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Introduction The Pell(no, not Bell) Numbers are a simple, Fibonacci-like sequence, defined by the following relation: $P_n=\begin{cases}0&\mbox{if }n=0;\\1&\mbox{if }n=1;\\2P_{n-1}+P_{n-2...
JavaScript (Node.js), 150 71 bytes f=(a,b)=>a[b.length]?f(b,a):b.filter(x=>a[y=a.indexOf(x)]=!~y).length-1 Try it online!
JavaScript (Node.js), 27 22 bytes -5 bytes thanks to Hakerh400 Direct computation f=n=>(n+(n*n+4)**.5)/2 $$\frac{n+\sqrt{n^2+4}}{2}$$ Try it online!
Brachylog, 8 bytes pᵐ≠{b|}ᵛ Try it online! Since it takes $5!*5! = 14400$ tries to determine that the first test case gives false, I've added some shorter ones... pᵐ Permute each ele...
Scala, 55 bytes (Stream from 1 map 1.0./scanLeft.0)(_+_)indexWhere _.<= Try it online! Written in a more sane manner: n => Stream.from(1).map(1.0/_).scanLeft(0.0)(_+_).indexWhere(_ &g...
Python 3, 20 bytes print(input()[::-1]) Try it online!
Scala (-language:postfixOps), 4 solutions 1. Just 256: 256 2. Casting a character: +'Ā' The character Ā is 256 in decimal, and the unary + turns it into an Int. 3. Hashcode of a string: ...
PowerShell, 4 solutions 99+99+9+9+9+9+9+9+4 355-33-33-33 16*16 [int][char]'Ā' Try it online!
Jelly, 4 bytes ¹Ƥċ" Try it online! " For each element of the input, ċ how many times does it occur in ¹Ƥ " the corresponding prefix of the input?
APL (Dyalog Unicode), 11 bytes (SBCS) 1 1∘⍉+\∘.=⍨ Try it online! This was a fun APL exercise. Working on figuring out how to get it down to 7 bytes.
BQN, 5 bytes 1+⊒ Try it here! 3 characters, but, as there's no SBCS codepage for BQN, it must be counted as UTF-8. Two of the three characters are just adding one to the built-in that almost ...
BQN, 20 bytesSBCS ∾2/0‿8⊏↓"∾2/0‿8⊏↓""" Run online! BQN's strings escape double quotes by doubling them, so that the string used to start with only contains a single quote, although the output ...
Python 3, 34 bytes f=lambda x:[f(y)for y in range(x)] Try it online! -9 bytes thanks to Moshi Unfortunately, I cannot use Python sets, because sets are unhashable and thus cannot be put int...
APL (ngn/apl), 6 bytes {∇¨⍳⍵} Try it online! Dyalog APL and dzaima/APL appear to not give empty vectors for ⍳0, so I used ngn/APL. I don't know how to turn on boxing there, though, so there'...
BQN, 18 bytesSBCS {⊑(⊑𝕨⊸⍷)¨⊸/∾⟜𝕩¨↑𝕨} Run online! A direct translation of Razetime's APL solution (my attempted improvement ⊣∾{⊢´/𝕨⊸«⊸≡¨↑𝕩}↓⊢ turns out to be not at all correct). The BQN solut...
Sed -E, 25 bytes Takes second input, , comma, first input (inputs cannot contain , commas). s/^(.*)(.*),(.*)\1$/\3\2/
Scala 3 and Python 3.8 (pre-release), 2 languages (385 bytes) def String():Any=0 def Int():Any=0 def f(s:String):Any=print(s.replace(chr(81),chr(34)*3+s+chr(34)*3)) class M: f("""def Strin...
Programs may output via exit code Exit codes are to programs what return values are to functions, so it makes sense that exit codes would be allowed.
Abusing native number types It's common to restrict inputs, outputs, intermediate algorithm values, etc. to fit into the native number type of the language. This can be a problem for languages li...
Numerical I/O may be given as a character code Input 64 may be given as @ instead. This mostly exists for languages like Brainfuck that only take input through character codes.
Unless explicitly asking for exactly two values, you may use any truthy/falsey values in decision problems For instance, considering the hypothetical challenge "Determine if a number is non-divisi...
Every outcome must be possible For example, if the challenge requires a random number from 1-5, all 5 numbers must be possible. It does not need to be uniformly random (unless otherwise specified)...
Storing information in the platform specs We should disallow requiring the program to be run on some platform to store information for the challenge. An extreme example would be Nullary, in which ...
Free access must not be finite There has to be a time-unlimited, free way to run the language in question.
Languages with free online interpreters are free This includes all languages currently on tio.run, for example.
