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!

• Wheat Wizard: Can we take in a string representing the array?
• Wheat Wizard: Will there be objects other than numbers and other arrays in the arrays
• user58988: What kind of numbers? Compex int, compex float, float int, int , not negative?
• ceased to turn counterclockwis: FTR and in the name of proper mathematical thinking: the array [[2]] does not contain a two.
• ATaco: @KevinCruijssen Given the sheer number of answers that ignore the commented restriction (And the age of the challenge), I'd rather now state that answers needn't worry about numbers other than positive integers. I'll adjust the challenge description to specify this.

Answer 1

Python 2, (削除) 43 (削除ここまで) 40 bytes

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

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---

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)

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• Adnan: That's a neat way to check whether an element is a list.
• Wheat Wizard: This is why I don't like that consensus. It really ruins python golfing.
• Wheat Wizard: However since you are going by exit code you don't need the all, anything other than an error is truthy.

Answer 2

Prolog (SWI), (削除) 43 (削除ここまで) 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

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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

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Technically, this could just be

2=

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

• Erik the Outgolfer: A list containing 0 is falsy? Oh man.
• Erik the Outgolfer: I don't think the 2-byte version is valid, since in the comments ATaco said that a unique output pair is valid.
• Stewie Griffin: I believe 2= fails for empty matrices, or?
• DJMcMayhem: @stewiegriffin That seems like a strange edge case to need to handle, but conveniently it does work: Try it online!
• Stewie Griffin: Yes, 2=p works fine. The shorter version in the end, 2=, doesn't. Also, "the strange edge cases" are two of the test cases. :-)

Answer 6

Mathics, 28 bytes

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

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• Greg Martin: I think that the input {0} is allowed; that would result in a false positive.

Answer 7

Retina, (削除) 13 (削除ここまで) 10 bytes

Thanks to Kritixi Lithos for saving 3 bytes.

\W|\b2
^$

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• Kevin Cruijssen: Seems to fail for lists that contain -2.

Answer 8

05AB1E, 4 bytes

 ̃YQP

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Explanation

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

• Erik the Outgolfer: Why wouldn't 2 work instead of Y?
• Emigna: @EriktheOutgolfer: 2 works as well. I just like the fact that there are no numbers in it :)

Answer 9

JavaScript (ES6), (削除) 22 (削除ここまで) (削除) 19 (削除ここまで) (削除) 23 (削除ここまで) 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]))

• Arnauld: Nice one! I wonder if it could be shortened some more, but I doubt it.
• Shaggy: Thanks, @Arnauld; still haven't figured out a way to improve on it.

Answer 10

APL (Dyalog), 5 bytes

∧/2=∊

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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.

• Pavel: Outgolfed, but I still really want to know what is being done here.

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(:)[]

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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.

• zbw: How about just not.all(`elem`"2,[]")?
• Ørjan Johansen: @zbw That fails because of numbers like 22.

Answer 14

Python 2, 38 bytes

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

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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, (削除) 28 (削除ここまで) (削除) 23 (削除ここまで) 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.

• Nick M: x.flatten.uniq==[2]
• ymbirtt: @NickM - that won't work on test cases like [] or [[],[]]. [2,*x].flatten.uniq==[2] is slightly longer
• G B: x.flatten|[2]==[2] would be shorter.
• ymbirtt: @GB and x.flatten-[2]==[] is shorter still. Thanks for the tip!
• G B: And yet regex wins: codegolf.stackexchange.com/a/120781/18535 :-)

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]))

• Leaky Nun: You need to count f= because you referred to it.
• Arnauld: @LeakyNun Indeed. Fixed.

Answer 17

MATL, 4 bytes

2-a~

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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 !.

• BLT: pryr::f(!any(unlist(x)-2)) saves a couple of bytes.
• Giuseppe: this is the same length as all(unlist(x)==2) as well.
• Giuseppe: or you could also say any(unlist(x)-2) which returns a consistent TRUE if there is a non-2 value in the flattened array and a consistent FALSE if all the values are 2...
• JAD: @Giuseppe Not sure if TRUE counts as falsey though :/
• Giuseppe: well, there's still not a consensus on meta, but codegolf.meta.stackexchange.com/a/2192/67312

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)

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• user45941: -1 byte: int!=type(x)and

Answer 20

Jelly, 4 bytes

F=2Ạ

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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, (削除) 14 (削除ここまで) 11 bytes

^(\W|2\b)+$

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• Aaron: \W doesn't seem such a good criteria : 2.2 is a number that isn't 2, yet I suppose it would match
• user41805: @Aaron I have just asked the OP on whether the array can containing decimal numbers. If they state that floating-point numbers will be present in the array, I will change my submission.
• Aaron: Yeah, I see RosLup asked the same question yesterday and hasn't got an answer yet. I hope OP will come soon to clarify !

Answer 22

05AB1E, 4 bytes

2‚ ̃Ë

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Answer 23

JavaScript (ES6), (削除) 53 (削除ここまで) (削除) 50 (削除ここまで) 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]))

• Arnauld: f([]) and f([[],[]]) should be truthy
• Arjun: @Arnauld Is it correct now?
• Arnauld: I think so. :-)
• Shaggy: Think you can save a couple of bytes with !c instead of c=="".
• Arjun: @Arnauld Thanks for pointing that out. This challenge was actually posted as a CMC in the Nineteenth byte. That CMC did not have anything to say regarding [[],[]] etc kind of test cases. When the challenge got posted on the main site, I quickly added my solution (It even asked me CAPTCHA!) without looking at rules! Thanks once again! :)

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!

• ymbirtt: Curses! Nice work. What's this x*?!!~ construct?

Answer 26

Jelly, 3 bytes

2ṁ=

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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, (削除) 126 (削除ここまで) (削除) 55 (削除ここまで) 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.

• Jakob: I hate to nullify all your work on the list solution, but couldn't you coerce to a string and use the string solution?
• Kevin Cruijssen: @Jakob You mean in the explanation? I am using a regex String solution at the moment. I've just kept my original List answer and it's explanation, because the String solution is a port. Are you asking to just remove the List solution? Or add an explanation for the String solution?
• Jakob: I mean that as long as you have a list solution you might as well shorten it by using the string solution in it. Like boolean c(java.util.List l){return(l+"").matches("^(\\W|2\\b)+$");} would work, right? Just wanted to point that out in case you were planning to further golf the list solution.
• Jakob: Oh and you can lose 2 bytes by removing ^ and $ in the regex, since String.matches only tests against the whole string.
• Kevin Cruijssen: @Jakob Removed the List answer entirely, converted to Java 8, and removed the ^...$. Forgot about that, even though I've used it quite a lot of times in the past..

Answer 28

Thunno 2, 4 bytes

Ḟ2=p

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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), (削除) 13 (削除ここまで) 10 bytes

2#2&

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 is \[VectorLessEqual].