Determine if an Array contains something other than 2

Question

Take an array which consists of positive integers or arrays, output if it only contains 2s.

Output should be a truthy or falsey value (Sorry if this destroys answers)

Truthy Test Cases

[2]
[2,2]
[[2],[2,2],2]
[]
[[],[]]

Falsey Test Cases

[1]
[22]
[2,2,2,1]
[[1,2],2]

Standard Loopholes Are forbidden.

Default IO rules apply.

Code-golf, Fewest bytes wins!

Answer 1

Python 2, 43 40 bytes

f=lambda l:l>=[]and all(map(f,l))or l==2

Try it online!

---

At time of posting this answer, it was still allowed per this meta consensus to output via throwing an error / not throwing an error. Therefore this answer at 26 bytes was valid:

f=lambda l:l==2or map(f,l)

Try it online!

Answer 2

Prolog (SWI), 43 33 bytes

I smell... recursion.

Thanks to Emigna and Leaky Nun for saving 10 bytes!

Code

a([]).
a([X|T]):-(X=2;a(X)),a(T).

Try it online! or Verify all test cases!

Explanation:

For non-Prolog users, a list is formatted in the following way: [Head | Tail].

The Head is the first element of the list, and tail is the remaining list. Test it here!. An important case here is that the tail of a list with 1 element is equal to []. You can test that here.

% State that an empty array is truthy.
a([]).

% If the list is not empty (covered by the previous line), we need to check
% whether the Head is equal to 2 or whether the head is truthy.
% After that, we only need to check if the remaining list is truthy.
a([Head | Tail]) :- (Head = 2; a(Head)), a(Tail).

Answer 3

Jelly, 4 bytes

F;2E

Try it online!

How it works

F;2E
F    flatten
 ;2  append 2
   E all elements are equal

Answer 4

Octave, 13 bytes

@(x)~any(x-2)

Verify all test cases.

This is an anonymous function taking one input argument, x. It subtracts 2 from all elements, checks if there are any non-zero elements. It negates the output to get true for cases where all values are zero.

This works because x-2 works for matrices of all sizes, including the empty matrix, [].

x-2 would be sufficient if there couldn't be empty matrices in the input.

Answer 5

MATL, 3 bytes

2=p

Try it online!

Technically, this could just be

2=

Since an array containing any zero elements is falsy, but this seems cheap.

Answer 6

Mathics, 28 bytes

Select[Flatten@#,#!=2&]=={}&

Try it online!

Answer 7

Retina, 13 10 bytes

Thanks to Kritixi Lithos for saving 3 bytes.

\W|\b2

^$

Try it online!

Answer 8

05AB1E, 4 bytes

˜YQP

Try it online!

Explanation

˜      # flatten list
 YQ    # check each element for equality to 2
   P   # product of list

Answer 9

JavaScript (ES6), 22 19 23 22 bytes

a=>!/[^2,]|22/.test(a)

---

Test it

f=
a=>!/[^2,]|22/.test(a)
console.log(" "+f([2])+": "+JSON.stringify([2]))
console.log(" "+f([2,2])+": "+JSON.stringify([2,2]))
console.log(" "+f([[2],[2,2],2])+": "+JSON.stringify([[2],[2,2],2]))
console.log(" "+f([])+": "+JSON.stringify([]))
console.log(" "+f([[],[]])+": "+JSON.stringify([[],[]]))
console.log(f([1])+": "+JSON.stringify([1]))
console.log(f([22])+": "+JSON.stringify([22]))
console.log(f([2,2,2,1])+": "+JSON.stringify([2,2,2,1]))
console.log(f([[1,2],2])+": "+JSON.stringify([[1,2],2]))

Answer 10

APL (Dyalog), 5 bytes

∧/2=∊

Try it online!

Explanation

∧/                         Only
  2=                       2s are equal to
    ∊                      any of the elements in the enlisted form of the right argument

Answer 11

Mathematica, 15 bytes

FreeQ[x_/;x!=2]

It also works in Mathics. Try it online!

Answer 12

Mathematica, 24 bytes

Cases[t=Flatten@#,2]==t&

Pure function returning True or False. After Flattening the nested array and calling it t, Cases[t,2] returns the list of elements that match the "pattern" 2, and ==t checks whether that's the whole list.

Mathematica, 29 bytes

(#//.{2->{},{{}..}->{}})=={}&

Not as short, but more fun. Starting from the input #, two replacement rules are applied until the result stops changing (//.): first, all 2s are replaced by {}s; and then any list whose entries are all empty sets ({{}..}) are replaced (repeatedly) by empty sets. If the rest is an empty set (=={}), we win.

Answer 13

Haskell, 36 bytes

An anonymous function, takes a String and returns a Bool.

Use as (all((==2).fst).(reads=<<).scanr(:)[]) "[2,2,2,1]"

all((==2).fst).(reads=<<).scanr(:)[]

Try it online!

How it works

• Haskell doesn't have builtin mixed-type lists, so we take a string as argument.
• scanr(:)[] generates a list of all suffixes of the string.
• (reads=<<) tries to parse a number at the beginning of each suffix, combining the successes into a list of tuples (n,restOfString).
• all((==2).fst) checks if all the parsed numbers are 2.

Answer 14

Python 2, 38 bytes

lambda l:l.strip('[],2')==l*('22'in l)

Try it online!

Takes in a string without spaces, outputs a bool.

Checks if removing all the characters '[],2' of l gives the empty string. Also checks that 22 is not a substring -- if it is, the input l is used in place of the empty string to compare to the result of removal, and that always fails.

Answer 15

Ruby, 28 23 22 bytes - 5 bytes saved by G B

->x{x.flatten-[2]==[]}

Despite "flatten" being really long, it's still shorter than regex based solutions or recursive stuff that has to rescue errors in the base case. Ruby's built-in conflation of sets and arrays, however, is amazingly useful sometimes.

Answer 16

JavaScript (ES6), 26 bytes

f=a=>a.map?a.every(f):a==2

Test cases

f=a=>a.map?a.every(f):a==2

console.log(f([2]))
console.log(f([2,2]))
console.log(f([[2],[2,2],2]))
console.log(f([]))
console.log(f([[],[]]))

console.log(f([1]))
console.log(f([22]))
console.log(f([2,2,2,1]))
console.log(f([[1,2],2]))

Answer 17

MATL, 4 bytes

2-a~

Try it online!

Breakdown:

           % Implicit input
2-         % Push 2 to the stack, and subtract from input
  a        % Any non-zero elements?
    ~      % Negate to get true for cases where all elements are zero.

---

Well, outgolfed. But I'm keeping this, since I'm quite happy I managed this all on my own (even though the task is super simple).

Answer 18

R, 28 bytes

function(x)!any(unlist(x)-2)

unlist(x) turns a (nested) list into a vector. Then 2 is subtracted from that vector. any converts (with a warning) numeric to logical and checks if there are any TRUEs. This is inverted with ! and output.

This works with nested lists because unlist by default works recursively to unlist all list entries of the initial list.

This also works with empty lists, because unlist(list()) becomes numeric(), an empty numerical vector. Coercion by any makes it logical(), which is interpreted as FALSE by any, and then reversed to TRUE by !.

Answer 19

Python 3, 55 bytes

No cheating. Uses nested list as input.

f=lambda a:all(type(x)!=int and f(x)for x in a if x!=2)

Try it online!

Answer 20

Jelly, 4 bytes

F=2Ạ

Try it online!

Slightly different than Leaky's algorithm.

Explanation:

F=2Ạ
F    Flatten
 =2  Check if equal to 2 (vectorizes)
   Ạ Check if there isn't any falsey value

Answer 21

Retina, 14 11 bytes

^(\W|2\b)+$

Try it online!

Answer 22

05AB1E, 4 bytes

2‚˜Ë

Try it online!

Answer 23

JavaScript (ES6), 53 50 48 bytes

_=>(_+"").split`,`.map(c=>!c?2:c).every(c=>c==2)

Saved 5 bytes, thanks to @Shaggy!

Test Cases :

let f =

_=>(_+"").split`,`.map(c=>!c?2:c).every(c=>c==2)

console.log(f([2]))
console.log(f([2,2]))
console.log(f([[2],[2,2],2]))
console.log(f([]))
console.log(f([[],[]]))

console.log(f([1]))
console.log(f([22]))
console.log(f([2,2,2,1]))
console.log(f([[1,2],2]))

Answer 24

05AB1E, 7 bytes

˜DOsg·Q

Try it online! or Try All Tests!

˜D      # Flatten and duplicate
  O     # Sum one copy
   sg·  # Get double the length of the other copy
      Q # Check if they are equal

Answer 25

Ruby, 21 bytes

->x{x*?!!~/[^2!]|22/}

Using a regex is actually shorter, because joining an array also flattens it.

How it works

->x{
    x*?!                -> Join array using an exclamation mark
        !~              -> String does not contain
          /[^2!]        -> characters different from '2' or '!'
                |       -> or
                 22/    -> '2' repeated at least twice
                    }

Try it online!

Answer 26

Jelly, 3 bytes

2ṁ⁼

Try it online!

How it works

2ṁ⁼  Main link. Argument: A (array)

2ṁ   Mold 2 like A. This replaces each number in A by 2.
  ⁼  Test if the result is equal to A.

Answer 27

Java 8, 126 55 27 bytes

s->s.matches("(\\W|2\\b)+")

Port of @KritixiLithos's amazing Retina answer, excluding the ^...$, since String#matches always matches the entire String and adds the ^...$ implicitly.

-2 bytes thanks to @Jakob for reminding me of ^...$ isn't necessary for String#matches.

Try it here.

Answer 28

Thunno 2, 4 bytes

Ḟ2=p

Try it online!

Explanation

Ḟ2=p  # Implicit input
Ḟ     # Flatten the list
 2=   # Check for == 2
   p  # All are true?
      # Implicit output

Answer 29

Nekomata + -e, 3 bytes

2-ž

Attempt This Online!

2-      Minus 2
  ž     Check if all elements are zero

Answer 30

Wolfram Language (Mathematica), 13 10 bytes

2#2&

Try it online!

 is \[VectorLessEqual].