Yorgey, Brent

A template is a class or a function that we can parameterize with a set of types or values.

Parameterized Types

The vector-of-doubles can be generalized to a vector-of-anything type by making it a template:

// `template<typename T>` can be read as "for all types T". Older code
// uses `template<class T>`, which is equivalent.
template<typename T>
class Vector {
 public:
  explicit Vector(int s);
  ~Vector() { delete[] elem; }

  // ... copy and move operations ...

  T& operator[](int i);               // For non-const Vectors
  const T& operator[](int i) const;   // For const Vectors
  int size() const { return sz; }

 private:
  T* elem;
  int sz;
};

A class is a user-defined type provided to represent a concept in the code of a program. Essentially, all language facilities beyond the fundamental types, operators, and statements exist to help define better class or to use them more conveniently.

This page contains remarks on Haskell that I encountered when working with source files that span multiple AoC 2021 problems.

and have Haskell solutions. It’ll be nice to compare how they solved the problems. I don’t want to end up perfecting the wrong approach!

Setting Up Haskell Env for AoC

To manage dependencies, Cabal and Stack are pretty popular. Stack incorporates the Cabal build system.

Day 1 - Advent of Code 2021: Sonar Sweep. Eric Wastl. adventofcode.com . Accessed Feb 18, 2022.

Part One

As the submarine drops below the surface of the ocean, it automatically performs a sonar sweep of the nearby sea floor. On a small screen, the sonar weep report (your puzzle input) appears: each line is a measurement of the sea floor depth as the sweep looks further and further away from the submarine.

Problem Statement

Let \(d(n)\) be defined as the sum of proper divisors of \(n\) (numbers less than \(n\) which divide evenly into \(n\)).

If \(d(a) = b\) and \(d(b) = a\), where \(a \neq b\), then \(a\) and \(b\) are an amicable pair and each of \(a\) and \(b\) are called amicable numbers.

For example, the proper divisors of \(220\) are \(1, 2, 4, 5, 10, 11, 20, 22, 44, 55, 110\); therefore \(d(220) = 284\). The proper divisors of \(284\) are \(1, 2, 4, 71, 142\); so \(d(284) = 220\).