# APOTHEM

Apache Project(s) of the month

## Apache Commons Numbers

Although Java is probably not the first programming language to come to mind for scientific computing, there are several projects and libraries that try to bridge the gap. One prominent example is from the Apache Commons project: Apache Commons Math, a group of specialized mathematics and statistics components, has been introduced in 2004 and is now at its 3.6.1 stable release, with version 4.0 still under development. The library includes a large number of components, ranging from linear algebra to geometry, from ordinary differential equations to complex numbers, and so on.

The difficulty in maintaining the balance between backwards compatibility and adding improvements or new functionalities has inspired the creation of lower level libraries that contain only specific components (numbers, geometric primitives, etc.) and, as such, are smaller and easier to maintain. The library that we will explore in this article, Apache Commons Numbers, is dedicated to different types of numbers (fractions, angles, complex numbers, etc.), while other libraries have been created in a similar fashion for geometry, statistics, and random numbers generation. Given that the documentation is still at a very early stage for Numbers, hopefully this review will be useful to the library developers too.

### Setting up

The Commons Numbers library is currently in beta stage, basically meaning that it is still being tested and subject to change. While we would usually start exploring a library from its stable version and possibly add some observations about the development version, since there is no stable release for Numbers we will use the beta version throughout.

As we have done in the Apache Commons CLI article, we will use the library by means of a pom.xml file. Since we need to add several components, it is more practical to also set a commons.numbers.version property that can be updated at once, for instance when a first release becomes available. The POM file may look like the following:

<groupId>blog.apothem</groupId>
<artifactId>apache-commons-numbers-example</artifactId>
<version>0.0.1-SNAPSHOT</version>

<properties>
<commons.numbers.version>1.0-beta1</commons.numbers.version>
</properties>

<dependencies>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-numbers-core</artifactId>
<version>${commons.numbers.version}</version> </dependency> <dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-numbers-angle</artifactId> <version>${commons.numbers.version}</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-numbers-arrays</artifactId>
<version>${commons.numbers.version}</version> </dependency> <dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-numbers-combinatorics</artifactId> <version>${commons.numbers.version}</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-numbers-complex</artifactId>
<version>${commons.numbers.version}</version> </dependency> <dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-numbers-fraction</artifactId> <version>${commons.numbers.version}</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-numbers-gamma</artifactId>
<version>${commons.numbers.version}</version> </dependency> <dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-numbers-primes</artifactId> <version>${commons.numbers.version}</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-numbers-quaternion</artifactId>
<version>\${commons.numbers.version}</version>
</dependency>
</dependencies>


Again, we will create a class with a main method and write the entire code there; the complete code is available as always in the companion Apothem resources repository.

For the time being, I am skipping two components of the library, namely Field and RootFinder, since it is more difficult to create easy examples for both; I might add them as an update later on.

### Modules

In this section we will explore each component with code examples.

#### Core

The Core package contains some of the interfaces used throughout the library, but most importantly it contains some number-related utilities as static methods within the core.ArithmeticUtils and core.Precision classes. For instance, given two integer numbers $$x = 256$$ and $$y = 6$$, we can:

• calculate the (unsigned) quotient and reminder of the division $$x / y$$:

int x = 256;
int y = 6;

System.out.printf("%d / %d = %d%n",
x, y, ArithmeticUtils.divideUnsigned(x, y));
System.out.printf("%d %% %d = %d%n",
x, y, ArithmeticUtils.remainderUnsigned(x, y));

256 / 6 = 42
256 % 6 = 4

• find the greatest common divisor (gcd) and the least common multiple (lcm) between $$x$$ and $$y$$:

System.out.printf("gcd(%d, %d) = %d%n",
x, y, ArithmeticUtils.gcd(x, y));
System.out.printf("lcm(%d, %d) = %d%n",
x, y, ArithmeticUtils.lcm(x, y));

gcd(256, 6) = 2
lcm(256, 6) = 768

• calculate the powers $$x^2$$ and $$x^y$$, where the second one needs a cast to BigInteger since the result is too large to be contained in a long:

System.out.printf("%d ^ %d = %d%n",
x, 2, ArithmeticUtils.pow(x, 2));
System.out.printf("%d ^ %d = %d%n",
x, y, ArithmeticUtils.pow(BigInteger.valueOf(x), y));

256 ^ 2 = 65536
256 ^ 6 = 281474976710656

• check whether $$x$$ and $$x + 1$$ are powers of 2:

System.out.printf("Is %d a power of 2? %b%n",
x, ArithmeticUtils.isPowerOfTwo(x));
System.out.printf("Is %d + 1 a power of 2? %b%n",
x, ArithmeticUtils.isPowerOfTwo(x + 1));

Is 256 a power of 2? true
Is 256 + 1 a power of 2? false


Using the methods from the core.Precision class, given a decimal number $$x = 1.23456789$$ we can:

• round it to 2, 4 and 6 places:

double x = 1.23456789;

for (int places : Arrays.asList(2, 4, 6)) {
System.out.printf("Rounding %f to %d places = %f%n",
x, places, Precision.round(x, places));
}

Rounding 1.234568 to 2 places = 1.230000
Rounding 1.234568 to 4 places = 1.234600
Rounding 1.234568 to 6 places = 1.234568

• check that it is equal to $$x + 10^{-6}$$ up to a certain precision $$\epsilon$$:

for (double eps : Arrays.asList(1e-3, 1e-6, 1e-9)) {
System.out.printf("%.9f == %.9f (up to %e)? %b%n",
x, x + 1e-6, eps, Precision.equals(x, x + 1e-6, eps));
}

1.234567890 == 1.234568890 (up to 1.000000e-03)? true
1.234567890 == 1.234568890 (up to 1.000000e-06)? true
1.234567890 == 1.234568890 (up to 1.000000e-09)? false

• check whether two NaNs are equal, using both the same equals method and the equalsIncludingNaN method which takes NaNs into account:

double nan = Double.NaN;
double inf = Double.POSITIVE_INFINITY;

System.out.printf("%f == %f? %b%n",
nan, nan, Precision.equals(nan, nan));
System.out.printf("%f - %f == %f - %f? %b%n",
inf, inf, inf, inf, Precision.equals(inf - inf, inf - inf));
System.out.printf("%f == %f (including NaN)? %b%n",
nan, nan, Precision.equalsIncludingNaN(nan, nan));
System.out.printf("%f - %f == %f - %f (including NaN)? %b%n",
inf, inf, inf, inf, Precision.equalsIncludingNaN(inf - inf, inf - inf));

NaN == NaN? false
Infinity - Infinity == Infinity - Infinity? false
NaN == NaN (including NaN)? true
Infinity - Infinity == Infinity - Infinity (including NaN)? true


#### Arrays

This package can be used for basic linear algebra operations, such as:

• calculating the norm of a vector (which is called SafeNorm but could probably just be called Norm, as there is no other norm defined):

double[] array1 = {0.1, 0.4, 0.1};

System.out.printf("Norm of %s = %f%n",
Arrays.toString(array1), SafeNorm.value(array1));

Norm of [0.1, 0.4, 0.1] = 0.424264

• calculating the cosine similiarity and the dot product between two vectors:

double[] array2 = {-0.1, -0.4, -0.1};

System.out.printf("Angle between %s and %s = %f%n",
Arrays.toString(array1), Arrays.toString(array2), CosAngle.value(array1, array2));
System.out.printf("dot(%s, %s) = %f%n",
Arrays.toString(array1), Arrays.toString(array2), LinearCombination.value(array1, array2));

Angle between [0.1, 0.4, 0.1] and [-0.1, -0.4, -0.1] = -1.000000
dot([0.1, 0.4, 0.1], [-0.1, -0.4, -0.1]) = -0.180000

• converting the index of a multidimensional array from a multidimensional index (e.g. (1, 3, 2)) to a unidimensional (linear) index (e.g. 23) and vice versa, with the conversion logic explained in the class Javadoc:

MultidimensionalCounter counter = MultidimensionalCounter.of(4, 4, 3);
int pos = 23;
int[] index = {1, 3, 2};

System.out.printf("Array of dimensions %s: position %d corresponds to index %s%n",
Arrays.toString(counter.getSizes()), pos, Arrays.toString(counter.toMulti(pos)));
System.out.printf("Array of dimensions %s: index %s corresponds to position %d%n",
Arrays.toString(counter.getSizes()), Arrays.toString(index), counter.toUni(index));

Array of dimensions [4, 4, 3]: position 23 corresponds to index [1, 3, 2]
Array of dimensions [4, 4, 3]: index [1, 3, 2] corresponds to position 23


It could be useful if some vector algebra functions were added (such as vector sum and scaling), although more sophisticated operations would probably be better left as part of a linear algebra-focused library.

#### Complex

This package contains definitions and methods for complex numbers, a type of number that is frequently used in physics and engineering. A complex number can be created in three different ways:

• using the cartesian coordinates, so $$z = a + b * i$$ would be expressed as the tuple $$(a, b)$$;
• using the polar coordinates, so $$z = \rho * (cos(\theta) + i * sin(\theta))$$ would be expressed as the tuple $$(\rho, \theta)$$ and, to convert from cartesian coordinates, $$\rho = \sqrt{a^2 + b^2}$$ and $$\theta = atan2(a, b)$$;
• using the cis (cos + i sin) form, so $$z = cos(x) + i * sin(x)$$ and, equivalently, $$z = e^{ix}$$ would be expressed by the single number $$x$$.

These three methods can be implemented as in the following example:

Complex z = Complex.ofCartesian(1, 1);
Complex z2 = Complex.ofPolar(Math.sqrt(2), Math.PI / 4);
Complex z3 = Complex.ofCis(Math.PI);

System.out.printf("z: %s, z2: %s, z3: %s%n",
z, z2, z3);

z: (1.0,1.0), z2: (1.0000000000000002,1.0), z3: (-1.0,1.2246467991473532E-16)


We can see that, although they should theoretically be the same, $$z$$ and $$z_2$$ are not considered equal due to numerical instability:

System.out.printf("%s == %s? %b%n",
z, z2, z.equals(z2));

(1.0,1.0) == (1.0000000000000002,1.0)? false


For this reason, it would be useful to have an "almost equals" method available for complex numbers too. However, there are many other methods that, given a complex number, can:

• extract its real and imaginary part, its absolute value and its argument;
• calculate its conjugate and its squared norm;
• calculate the value of several trigonometric functions.

Examples:

System.out.printf("Given z = %s:%n", z);
System.out.printf("\tRe(z) = %s, Im(z) = %s%n",
z.real(), z.imag());
System.out.printf("\tabs(z) = %s, arg(z) = %s%n",
z.abs(), z.arg());
System.out.printf("\tconj(z) = %s%n", z.conj());
System.out.printf("\tnorm(z) = %f%n", z.norm());
System.out.printf("\tasin(z) = %s%n", z.asin());
System.out.printf("\tacos(z) = %s%n", z.acos());
System.out.printf("\tatan(z) = %s%n", z.atan());
System.out.printf("\tasinh(z) = %s%n", z.asinh());
System.out.printf("\tacosh(z) = %s%n", z.acosh());
System.out.printf("\tatanh(z) = %s%n", z.atanh());

Given z = (1.0,1.0):
Re(z) = 1.0, Im(z) = 1.0
abs(z) = 1.4142135623730951, arg(z) = 0.7853981633974483
conj(z) = (1.0,-1.0)
norm(z) = 2.000000
asin(z) = (0.6662394324925152,1.0612750619050357)
acos(z) = (0.9045568943023814,-1.0612750619050357)
atan(z) = (1.0172219678978514,0.40235947810852507)
asinh(z) = (1.0612750619050357,0.6662394324925152)
acosh(z) = (1.0612750619050357,0.9045568943023814)
atanh(z) = (0.40235947810852507,1.0172219678978514)


Furthermore, given two complex numbers, it is possible to add, subtract, multiply, and divide them, and raise one to the power of the other one:

System.out.printf("Given z = %s anz z2 = %s:%n", z, z2);
System.out.printf("\tz + z2 = %s%n", z.add(z2));
System.out.printf("\tz - z2 = %s%n", z.subtract(z2));
System.out.printf("\tz * z2 = %s%n", z.multiply(z2));
System.out.printf("\tz / z2 = %s%n", z.divide(z2));
System.out.printf("\tz ^ z2 = %s%n", z.pow(z2));

Given z = (1.0,1.0) anz z2 = (1.0000000000000002,1.0):
z + z2 = (2.0,2.0)
z - z2 = (-2.220446049250313E-16,0.0)
z * z2 = (2.220446049250313E-16,2.0)
z / z2 = (0.9999999999999998,1.1102230246251563E-16)
z ^ z2 = (0.273957253830121,0.5837007587586148)


Using the previously seen Precision.equals method, we can even show Euler's identity for $$z = e^{i\pi}$$:

double eps = 1e-15;
System.out.printf("Re(%s) == -1 up to %s? %b%n",
z3, eps, Precision.equals(z3.real(), -1, eps));
System.out.printf("Im(%s) == 0 up to %s? %b%n",
z3, eps, Precision.equals(z3.imag(), 0, eps));

Re((-1.0,1.2246467991473532E-16)) == -1 up to 1.0E-15? true
Im((-1.0,1.2246467991473532E-16)) == 0 up to 1.0E-15? true


#### Angle

This package can be used to define angles from different units of measurement, namely degrees, radians, and turns (where 1 turn = 360 degrees):

PlaneAngle alpha = PlaneAngle.ofDegrees(180);
PlaneAngle beta = PlaneAngle.ofRadians(Math.PI);

System.out.printf("Angle alpha in degrees: %f%n",
alpha.toDegrees());
System.out.printf("Angle alpha in radians: %f%n",
System.out.printf("Angle alpha in turns: %f%n",
alpha.toTurns());

System.out.printf("Angle beta in degrees: %f%n",
alpha.toDegrees());
System.out.printf("Angle beta in radians: %f%n",
System.out.printf("Angle beta in turns: %f%n",
alpha.toTurns());

Angle alpha in degrees: 180.000000
Angle alpha in radians: 3.141593
Angle alpha in turns: 0.500000
Angle beta in degrees: 180.000000
Angle beta in radians: 3.141593
Angle beta in turns: 0.500000


Angles can also be compared for equality:

System.out.printf("Are angles alpha and beta equal? %b%n",
alpha.equals(beta));

Are angles alpha and beta equal? true


The package is currently missing operations between angles, such as addition and multiplication by scalars.

#### Fraction

This package can be used to define fractions, where instead of a (possibly approximate) decimal value we want to use a rational number and perform operations on it. Fractions can be created from a numerator and a denominator or from a decimal value, in which case the denominator can optionally be limited to a maximum (useful to adjust the precision of the conversion):

List<Fraction> fractions = Arrays.asList(
Fraction.of(4, 9),
Fraction.of(1, 2),
Fraction.from(0.99245),
Fraction.from(0.99245, 10000000));

System.out.printf("Fractions: %s%n",
fractions.stream().map(Fraction::toString).collect(Collectors.joining(" ; ")));

Fractions: 4 / 9 ; 1 / 2 ; 263 / 265 ; 19849 / 20000


A fraction can be:

• decomposed into numerator and denominator;
• converted to a decimal number;
• converted to its reciprocal;
• negated;
• raised to a power.

Examples:

System.out.printf("The fraction %s has numerator %d and denominator %d%n",
fractions.get(0), fractions.get(0).getNumerator(), fractions.get(0).getDenominator());
System.out.printf("%s as a decimal number: %f%n",
fractions.get(0), fractions.get(0).doubleValue());
System.out.printf("Reciprocal of %s: %s%n",
fractions.get(0), fractions.get(0).reciprocal());
System.out.printf("Additive inverse of %s: %s%n",
fractions.get(0), fractions.get(0).negate());
System.out.printf("(%s) ^ 3 = %s%n",
fractions.get(0), fractions.get(0).pow(3));

The fraction 4 / 9 has numerator 4 and denominator 9
4 / 9 as a decimal number: 0.444444
Reciprocal of 4 / 9: 9 / 4
Additive inverse of 4 / 9: -4 / 9
(4 / 9) ^ 3 = 64 / 729


A fraction can be added, subtracted, multiplied, and divided by another one:

System.out.printf("(%s) + (%s) = %s%n",
System.out.printf("(%s) - (%s) = %s%n",
fractions.get(0), fractions.get(1), fractions.get(0).subtract(fractions.get(1)));
System.out.printf("(%s) * (%s) = %s%n",
fractions.get(0), fractions.get(1), fractions.get(0).multiply(fractions.get(1)));
System.out.printf("(%s) / (%s) = %s%n",
fractions.get(0), fractions.get(1), fractions.get(0).divide(fractions.get(1)));

(4 / 9) + (1 / 2) = 17 / 18
(4 / 9) - (1 / 2) = -1 / 18
(4 / 9) * (1 / 2) = 2 / 9
(4 / 9) / (1 / 2) = 8 / 9


#### Primes

This package is dedicated to prime numbers, a class of numbers with many applications especially in cryptography. The Primes class has methods to determine whether a number is prime, to find the closest prime to a number, and to decompose a number into prime factors:

for (int n : Arrays.asList(2, 4, 42)) {
System.out.printf("Is %d prime? %b%n",
n, Primes.isPrime(n));
System.out.printf("What is the closest prime to %d? %d%n",
n, Primes.nextPrime(n)); // Note: why is it 2 when n = 2?
System.out.printf("Prime factors of %d: %s%n",
n, Primes.primeFactors(n));
}

Is 2 prime? true
What is the closest prime to 2? 2
Prime factors of 2: [2]
Is 4 prime? false
What is the closest prime to 4? 5
Prime factors of 4: [2, 2]
Is 42 prime? false
What is the closest prime to 42? 43
Prime factors of 42: [2, 3, 7]


#### Quaternion

This package introduces the concept of quaternion, an extension to complex numbers that is widely used in computer graphics and robotics due to its convenience in implementing rotations around multiple axes. A quaternion is composed of two parts, a scalar and a tridimensional vector; it can therefore be created with different methods, where the simplest one sets the scalar part to zero (thus creating a so-called pure quaternion):

List<Quaternion> quaternions = Arrays.asList(
Quaternion.of(new double[]{1, 1, 1}),
Quaternion.of(2, new double[]{1, 1, 1}),
Quaternion.of(0.5, 1, 1, 1));


Each quaternion can be:

• checked for "pureness";
• conjugated;
• inverted;
• normalized (as a versor) or converted to positive polar form (defined as "the unit quaternion with positive scalar part", although this could be made more clear);
• divided by a scalar, for instance by its own norm (to check that the normalization method yields the correct result).

Examples:

for (Quaternion q : quaternions) {
System.out.printf("Quaternion %s%n", q);
System.out.printf("\tIs it pure (up to 1e-10)? %b%n",
q.isPure(1e-10));
System.out.printf("\tConjugate: %s%n",
q.conjugate());
System.out.printf("\tInverse: %s%n",
q.inverse());
System.out.printf("\tNormalized form: %s%n",
q.normalize());
System.out.printf("\tPositive polar form: %s%n",
q.positivePolarForm());
System.out.printf("\tQuaternion divided by its norm %f: %s%n",
q.norm(), q.divide(q.norm()));
}

Quaternion [0.0 1.0 1.0 1.0]
Is it pure (up to 1e-10)? true
Conjugate: [0.0 -1.0 -1.0 -1.0]
Inverse: [0.0 -0.3333333333333333 -0.3333333333333333 -0.3333333333333333]
Normalized form: [0.0 0.5773502691896258 0.5773502691896258 0.5773502691896258]
Positive polar form: [0.0 0.5773502691896258 0.5773502691896258 0.5773502691896258]
Quaternion divided by its norm 1.732051: [0.0 0.5773502691896258 0.5773502691896258 0.5773502691896258]
Quaternion [2.0 1.0 1.0 1.0]
Is it pure (up to 1e-10)? false
Conjugate: [2.0 -1.0 -1.0 -1.0]
Inverse: [0.2857142857142857 -0.14285714285714285 -0.14285714285714285 -0.14285714285714285]
Normalized form: [0.7559289460184544 0.3779644730092272 0.3779644730092272 0.3779644730092272]
Positive polar form: [0.7559289460184544 0.3779644730092272 0.3779644730092272 0.3779644730092272]
Quaternion divided by its norm 2.645751: [0.7559289460184544 0.3779644730092272 0.3779644730092272 0.3779644730092272]
Quaternion [0.5 1.0 1.0 1.0]
Is it pure (up to 1e-10)? false
Conjugate: [0.5 -1.0 -1.0 -1.0]
Inverse: [0.15384615384615385 -0.3076923076923077 -0.3076923076923077 -0.3076923076923077]
Normalized form: [0.2773500981126146 0.5547001962252291 0.5547001962252291 0.5547001962252291]
Positive polar form: [0.2773500981126146 0.5547001962252291 0.5547001962252291 0.5547001962252291]
Quaternion divided by its norm 1.802776: [0.2773500981126146 0.5547001962252291 0.5547001962252291 0.5547001962252291]


Two quaternions can be added, subtracted, and multiplied (the division is only allowed by a decimal number):

System.out.printf("%s + %s = %s%n",
System.out.printf("%s - %s = %s%n",
quaternions.get(0), quaternions.get(1), quaternions.get(0).subtract(quaternions.get(1)));
System.out.printf("%s * %s = %s%n",
quaternions.get(0), quaternions.get(1), quaternions.get(0).multiply(quaternions.get(1)));

[0.0 1.0 1.0 1.0] + [2.0 1.0 1.0 1.0] = [2.0 2.0 2.0 2.0]
[0.0 1.0 1.0 1.0] - [2.0 1.0 1.0 1.0] = [-2.0 0.0 0.0 0.0]
[0.0 1.0 1.0 1.0] * [2.0 1.0 1.0 1.0] = [-3.0 2.0 2.0 2.0]


and it is also possible to calculate the dot product between them:

System.out.printf("dot(%s, %s) = %s%n",
quaternions.get(0), quaternions.get(1), quaternions.get(0).dot(quaternions.get(1)));

dot([0.0 1.0 1.0 1.0], [2.0 1.0 1.0 1.0]) = 3.0


For more information on how quaternion algebra works, you can check the Wikipedia page.

Another interesting class from this package is Slerp, where the only available method apply is used to create a spherical linear interpolation (or slerp) between two quaternions, with the purpose of smoothly transforming the first into the second (useful for instance in 3D-rotation animation tasks). A slerp from the special quaternion $$\boldsymbol{i} = (0, 1, 0, 0)$$ to the other special quaternion $$\boldsymbol{j} = (0, 0, 1, 0)$$ can be created as follows:

Slerp slerp = new Slerp(Quaternion.I, Quaternion.J);


We want the transition to take place in intervals of 0.1, so we use a for loop:

System.out.printf("Slerp demo%n");
for (double t = 0; t <= 1.0; t += 0.1) {
System.out.printf("\tQuaternion at t = %f: %s%n",
t, slerp.apply(t));
}

Slerp demo
Quaternion at t = 0.000000: [0.0 1.0 0.0 0.0]
Quaternion at t = 0.100000: [0.0 0.9876883405951378 0.15643446504023087 0.0]
Quaternion at t = 0.200000: [0.0 0.9510565162951536 0.30901699437494745 0.0]
Quaternion at t = 0.300000: [0.0 0.8910065241883679 0.45399049973954686 0.0]
Quaternion at t = 0.400000: [0.0 0.8090169943749475 0.5877852522924731 0.0]
Quaternion at t = 0.500000: [0.0 0.7071067811865476 0.7071067811865476 0.0]
Quaternion at t = 0.600000: [0.0 0.5877852522924731 0.8090169943749475 0.0]
Quaternion at t = 0.700000: [0.0 0.45399049973954686 0.8910065241883679 0.0]
Quaternion at t = 0.800000: [0.0 0.3090169943749475 0.9510565162951535 0.0]
Quaternion at t = 0.900000: [0.0 0.15643446504023104 0.9876883405951378 0.0]
Quaternion at t = 1.000000: [0.0 1.743934249004316E-16 1.0 0.0]


#### Combinatorics

This package contains several classes for common combinatorial concepts such as factorials (to calculate the number of permutations of $$n$$ objects) and combinations (to calculate the number of ways $$n$$ objects can be arranged in groups of $$k$$ objects). For example, given $$n = 10$$, we can calculate the factorial (as a long and as a double) and log-factorial of $$n$$:

int n = 10;

System.out.printf("%d! = %d%n",
n, Factorial.value(n));
System.out.printf("%d! = %f (double)%n",
n, FactorialDouble.create().value(n));
System.out.printf("log(%d!) = %f%n",
n, LogFactorial.create().value(n));

10! = 3628800
10! = 3628800.000000 (double)
log(10!) = 15.104413


Furthermore, given $$k = 3$$, we can calculate the binomial coefficient $$\binom{n}{k}$$ (i.e. the number of $$k$$-combinations of $$n$$ objects), both as a long and as a double, and its logarithm:

int k = 3;

System.out.printf("Binomial coefficient (%d %d) = %d%n",
n, k, BinomialCoefficient.value(n, k));
System.out.printf("Binomial coefficient (%d %d) = %f (double)%n",
n, k, BinomialCoefficientDouble.value(n, k));
System.out.printf("Log of the binomial coefficient (%d %d) = %f%n",
n, k, LogBinomialCoefficient.value(n, k));

Binomial coefficient (10 3) = 120
Binomial coefficient (10 3) = 120.000000 (double)
Log of the binomial coefficient (10 3) = 4.787492


We can also calculate the combinations themselves, take their number and show some of them:

Combinations comb = Combinations.of(n, k);
List<int[]> combList = new ArrayList<>();
for (int[] element : comb) {
}

System.out.printf("Number of combinations C(%d, %d): %d%n",
n, k, combList.size());
System.out.printf("First three combinations: %s, %s, %s%n",
Arrays.toString(combList.get(0)), Arrays.toString(combList.get(1)),
Arrays.toString(combList.get(2)));

Number of combinations C(10, 3): 120
First three combinations: [0, 1, 2], [0, 1, 3], [0, 2, 3]


One important thing that must be noted here is the potentially large size of the numbers involved. The factorial grows very quickly, so the numbers for which the factorial can fit in a long are very few (just up to $$n = 20$$); the binomial grows quickly as well, so its input is also limited (in fact it can be calculated only for $$n <= 66$$). This problem can be alleviated by using approximations, either using the Double version of both classes (i.e. FactorialDouble and BinomialCoefficientDouble) or using their logarithmic versions (i.e. LogFactorial and LogBinomialCoefficient), which in turn use the LogGamma class from the Gamma package. It would be interesting to see also a version of Factorial and BinomialCoefficient that makes use of BigIntegers to allow for exact calculation on larger numbers.

#### Gamma

This package is very specific to a certain class of mathematical functions derived from the gamma function, including:

• the error function and its inverse:

for (double value : Arrays.asList(1., 2., 3.)) {
System.out.printf("erf(%f) = %f%n",
value, Erf.value(value));
}

for (double value : Arrays.asList(0.9999, 0.9953, 0.8427)) {
System.out.printf("erf^(-1)(%f) = %f%n",
value, InverseErf.value(value));
}

erf(1.000000) = 0.842701
erf(2.000000) = 0.995322
erf(3.000000) = 0.999978
erf^(-1)(0.999900) = 2.751064
erf^(-1)(0.995300) = 1.998925
erf^(-1)(0.842700) = 0.999998

• the gamma function itself, its derivatives digamma and trigamma, and its logarithm log-gamma, which are calculated in two interesting points to show their values (with $$g$$ being the Euler–Mascheroni constant):

System.out.printf("Gamma(1) = %f%n",
Gamma.value(1));
System.out.printf("Gamma(0.5) = %f (sqrt(pi))%n",
Gamma.value(0.5));

System.out.printf("Digamma(1) = %f (-g)%n",
Digamma.value(1));
System.out.printf("Digamma(0.5) = %f (-2 * ln(2) - g)%n",
Digamma.value(0.5));

System.out.printf("Trigamma(1) = %f (pi ** 2 / 6)%n",
Trigamma.value(1));
System.out.printf("Trigamma(0.5) = %f (pi ** 2 / 2)%n",
Trigamma.value(0.5));

System.out.printf("log-gamma(1) = %f%n",
LogGamma.value(1));
System.out.printf("log-gamma(0.5) = %f (log(sqrt(pi)))%n",
LogGamma.value(0.5));

gamma(1) = 1.000000
gamma(0.5) = 1.772454 (sqrt(pi))
digamma(1) = -0.577216 (-gamma)
digamma(0.5) = -1.963510 (-2 * ln(2) - gamma)
trigamma(1) = 1.644934 (pi ** 2 / 6)
trigamma(0.5) = 4.934802 (pi ** 2 / 2)
log-gamma(1) = -0.000000
log-gamma(0.5) = 0.572365 (log(sqrt(pi)))


Among the packages we have seen so far, this seems less connected to the concept of "number" and closer to the concept of "function".

### Conclusions

The Commons Numbers library is a good step towards a light, easily-embeddable and general purpose number library, with many common number types (especially fractions and complex numbers) and useful utilities such as the Precision class. The library makes these "special" numbers quite easy to use, although Java itself is still missing a much desired feature (operator overloading) that would make numeric computing even more appealing.

The library could be extended to add more functionalities to some number types (e.g. more operations for angles and vectors, close equality check for complex numbers) and a better integration with BigIntegers, but the current beta version seems very promising.