Class inheritance and generics/templates makes it possible to write code with nice abstractions very similar to the abstractions used in math, particularly in abstract algebra, where a computation can be described for an entire class of algebraic structures sharing some similar properties (eg. groups and rings). This has been described nicely in the book “From Mathematics to Generic Programming” by Alexander A. Stepanov and Daniel E. Rose.

Inspired by this book, I have implemented a library for computations on abstract algebraic structures such as groups, rings and fields. The library is called Ruffini (named after the italian mathematician Paolo Ruffini) and is developed in Java using generics to achieve the same kind of abstraction as in abstract algebra, eg. that you do not specify what specific algebraic structure and elements are used, but only what abstract structure it has, eg. that it is a group or a ring.

Abstract algebraic structures are defined in Ruffini by a number of interfaces extending each other, each describing operations on elements of some set represented by a generic class E. The simplest such interface is a semigroup:

public interface Semigroup<E> {

  /**
   * Return the result of the product <i>ab</i> in this 
   * semigroup.
   * 
   * @param a An element <i>a</i> in this algebraic structure.
   * @param b Another element <i>b</i> in this algebraic 
   *          structure.
   * @return The product of the two elements, <i>ab</i>.
   */
  public E multiply(E a, E b);

}

The semigroup is extended by a monoid interface which adds a getIdentity() method and by a group interface which adds an invert(E) method. Continuing like this, we end up with interfaces for more complicated structures like rings, fields and vector spaces.

Now, each of these interfaces describes functions that can be applied to a generic type E (eg. that given two instances of E, the multiply method above returns a new instance of type E). As in abstract algebra, these functions can be used to describe complicated computations without specifying exactly what algebraic structure is being used.

Ruffini currently implements a number of algorithms that is defined and described for the abstract structures, including the discrete Fourier transform, the Euclidean algorithm, computing Gröbner bases, the Gram-Schmidt process and Gaussian elimination. The abstraction makes it possible to write the algorithms only once in a clear, while still being usable on any of the algebraic structures defined in the library, such as integers, rational numbers, integers modulo n, finite fields, polynomial rings and matrix rings. Below is the code for the Gram-Schmidt process:

public List<V> apply(List<V> vectors) {
  List<V> U = new ArrayList<>();

  Projection<V, S> projection = 
      new Projection<>(vectorSpace, innerProduct);

  for (V v : vectors) {
    for (V u : U) {
      V p = projection.apply(v, u);
      v = vectorSpace.subtract(v, p);
    }
    U.add(v);
  }
  return U;
}

Here V and S are the generic types for resp. the vectors and scalars of the underlying vector space, and Projection is a class defining projection given a specific vector space and an inner product.

The library is can be downloaded from GitHub: https://github.com/jonas-lj/Ruffini.

Given a list of items each with a value and a weight, the Knapsack problem seeks to find the set of items with the largest combined value within a given weight limit. There are a nice dynamic programming solution which I decided to implement in a spread sheet. I used Google Sheets but the solution is exported as an excel-sheet.

The solution builds a Knapsack table for round 0,1,…,limit. In each round r the solution for the problem with limit r is constructed as a column in the table, so the table has to be as wide as the maximum limit. Once the table is built, the solution can be found using backtracking. This is all described pretty well on Wikipedia, https://en.wikipedia.org/wiki/Knapsack_problem#0/1_knapsack_problem.

The main challenge was to translate this algorithm from procedural pseudocode to a spreadsheet. Building the table is simple enough (once you learn the OFFSET command in Excel which allows you to add or subtract a variable number of rows and columns from a given position), but the backtracking was a bit more tricky.

Assuming that the weights are stored in column A from row 3 and the corresponding values are stored in column B, the table starts in column D. The round numbers are stored in row 1 from columnD. Row 2 are just 0’s and all other entries are (the one below is from D3):

=IF($A3>D$1; D2; MAX(D2; OFFSET(INDIRECT(ADDRESS(ROW(); COLUMN()));-1;-$A3) + $B3))

The table stops after round 40, so we find the solution with weight at most 40.

The backtracking to find the actual solution after building the table is done in column AT and AU. In the first column, the number of weight spent after 40 rounds are calculated from bottom to top row using the formula (from AT3):

=IF(OFFSET(INDIRECT(ADDRESS(ROW(); COLUMN()));0;-AT23-2) <> OFFSET(INDIRECT(ADDRESS(ROW(); COLUMN()));-1;-AT23-2); A22 + AT23; AT23)

The solution is shown in column AU where for each row we simply check if the accumulated weight increased with this item.

Given a set of linearly independent vectors, the Gram-Schmidt process produces a new set of vectors spanning the same subset, but the new set of vectors are now mutually orthogonal. The algorithm is usually stated as working over a vector space, but will in this post be altered to work over a module which is a vector space where the scalars are elements in a ring instead of a field. This could for instance be the case if we are given a set of integer vectors we wish to orthogonalise.

The original version can be described as follows: Given a set of vectors v_1, \ldots, v_n we define u_1 = v_1 and

u_k = v_k - \text{proj}_{u_1}(v_k) - \text{proj}_{u_2}(v_k) - \cdots - \text{proj}_{u_{k-1}}(v_k)

for k = 2,3,\ldots,n. The projection operator is defined as

\text{proj}_u(v) = \frac{\langle u, v \rangle}{\langle u, u \rangle} u.

If we say that all v_i have integer entries, we see that u_i must have rational entries, and simply scaling each vector with a multiple of all the denominators of the entries will give a vector parallel to the original vector but with integer entries. But what if we are interested in an algorithm that can only represent integers?

The algorithm is presented below. Note that the algorithm works over any module with inner product (aka a Hilbert Module).

# Given: Set of vectors V.
# Returns: A set of mutually orthogonal vectors spanning 
# the same subspace as the vectors in V.

U := Ø
k := 1

while V ≠ Ø do
  poll vector v from V
  w := kv

  for (u,m) in U do
    w -= m〈v,u〉u
  
  # Optional: Divide all entries in w by gcd of entries

  if V ≠ Ø do
    n :=〈w,w〉
    for (u,m) in U do
      m *= n
    Put (w,k) in U
    k *= n
  else do
    # No more iterations after this one
    Put (w,k) in U

return first coordinates of elements in U

When used on integers, the entries in the vectors grow very fast, but this may be avoided by dividing w by the greatest common divisor of the entries.