# Popular problem

From Esolang

Some problems, of varying degrees of complexity, are very popular targets for esoteric programmers. Programs solving these problems are usually the first, and often the only, programs to be implemented in a new esoteric programming language.

Some of the popular problems are:

- Hello, world! - Print "Hello, world!".
- 99 bottles of beer - Print the text of the "99 bottles of beer" song.
- Quine - Output program's own source code.
- Fibonacci sequence - Output the Fibonacci numbers (1,1,2,3,5,8,13...) using either recursion or iteration.
- Factorials - Calculate
*n*! for an arbitrary*n*, or output the sequence (1!,2!,3!,...), where*n*! = 1*2*...**n*. - Prime numbers - Output the sequence of prime numbers (2,3,5,7,11,13,17...) using e.g. the Sieve of Eratosthenes (or just brute force.)
- Cat program - Copy standard input to standard output.
- Digital root calculator - Calculate and print out the digital root of given input.
- Mandelbrot set - Output the Mandelbrot fractal, usually in ASCII pseudographics.
- Equivalency simulation - Interpret a language with a known computational class, e.g. Brainfuck, to prove that the language is in the same class, e.g. Turing-complete.
- Interpreter - Interpret Befunge or some other esoteric programming language not to demonstrate anything in particular, just for fun. An especially popular choice, for fairly esoteric reasons, is Deadfish.
- Bootstrap - Interpret or compile the language in which the program is written.
- ROT13 encoder/decoder - Apply the ROT13 cipher to the input text.
- Pi calculator - Calculate π to an arbitrary precision.
- Truth-machine - Get input, if 0, output '0' and quit, if 1, output '1' infinitely.

Other popular types of programs are:

- Palindromes - Programs that read the same forwards and backwards.
- Polyglots - Programs that run in multiple languages.