()
Returns a random boolean from a Bernoulli distribution with success probability 1/2.
static boolean bernoulli(double p)
Returns a random boolean from a Bernoulli distribution with success probability p.
static double cauchy()
Returns a random real number from the Cauchy distribution.
static int discrete(double[] probabilities)
Returns a random integer from the specified discrete distribution.
static int discrete(int[] frequencies)
Returns a random integer from the specified discrete distribution.
static double exp(double lambda)
Returns a random real number from an exponential distribution with rate λ.
static double gaussian()
Returns a random real number from a standard Gaussian distribution.
static double gaussian(double mu, double sigma)
Returns a random real number from a Gaussian distribution with mean μ and standard deviation σ.
static int geometric(double p)
Returns a random integer from a geometric distribution with success probability p.
static long getSeed()
Returns the seed of the pseudorandom number generator.
static void main(String[] args)
Unit tests the methods in this class.
static double pareto()
Returns a random real number from the standard Pareto distribution.
static double pareto(double alpha)
Returns a random real number from a Pareto distribution with shape parameter α.
static int[] permutation(int n)
Returns a uniformly random permutation of n elements.
static int[] permutation(int n, int k)
Returns a uniformly random permutation of k of n elements.
static int poisson(double lambda)
Returns a random integer from a Poisson distribution with mean λ.
static double random()
Deprecated.
Replaced by uniform().
static void setSeed(long s)
Sets the seed of the pseudorandom number generator.
static void shuffle(char[] a)
Rearranges the elements of the specified array in uniformly random order.
static void shuffle(double[] a)
Rearranges the elements of the specified array in uniformly random order.
static void shuffle(double[] a, int lo, int hi)
Rearranges the elements of the specified subarray in uniformly random order.
static void shuffle(int[] a)
Rearranges the elements of the specified array in uniformly random order.
static void shuffle(int[] a, int lo, int hi)
Rearranges the elements of the specified subarray in uniformly random order.
static void shuffle(Object[] a)
Rearranges the elements of the specified array in uniformly random order.
static void shuffle(Object[] a, int lo, int hi)
Rearranges the elements of the specified subarray in uniformly random order.
static double uniform()
Returns a random real number uniformly in [0, 1).
static double uniform(double a, double b)
Returns a random real number uniformly in [a, b).
static int uniform(int n)
Returns a random integer uniformly in [0, n).
static int uniform(int a, int b)
Returns a random integer uniformly in [a, b).
static long uniform(long n)
Returns a random long integer uniformly in [0, n).
• ### Method Detail

• #### setSeed

public static void setSeed(long s)
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.
Parameters:
s - the seed
• #### getSeed

public static long getSeed()
Returns the seed of the pseudorandom number generator.
Returns:
the seed
• #### uniform

public static double uniform()
Returns a random real number uniformly in [0, 1).
Returns:
a random real number uniformly in [0, 1)
• #### uniform

public static int uniform(int n)
Returns a random integer uniformly in [0, n).
Parameters:
n - number of possible integers
Returns:
a random integer uniformly between 0 (inclusive) and n (exclusive)
Throws:
IllegalArgumentException - if n <= 0
• #### uniform

public static long uniform(long n)
Returns a random long integer uniformly in [0, n).
Parameters:
n - number of possible long integers
Returns:
a random long integer uniformly between 0 (inclusive) and n (exclusive)
Throws:
IllegalArgumentException - if n <= 0
• #### random

@Deprecated
public static double random()
Deprecated. Replaced by uniform().
Returns a random real number uniformly in [0, 1).
Returns:
a random real number uniformly in [0, 1)
• #### uniform

public static int uniform(int a,
int b)
Returns a random integer uniformly in [a, b).
Parameters:
a - the left endpoint
b - the right endpoint
Returns:
a random integer uniformly in [a, b)
Throws:
IllegalArgumentException - if b <= a
IllegalArgumentException - if b - a >= Integer.MAX_VALUE
• #### uniform

public static double uniform(double a,
double b)
Returns a random real number uniformly in [a, b).
Parameters:
a - the left endpoint
b - the right endpoint
Returns:
a random real number uniformly in [a, b)
Throws:
IllegalArgumentException - unless a < b
• #### bernoulli

public static boolean bernoulli(double p)
Returns a random boolean from a Bernoulli distribution with success probability p.
Parameters:
p - the probability of returning true
Returns:
true with probability p and false with probability p
Throws:
IllegalArgumentException - unless 0p1.0
• #### bernoulli

public static boolean bernoulli()
Returns a random boolean from a Bernoulli distribution with success probability 1/2.
Returns:
true with probability 1/2 and false with probability 1/2
• #### gaussian

public static double gaussian()
Returns a random real number from a standard Gaussian distribution.
Returns:
a random real number from a standard Gaussian distribution (mean 0 and standard deviation 1).
• #### gaussian

public static double gaussian(double mu,
double sigma)
Returns a random real number from a Gaussian distribution with mean μ and standard deviation σ.
Parameters:
mu - the mean
sigma - the standard deviation
Returns:
a real number distributed according to the Gaussian distribution with mean mu and standard deviation sigma
• #### geometric

public static int geometric(double p)
Returns a random integer from a geometric distribution with success probability p.
Parameters:
p - the parameter of the geometric distribution
Returns:
a random integer from a geometric distribution with success probability p; or Integer.MAX_VALUE if p is (nearly) equal to 1.0.
Throws:
IllegalArgumentException - unless p >= 0.0 and p <= 1.0
• #### poisson

public static int poisson(double lambda)
Returns a random integer from a Poisson distribution with mean λ.
Parameters:
lambda - the mean of the Poisson distribution
Returns:
a random integer from a Poisson distribution with mean lambda
Throws:
IllegalArgumentException - unless lambda > 0.0 and not infinite
• #### pareto

public static double pareto()
Returns a random real number from the standard Pareto distribution.
Returns:
a random real number from the standard Pareto distribution
• #### pareto

public static double pareto(double alpha)
Returns a random real number from a Pareto distribution with shape parameter α.
Parameters:
alpha - shape parameter
Returns:
a random real number from a Pareto distribution with shape parameter alpha
Throws:
IllegalArgumentException - unless alpha > 0.0
• #### cauchy

public static double cauchy()
Returns a random real number from the Cauchy distribution.
Returns:
a random real number from the Cauchy distribution.
• #### discrete

public static int discrete(double[] probabilities)
Returns a random integer from the specified discrete distribution.
Parameters:
probabilities - the probability of occurrence of each integer
Returns:
a random integer from a discrete distribution: i with probability probabilities[i]
Throws:
IllegalArgumentException - if probabilities is null
IllegalArgumentException - if sum of array entries is not (very nearly) equal to 1.0
IllegalArgumentException - unless probabilities[i] >= 0.0 for each index i
• #### discrete

public static int discrete(int[] frequencies)
Returns a random integer from the specified discrete distribution.
Parameters:
frequencies - the frequency of occurrence of each integer
Returns:
a random integer from a discrete distribution: i with probability proportional to frequencies[i]
Throws:
IllegalArgumentException - if frequencies is null
IllegalArgumentException - if all array entries are 0
IllegalArgumentException - if frequencies[i] is negative for any index i
IllegalArgumentException - if sum of frequencies exceeds Integer.MAX_VALUE (231 - 1)
• #### exp

public static double exp(double lambda)
Returns a random real number from an exponential distribution with rate λ.
Parameters:
lambda - the rate of the exponential distribution
Returns:
a random real number from an exponential distribution with rate lambda
Throws:
IllegalArgumentException - unless lambda > 0.0
• #### shuffle

public static void shuffle(Object[] a)
Rearranges the elements of the specified array in uniformly random order.
Parameters:
a - the array to shuffle
Throws:
IllegalArgumentException - if a is null
• #### shuffle

public static void shuffle(double[] a)
Rearranges the elements of the specified array in uniformly random order.
Parameters:
a - the array to shuffle
Throws:
IllegalArgumentException - if a is null
• #### shuffle

public static void shuffle(int[] a)
Rearranges the elements of the specified array in uniformly random order.
Parameters:
a - the array to shuffle
Throws:
IllegalArgumentException - if a is null
• #### shuffle

public static void shuffle(char[] a)
Rearranges the elements of the specified array in uniformly random order.
Parameters:
a - the array to shuffle
Throws:
IllegalArgumentException - if a is null
• #### shuffle

public static void shuffle(Object[] a,
int lo,
int hi)
Rearranges the elements of the specified subarray in uniformly random order.
Parameters:
a - the array to shuffle
lo - the left endpoint (inclusive)
hi - the right endpoint (exclusive)
Throws:
IllegalArgumentException - if a is null
IllegalArgumentException - unless (0 <= lo) && (lo < hi) && (hi <= a.length)
• #### shuffle

public static void shuffle(double[] a,
int lo,
int hi)
Rearranges the elements of the specified subarray in uniformly random order.
Parameters:
a - the array to shuffle
lo - the left endpoint (inclusive)
hi - the right endpoint (exclusive)
Throws:
IllegalArgumentException - if a is null
IllegalArgumentException - unless (0 <= lo) && (lo < hi) && (hi <= a.length)
• #### shuffle

public static void shuffle(int[] a,
int lo,
int hi)
Rearranges the elements of the specified subarray in uniformly random order.
Parameters:
a - the array to shuffle
lo - the left endpoint (inclusive)
hi - the right endpoint (exclusive)
Throws:
IllegalArgumentException - if a is null
IllegalArgumentException - unless (0 <= lo) && (lo < hi) && (hi <= a.length)
• #### permutation

public static int[] permutation(int n)
Returns a uniformly random permutation of n elements.
Parameters:
n - number of elements
Returns:
an array of length n that is a uniformly random permutation of 0, 1, ..., n-1
Throws:
IllegalArgumentException - if n is negative
• #### permutation

public static int[] permutation(int n,
int k)
Returns a uniformly random permutation of k of n elements.
Parameters:
n - number of elements
k - number of elements to select
Returns:
an array of length k that is a uniformly random permutation of k of the elements from 0, 1, ..., n-1
Throws:
IllegalArgumentException - if n is negative
IllegalArgumentException - unless 0 <= k <= n
• #### main

public static void main(String[] args)
Unit tests the methods in this class.
Parameters:
args - the command-line arguments