Find the newest element

Question

Given a finite sequence, each of its elements has a first occurrence. Your task is to output the symbol whose first occurrence is last in the sequence. If the elements represent posts and reposts of memes over time, we want to see the newest meme!

Example

Input: A-BANAL-BANANA-LAB

A first occurs at position 0 (it occurs at positions 0,3,5,9,11,13,16)
- first occurs at position 1
B first occurs at position 2
N first occurs at position 4
L first occurs at position 6

max(0,1,2,4,6) = 6, so

Output: L

Input/output format

You can assume that the input sequence is nonempty, and is either a string of printable non-whitespace ASCII characters or a list of nonnegative integers in any reasonable form (your choice). You will output one element of the input sequence.

And of course

This is code-golf, so the shortest solution in each language wins.

I downvoted because the trivial solution to this is almost the same task as removing duplicates from a list, which is already a task on this site. (Not to say that's the only way, but it's the trivial and usually shortest way.) — noodle person, Commented Jul 28 at 21:05

Arnauld · Accepted Answer · 2024-07-28 10:22:56Z

14

JavaScript (ES6), 24 bytes

Note that it works because Set is guaranteed to maintain the insertion order of its elements.

s=>[...new Set(s)].pop()

Try it online!

Commented

s =>         // s = input string
[...         // turn into an array ...
  new Set(s) //   ... the set generated from s
             //       e.g. "BANANA" -> Set {'B','A','N'}
]            //
.pop()       // return the last element

edited Jul 28 at 10:22

answered Jul 28 at 10:15

Arnauld

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xnor · Accepted Answer · 2024-07-28 10:26:53Z

14

Python, 26 bytes

lambda s:max(s,key=s.find)

Try it online!

Takes a string as input. s.find(c) outputs the first index in s where the character c appears, so using s.find as a key for max returns the character whose first occurrence in s comes latest.

answered Jul 28 at 10:26

xnor

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Adám · Accepted Answer · 2024-07-28 09:56:11Z

7

APL (Dyalog Vision), 2 bytes

⊇∪

⊇ last ∪ unique

answered Jul 28 at 9:56

Adám

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doubleunary · Accepted Answer · 2024-07-28 15:13:02Z

6

Google Sheets, 36 bytes

=chooserows(unique(tocol(A:A,1)),-1)

64 bytes:

=+let(β,A:A,γ,sort(row(β)),sort(β,1=countifs(β,β,γ,"<="&γ),,γ,))

...or:

=chooserows(filter(A:A,1=map(A:A,lambda(β,countif(A1:β,β)))),-1)

Put the array in column A1:A and the formula in cell B1.

screenshot

edited Jul 28 at 15:13

answered Jul 28 at 10:22

doubleunary

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1

\$\begingroup\$ For 35 you could be explicit about your range of elements using =INDEX(A:A,MAX(XMATCH(A1:A17,A:A))), and if we assume there is not going to be a pretty much endless string, then for 27 =CHOOSEROWS(UNIQUE(A:A),-2) \$\endgroup\$
– JvdV
Commented Jul 28 at 18:06
\$\begingroup\$ @JvdV yes... but I don't want to hardcode the range, nor assume that the data never fills the whole sheet down to the last row. \$\endgroup\$
– doubleunary
Commented Jul 28 at 19:14
\$\begingroup\$ Yeah it feels better your way for sure. 👍 \$\endgroup\$
– JvdV
Commented Jul 29 at 6:05

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emanresu A · Accepted Answer · 2024-07-28 09:50:24Z

5

Vyxal, 2 bytes

Ut

Try it Online!

 t # last
U  # unique element

answered Jul 28 at 9:50

emanresu A

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1

\$\begingroup\$ Answered when the question was on 1 view :D \$\endgroup\$
– aeh5040
Commented Jul 28 at 9:52

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ovs · Accepted Answer · 2024-07-28 10:16:50Z

5

C (tcc), 57 bytes

k;f(char*s){for(int v[128]={};*s;s++)k=v[*s]++?k:*s;k=k;}

Try it online!

answered Jul 28 at 10:16

ovs

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chunes · Accepted Answer · 2024-07-28 10:58:00Z

5

Uiua ^SBCS, 3 bytes

⊢⇌◴

Try it!

⊢⇌◴⁡‎‎⁪⁡⁪⁠⁪⁣⁪‏‏⁡⁠⁡‌⁢‎‎⁪⁡⁪⁠⁪⁡⁪‏⁠‎⁪⁡⁪⁠⁪⁢⁪‏‏⁡⁠⁡‌
  ◴  # ‎⁡deduplicate
⊢⇌   # ‎⁢last

answered Jul 28 at 10:58

chunes

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Andrew Bayly · Accepted Answer · 2024-07-28 20:36:47Z

5

JavaScript (Node.js), 58 45 38 bytes

s=>[...s].map(a=e=>a[e]?0:a[e]=b=e)&&b

Try it online!

edited Jul 28 at 20:36

answered Jul 28 at 10:41

Andrew Bayly

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2

\$\begingroup\$ You can use the callback function as your object (38 bytes). \$\endgroup\$
– Arnauld
Commented Jul 28 at 20:20

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user314159 · Accepted Answer · 2024-07-28 12:18:23Z

4

PowerShell, 27 bytes

($args|% *ay|select -u)[-1]

Try it online!

Ungolfed:

($args | ForEach-Object -Member ToCharArray | Select-Object -Unique)[-1]

Converts the input string to a char array by calling the string's member "ToCharArray", then selecting only unique characters, and picking the last char.

answered Jul 28 at 12:18

user314159

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akamayu · Accepted Answer · 2024-07-28 13:34:36Z

4

K (ngn/k), 4 bytes

The last *| unique ? element.

*|?:

Try it online!

answered Jul 28 at 13:34

akamayu

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Unrelated String · Accepted Answer · 2024-07-28 15:48:47Z

4

Brachylog, 2 bytes

dt

Try it online!

Nekomata, 2 bytes

ul

Attempt This Online!

More ~~boring~~ banal nub-tail solutions.

edited Jul 28 at 15:48

answered Jul 28 at 15:41

Unrelated String

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JvdV · Accepted Answer · 2024-07-28 18:14:55Z

4

Excel ms365, 22 bytes

=@TAKE(UNIQUE(A:A),-2)

answered Jul 28 at 18:14

JvdV

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SamR · Accepted Answer · 2024-07-29 09:40:43Z

4

R 4.4.0, 21 bytes

\(x)tail(unique(x),1)

Attempt This Online!

Takes as input a vector of utf-8 code points. Really not fancy, basically just a port of Arnauld's JS answer (and several others as well).

Although R's base::unique() is not documented to guarantee insertion order, the implementation will do so. It loops through every value of a vector in order and checks if it has been seen before, appending to a new vector any which have not been. While I suppose it's possible this implementation will not remain forever, it will work in R 4.4.0. Also, the git blame indicates this has been part of the R source for 26 years - I don't imagine it would be changed lightly in future versions.

edited Jul 29 at 9:40

answered Jul 29 at 9:02

SamR

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2

\$\begingroup\$ This is 22 bytes and guaranteed to work \$\endgroup\$
– Giuseppe
Commented Jul 31 at 20:00
\$\begingroup\$ @Giuseppe I think this is also guaranteed to work isn't it? At least in this and previous versions of R and until they change they R source. Though your approach is really nice. \$\endgroup\$
– SamR
Commented Aug 1 at 7:36
\$\begingroup\$ Yeah, I think yours will work, but I offer the 22 bytes as an alternative you could include in your answer as well; I can post it as my own as well if you don't want to add it here. \$\endgroup\$
– Giuseppe
Commented Aug 1 at 15:29
\$\begingroup\$ @Giuseppe it's really nice but I can't take credit for it - it's a pretty different approach to mine I think (and more elegant as a solution) so feel free to post it as your own. \$\endgroup\$
– SamR
Commented Aug 1 at 15:41

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Xcali · Accepted Answer · 2024-07-29 15:29:17Z

4

Perl 5 `-MList::Util=uniq -pF`, 15 bytes

$_=(uniq@F)[-1]

Try it online!

answered Jul 29 at 15:29

Xcali

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Dom Hastings · Accepted Answer · 2024-07-29 20:18:39Z

4

Perl 5 + `-0513pF`, 22 bytes

-3 bytes thanks to @Xcali!

$\=chop;/${\}/&&redo}{

Try it online!

edited Jul 29 at 20:18

answered Jul 28 at 20:44

Dom Hastings

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\$\begingroup\$ Slightly shorter, and doesn't use an additional module like my posted answer: Try it online! \$\endgroup\$
– Xcali
Commented Jul 29 at 18:15
\$\begingroup\$ Oh amazing work, thank you! Your solution is super clever too, nice - +1! \$\endgroup\$
– Dom Hastings
Commented Jul 29 at 20:19

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matteo_c · Accepted Answer · 2024-07-28 11:51:56Z

3

Python 3, 36 bytes

f=lambda x:list({a:0for a in x})[-1]

Try it online!

answered Jul 28 at 11:51

matteo_c

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7

\$\begingroup\$ No need for f= here since it's not recursive. Save three using list splatting (list(...) -> [*...]), and another forming the dictionary from the sequence of elements paired with themselves ({a:0for a in x} -> {a:a for a in x} -> dict(zip(x,x))) - TIO \$\endgroup\$
– Jonathan Allan
Commented Jul 28 at 15:35

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Neil · Accepted Answer · 2024-07-28 13:41:16Z

3

Retina 0.8.2, 8 bytes

D`.
!`.$

Try it online! Takes input as a string. Explanation:

D`.

Keep only the first of each duplicated character.

!`.$

Keep the last remaining character.

answered Jul 28 at 13:41

Neil

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Wheat Wizard · Accepted Answer · 2024-07-28 13:48:33Z

Haskell + hgl, 5 bytes

gj<nb

Attempt This Online!

Explanation

Nub and get the last element.

Parser, 16 bytes

gk$(h'>~nxy)<*h'

Attempt This Online!

Since this challenge is pretty trivial to do using the normal library, I thought I'd try a parser only version.

Explanation

We create a parser which matches all characters that are not preceded by a copy of themselves.

h': Get some prefix.
nxy: Parse a character not present in the prefix.
h': Parse some number of characters.
gk: Get the highest priority parse.

The priority here is determined by h' with the longest parse having the highest priority.

Reflection

The normal version of this can't reasonably be improved. Improvements can be made based on the parser version, but it will never be on par with the other version for this task.

gk gets the first complete parse. There should be a parser consumer which just gets the first parse. This would make the <*h' unnecessary and save a bunch of bytes here.
(h'*>) and (<*h') could probably use shortcuts. I use the former here. This would be superseded here by the above improvement, but would probably be useful in the long term.
I use (h'>~) here and here and the very similar (h_>~) here and here (the latter is now superseded by (h_<~)). There is some sort of common pattern here that seems likely to continue to pop up. I should have shortcuts for a bunch of these variants.

There are also some things I noticed which wouldn't save bytes here but should be done.

nχ and nxy could use versions precomposed with my and so.

Neil · Accepted Answer · 2024-07-28 13:55:54Z

3

Charcoal, 8 bytes

§θ⌈Ｅθ⌕θι

Try it online! Link is to verbose version of code. Explanation: Turned out to be a port of @xnor's answer.

    θ      Input string
   Ｅ       Map over characters
     ⌕     Index of
       ι   Current character
      θ    In input string
  ⌈        Take the maximum
§          Index into
 θ         Input string
           Implicitly print

answered Jul 28 at 13:55

Neil

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Suever · Accepted Answer · 2024-07-28 17:35:44Z

3

MATL, 3 bytes

uJ)

Try it at matl.io

Explanation

     % Implicitly fetch input
u    % Find unique elements 
J)   % Get the last element
     % Print implicitly

answered Jul 28 at 17:35

Suever

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bb94 · Accepted Answer · 2024-07-28 20:50:10Z

3

Setanta, 65 bytes

gniomh(s){le i idir(fad@s-1,-1)ma i<=aimsigh@s(s[i]) toradh s[i]}

try-setanta.ie link

answered Jul 28 at 20:50

bb94

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Aiden Chow · Accepted Answer · 2024-08-01 02:04:54Z

3

Desmos, 26 bytes

f(L)=U[U.count]
U=L.unique

Try It On Desmos!

Try It On Desmos! - Prettified

answered Aug 1 at 2:04

Aiden Chow

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Giuseppe · Accepted Answer · 2024-08-01 16:02:36Z

3

R, 22 bytes

\(x)x[max(match(x,x))]

Attempt This Online!

A 1 byte longer solution than SamR's, but not implementation-dependent.

answered Aug 1 at 16:02

Giuseppe

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RubenVerg · Accepted Answer · 2024-07-28 10:13:35Z

2

Vyxal 3, 2 bytes

ut

try

t last of
u unique elements
of the input

answered Jul 28 at 10:13

RubenVerg

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chunes · Accepted Answer · 2024-07-28 11:23:37Z

2

Arturo, 15 bytes

$=>[last∪<=&]

Explanation

$=>[]  ; a function where input is assigned to &
last   ; last element of...
∪      ; union between...
<=&    ; input and itself

This is two bytes shorter than last unique&, but requires the latest version for ∪, hence it won't work online yet.

answered Jul 28 at 11:23

chunes

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Shaggy · Accepted Answer · 2024-07-28 18:39:07Z

2

Japt v2.0a0 `-h`, 1 byte

â

Try it

answered Jul 28 at 18:39

Shaggy

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David Conrad · Accepted Answer · 2024-07-29 14:35:39Z

2

Java 21, 35 bytes

s->new LinkedHashSet<>().getLast()

A Function<List<T>, T> for some T. Requires Java 21 which does not seem to be available yet on Try It Online or Attempt This Online.

answered Jul 29 at 14:35

David Conrad

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Fmbalbuena · Accepted Answer · 2024-08-04 21:29:50Z

2

K (ngn/k), 14 bytes

{x@|/x?!1+|/x}

Try it online!

Explanation

{x@|/x?!1+|/x}
{            } function
       !1+     enumerate from 0 to
          |/x  the max element of the list
     x?        find every element index
   |/          pick the newest element index
 x@            index

answered Aug 4 at 21:29

Fmbalbuena

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\$\begingroup\$ 6? \$\endgroup\$
– ASCII-only
Commented Aug 5 at 12:18

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Швеев Алексей · Accepted Answer · 2024-08-05 00:26:27Z

2

Stack control, 3 characters:

∪⟄⟸

Explanation:

∪ - union (removes all duplicates)
⟄ - pop last element from array
⟸ - hard shift (removes original array)

edited Aug 5 at 0:26

answered Aug 3 at 20:50

Швеев Алексей

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south · Accepted Answer · 2024-08-05 02:30:06Z

2

J, 5 bytes

_1{~.

Nothing new.

Attempt This Online!

_1{~.
   ~.   NB. nub removes duplicates
_1{     NB. take the last item

answered Aug 5 at 2:30

south

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Find the newest element

Example

Input/output format

And of course

39 Answers 39

JavaScript (ES6), 24 bytes

Commented

Python, 26 bytes

APL (Dyalog Vision), 2 bytes

Google Sheets, 36 bytes

Vyxal, 2 bytes

C (tcc), 57 bytes

Uiua SBCS, 3 bytes

JavaScript (Node.js), 58 45 38 bytes

PowerShell, 27 bytes

K (ngn/k), 4 bytes

Brachylog, 2 bytes

Nekomata, 2 bytes

Excel ms365, 22 bytes

R 4.4.0, 21 bytes

Perl 5 -MList::Util=uniq -pF, 15 bytes

Perl 5 + -0513pF, 22 bytes

Python 3, 36 bytes

Retina 0.8.2, 8 bytes

Haskell + hgl, 5 bytes

Explanation

Parser, 16 bytes

Explanation

Reflection

Charcoal, 8 bytes

MATL, 3 bytes

Setanta, 65 bytes

Desmos, 26 bytes

R, 22 bytes

Vyxal 3, 2 bytes

Arturo, 15 bytes

Explanation

Japt v2.0a0 -h, 1 byte

Java 21, 35 bytes

K (ngn/k), 14 bytes

Explanation

Stack control, 3 characters:

Explanation:

J, 5 bytes

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Perl 5 + `-0513pF`, 22 bytes

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