### Add new LFE exercises

One of the easiest ways to add new exercises to the LFE track is to translate them from other language tracks on Exercism. This page lists all of the problem specifications in the common pool of exercises which have not yet been implemented in LFE.

The icon links to the language-independent description of the problem.

The icon links to the canonical test data.

We also link to all the implementations of each exercise in other languages, which you can use for inspiration.

Once you've picked an exercise, follow the instructions in this guide and submit your pull request to the LFE track repository on GitHub.

### Hello World

The classical introductory exercise. Just say "Hello, World!"

### Raindrops

Convert a number to a string, the contents of which depend on the number's factors.

### Triangle

Determine if a triangle is equilateral, isosceles, or scalene.

### Roman Numerals

Write a function to convert from normal numbers to Roman Numerals.

### Pangram

Determine if a sentence is a pangram.

### Scrabble Score

Given a word, compute the scrabble score for that word.

### Acronym

Convert a long phrase to its acronym

### Collatz Conjecture

Calculate the number of steps to reach 1 using the Collatz conjecture

### Isogram

Determine if a word or phrase is an isogram.

### Pascals Triangle

Compute Pascal's triangle up to a given number of rows.

### Robot Name

Manage robot factory settings.

### Nth Prime

Given a number n, determine what the nth prime is.

### Sieve

Use the Sieve of Eratosthenes to find all the primes from 2 up to a given number.

### All Your Base

Convert a number, represented as a sequence of digits in one base, to any other base.

### Perfect Numbers

Determine if a number is perfect, abundant, or deficient based on Nicomachus' (60 - 120 CE) classification scheme for natural numbers.

### Binary Search

Implement a binary search algorithm.

### Prime Factors

Compute the prime factors of a given natural number.

### Bracket Push

Make sure the brackets and braces all match.

### Crypto Square

Implement the classic method for composing secret messages called a square code.

### Say

Given a number from 0 to 999,999,999,999, spell out that number in English.

### Pig Latin

Implement a program that translates from English to Pig Latin

### Queen Attack

Given the position of two queens on a chess board, indicate whether or not they are positioned so that they can attack each other.

### Run Length Encoding

Implement run-length encoding and decoding.

### Secret Handshake

Given a decimal number, convert it to the appropriate sequence of events for a secret handshake.

### Flatten Array

Take a nested list and return a single list with all values except nil/null

### Minesweeper

Add the numbers to a minesweeper board

### Sublist

Write a function to determine if a list is a sublist of another list.

### Wordy

Parse and evaluate simple math word problems returning the answer as an integer.

### Two Fer

Create a sentence of the form "One for X, one for me."

### Binary Search Tree

Insert and search for numbers in a binary tree.

### Diamond

Given a letter, print a diamond starting with 'A' with the supplied letter at the widest point.

### List Ops

Implement basic list operations

### Custom Set

Create a custom set type.

### Ocr Numbers

Given a 3 x 4 grid of pipes, underscores, and spaces, determine which number is represented, or whether it is garbled.

### Rotational Cipher

Create an implementation of the rotational cipher, also sometimes called the Caesar cipher.

### Saddle Points

Detect saddle points in a matrix.

### Change

Correctly determine change to be given using the least number of coins

### Linked List

Implement a doubly linked list

### Palindrome Products

Detect palindrome products in a given range.

### Pythagorean Triplet

There exists exactly one Pythagorean triplet for which a + b + c = 1000. Find the product a * b * c.

### Food Chain

Generate the lyrics of the song 'I Know an Old Lady Who Swallowed a Fly'

### Isbn Verifier

Check if a given string is a valid ISBN-10 number.

### Kindergarten Garden

Given a diagram, determine which plants each child in the kindergarten class is responsible for.

### Matrix

Given a string representing a matrix of numbers, return the rows and columns of that matrix.

### Protein Translation

Translate RNA sequences into proteins.

### React

Implement a basic reactive system.

### Simple Cipher

Implement a simple shift cipher like Caesar and a more secure substitution cipher

### Simple Linked List

Write a simple linked list implementation that uses Elements and a List

### Transpose

Take input text and output it transposed.

### Forth

Implement an evaluator for a very simple subset of Forth

### Alphametics

Write a function to solve alphametics puzzles.

### Connect

Compute the result for a game of Hex / Polygon

### House

Output the nursery rhyme 'This is the House that Jack Built'.

### Twelve Days

Output the lyrics to 'The Twelve Days of Christmas'

### Complex Numbers

Implement complex numbers.

### Dominoes

Make a chain of dominoes.

### Spiral Matrix

Given the size, return a square matrix of numbers in spiral order.

### Tournament

Tally the results of a small football competition.

### Book Store

To try and encourage more sales of different books from a popular 5 book series, a bookshop has decided to offer discounts of multiple-book purchases.

### Poker

Pick the best hand(s) from a list of poker hands.

### Rail Fence Cipher

Implement encoding and decoding for the rail fence cipher.

### Proverb

For want of a horseshoe nail, a kingdom was lost, or so the saying goes. Output the full text of this proverbial rhyme.

### Zipper

Creating a zipper for a binary tree.

### Armstrong Numbers

Determine if a number is an Armstrong number

### Diffie Hellman

Diffie-Hellman key exchange.

### Rectangles

Count the rectangles in an ASCII diagram.

### Two Bucket

Given two buckets of different size, demonstrate how to measure an exact number of liters.

### Variable Length Quantity

Implement variable length quantity encoding and decoding.

### Markdown

Refactor a Markdown parser

### Pov

Reparent a graph on a selected node

### Word Search

Create a program to solve a word search puzzle.

### Error Handling

Implement various kinds of error handling and resource management

### Grep

Search a file for lines matching a regular expression pattern. Return the line number and contents of each matching line.

### Scale Generator

Generate musical scales, given a starting note and a set of intervals.

### Dot Dsl

Write a Domain Specific Language similar to the Graphviz dot language

### Go Counting

Count the scored points on a Go board.

### Sgf Parsing

Parsing a Smart Game Format string.

### Tree Building

Refactor a tree building algorithm.

### Hangman

Implement the logic of the hangman game using functional reactive programming.

### Ledger

Refactor a ledger printer.

### Lens Person

Use lenses to update nested records (specific to languages with immutable data).

### Paasio

Report network IO statistics