# Implement Lazy Drop Sort

This challenge already describes dropsort. However, I'm kinda lazy and I really only need my array to be a bit more sorted than before, it doesn't need to be sorted all the way.

In Drop Sort, we drop every element less than any element before it. In Lazy Drop Sort, we drop every element less than the one strictly preceding it.

Here's an example. Consider the following array:

8 6 9 9 7 2 3 8 1 3


Let's mark every element less than the one before it.

8 6 9 9 7 2 3 8 1 3
^     ^ ^     ^


Notice how neither 3 was marked, nor the last 8. They are all larger than the single element to the left of them.

Completing the algorithm, removing the marked elements, we get:

8 9 9 3 8 3


That basically looks more sorted. Kinda. I'm lazy.

Input is an array of at least 1 positive integer between 1 and 9, so you can take a string of digits as well.

This is , fewest bytes wins!

1
1

1 2 3
1 2 3

5 3 1
5

1 2 3 2 1
1 2 3

1 1 1 9 9 9 1 1 1 9 9 9 1 1 1
1 1 1 9 9 9 1 1 9 9 9 1 1

9 9
9 9

5 2 4 2 3
5 4 3

• Can it be a function or it must be a complete program? Mar 23, 2018 at 0:40
• @rafa11111 Either is fine Mar 23, 2018 at 0:44
• In the case it is a function, can the input array be hardcoded in the main program? And can the length of the array be passed as input to the function? Mar 23, 2018 at 1:56
• @rafa11111 The input can't be hardcoded in the function itself. It doens't matter how the function gets this input in your test program. You can take an array length only if you're using C/C++ or another language where that's the only way to determine an array's length. Mar 23, 2018 at 2:08

# JavaScript (ES6), 28 25 bytes

Saved 3 bytes thanks to @Shaggy

a=>a.filter(n=>~-a<(a=n))


Try it online!

• n=>p<=n would have looked awesome ;-) Mar 22, 2018 at 15:49
• @ETHproductions For +4 bytes, (n=p)=>p<=(p=n) works fine ;) Mar 22, 2018 at 15:52
• this answer is blowing my mind, why doesn't this explode when trying to access p for the first time, when it's not defined yet? Mar 22, 2018 at 16:26
• @Shaggy That looks safe. Will update when I'm back in front of a computer. Thanks! Mar 22, 2018 at 17:44
• @Pavel a is initially set to the input array and a-1 would result in NaN (unless it contains a single integer, in which case it is coerced to this integer). Mar 22, 2018 at 19:51

# Husk, 4 bytes

m←ġ<


Try it online!

### Explanation

m←ġ<
ġ<    Group the numbers into decreasing sequences
m←      Keep the first element of each sequence


# R, 27 bytes

(l=scan())[c(T,diff(l)>=0)]


Try it online!

# MATL, 9 8 bytes

Saved one byte thanks to Giuseppe.

0yd0<h~)


Try it online!

Explanation:

0                 % Push a zero
y                % Implicitly grab the input and duplicate it.
% Stack: [8 6 9 9 7 2 3 8 1 3], 0, [8 6 9 9 7 2 3 8 1 3]
d               % The difference between each number of the last element:
% Stack: [8 6 9 9 7 2 3 8 1 3], 0, [-2, 3, 0, -2, -5, 1, 5, -7, 2]
0<             % Which are negative?
% Stack: [8 6 9 9 7 2 3 8 1 3], 0, [1 0 0 1 1 0 0 1 0]
h            % Concatenate. Stack: [8 6 9 9 7 2 3 8 1 3], [0 1 0 0 1 1 0 0 1 0]
~           % Negate. Stack: [8 6 9 9 7 2 3 8 1 3], [1 0 1 1 0 0 1 1 0 1]
)          % Index. Stack: [8 9 9 3 8 3]


# Perl 5.10.0 + -nl, 16 bytes

$f>$_||say;$f=$_


Try it online!

• Translation to Perl 6 perl6 -ne '$/>$_||.say;$/=$_' Mar 22, 2018 at 22:46
• @Brad perl6 is a different language (it's not even backwards-compatible) Post it! Mar 23, 2018 at 5:44
• I wrote one that was more idiomatic Perl 6, but it was longer. Also one of the reasons I post here is to show off the language, and to explain it. Posting that translation does nothing but show that it is a slightly more verbose version of Perl. Basically it satisfies none of the reasons why I post on this site. Mar 24, 2018 at 15:58

f s=[b|(a,b)<-zip(0:s)s,a<=b]


just a simple list comprehension.

# Japt, 8 7 bytes

Saved 1 byte thanks to @Oliver

k@>(U=X


Test it online!

Alternatives:

f@T§(T=X
k@ä>0 gY
i0 ò> mÅ c

• Came up with the exact same solution :) Mar 22, 2018 at 17:39

# Stax, 5 bytes

âÿ╠╦░


Run and debug this online

Unpacking, ungolfing, and commenting the code, we get this.

Z   Push a zero under the input
f   Use the rest of the program as a filter on the input.  Output passing elements.
>   Current element is greater than previous?
_~  Push current element to the input stack; when the main stack is empty, pops fall back to this
!   Logical not; applies to the result of the greater-than


Run this one

The ordering of the instructions is awkward but there's a reason for it. Stax source code packing doesn't always yield the same size output for the same size input. Basically, you have a chance to save a byte if the last character of source has a lower character code. Well, ! has one of the lowest codes you can get for a printable character. (33 specifically) Many 6 byte ASCII stax programs can't pack any smaller. But if they end with a !, then they can. So the reason for this particular ordering of instructions is to ensure that the logical not ends up at the end of the program.

# J, 12 Bytes

#~1,2&(<:/\)


### Explanation:

#~1,2&(<:/\)    | Whole function, executed as a hook
<:/      | Distribute <: (greater or equal) over an array
2&(   \)    | Apply to each sub array of length 2
1,            | Append a 1 to the front
#~              | Choose elements from the original array


### Examples:

    2&(<:/\) 8 6 9 9 7 2 3 8 1 3
0 1 1 0 0 1 1 0 1
1,2&(<:/\) 8 6 9 9 7 2 3 8 1 3
1 0 1 1 0 0 1 1 0 1
(1 0 1 1 0 0 1 1 0 1) # 8 6 9 9 7 2 3 8 1 3
8 9 9 3 8 3
f =: #~1,2&(<:/\)
f 8 6 9 9 7 2 3 8 1 3
8 9 9 3 8 3


Try it online!

# Jelly, 6 bytes

>Ɲ0;¬×


I/O is on strings.

Try it online!

• I'm curious, why operate on strings and not arrays? I was told Jelly is bad at strings. Mar 22, 2018 at 19:01
• It is. × shouldn't work for character repetition, but it does. Mar 22, 2018 at 19:07

# Java 8, 6655 48 bytes

l->{for(int i=0;;)if(i>(i=l.next()))l.remove();}


-11 bytes after a tip from @OlivierGrégoire.
-7 more bytes thanks to @OlivierGrégoire.

Explanation:

Try it online.

l->{                     // Method with Integer-ListIterator parameter and no return-type
for(int i=0;;)         //  Loop over all items
if(i>(i=l.next()))   //   If the current item is larger than the next
l.remove();}       //    Remove this next item

• Why does everyone start using ~0 when it's basically -1. Personally I would choose the more intuitive solution if the byte-count is the same length (except for while(...) vs for(;...;), in which case I prefer the for. Thanks for another -7 bytes, though. :) Mar 22, 2018 at 18:21
• It's because I'm bad with 2-complement... I'm so bad I wanted to mean Integer.MIN_VALUE (which is then 1<<31, I guess...) ;-) Mar 22, 2018 at 18:32

# Octave, 21 bytes

@(x)x(~[0,diff(x)<0])


Try it online!

Explanation:

Take a vector x as input, and create a vector [0, diff(x)<0], where diff(x) is a vector with the difference between all adjacent elements. Keep only those that are negative by comparing it to zero, giving us a list of all the elements we want to drop.

We then select the elements from the input vector that we want to keep.

# V, 25 bytes

òjälá k$yl+@"òç-/d ç /dw  Try it online! Hexdump: 00000000: f26a e46c e120 6b24 796c 2b40 2218 f2e7 .j.l. k$yl+@"...
00000010: 2d2f 640a e720 2f64 77                   -/d.. /dw


Worst language for the job. But I did it for a dare.

• Side note: ojalá is Spanish for hopefully. Mar 22, 2018 at 21:45
• @dennis That's cool. What is k$yl+@"òç-/d Spanish for? Mar 22, 2018 at 21:57 • k$yl+@"òç-/d might be liberally translated as Ouch, who the hell left that cupboard door open? Mar 22, 2018 at 22:32

# Triangularity, 71 bytes

.....).....
....IEL....
...)rFD)...
..2+)IE)w..
.+h)2_stDO.
={M)IEm}...


Try it online!

## How it works?

)IEL)rFD)2+)IE)w+h)2_stDO={M)IEm} – Full program.
)IE                               – Get the 0th input I and evaluate it.
L)r                            – And push the range [0 ... length of I).
F                   {       – Filter the integers in this range which satisfy:
D)2+)IE)w+h)2_stDO=        – This condition. Runs each element E on a separate
stack and discard those that don't meet the criteria.
D)2+                       – Duplicate and add 2 to the second copy.
)IE                    – Retrieve I again.
)                   – Push a 0 onto the stack.
w                  – Wrap the 0 in a list. [0]
+                 – Prepend it to I.
h                – Head. Trim the elements after index E+2.
)2_             – Literal -2.
st           – Tail.
DO=        – Check whether the result is invariant over sorting.
M)IEm} – Last part: indexing into the input.
M    } – For each index that satisfies the conditions:
)IEm  – Retrieve the element of I at that position.

• Out of curiosity (since you are the creator of Triangularity): why not do something similar as Hexagony / Cubically, where a piece of code is automatically filled with the no-op dots? So this program would be )IEL)rFD)2+)IE)w+h)2_stDO={M)IEm} which would expand to your current answer? Mar 22, 2018 at 16:20
• @KevinCruijssen Because I was actually planning to make Triangularity a 2D esolang, but I gave up on the idea so I just sticked to my previous template. I think I'll make some major changes soon, when I release Triangularity v2. (Also it's kinda fun to golf in it in its current form, because a simple 1-byte save inline might instead save you 20 :D... It also applies retroactively when fixing stuff though :C) Mar 22, 2018 at 16:25
• Well, even if you do plan on releasing it as a 2D esolang, my comment still stands (somewhat). )IEL)rFD)2+)IE)w+h)2_stDO={M)IEm} would be your code, it would expand to your current template, and then do the 2D commands on that expanded template. EDIT: .....).....\n....IEL....\n...)rFD)...\n..2+)IE)w..\n.+h)2_stDO.\n={M)IEm}... and .....).........IEL.......)rFD).....2+)IE)w...+h)2_stDO.={M)IEm}... and )IEL)rFD)2+)IE)w+h)2_stDO={M)IEm} would all three be the exact same program. Mar 22, 2018 at 16:28

# Python, 40 bytes

f=lambda h,*t:t and h+f(*t)[h>t[0]:]or h


Try it online!

Input as tuple of characters.

# Python 3, 41 bytes

p=''
for x in input():x<p or print(x);p=x


Try it online!

String input.

# Python 2, 41 bytes

for x in input():
if x>=id:print x
id=x


Try it online!

String input, just because strings are greater than id but numbers are smaller.

# Wolfram Language (Mathematica), 33 bytes

Pick[#,Arg[#-{0}~Join~Most@#],0]&


Try it online!

## How it works

The code # - {0}~Join~Most@# turns an array {a,b,c,d,e,f} into {a,b-a,c-b,d-c,e-d,f-e}. Applying Arg to this sets negative numbers to Pi and nonnegative numbers to 0.

Pick[#, ..., 0]& picks out the entries of # where ... has a 0: in our case, exactly the elements that yield a nonnegative number when you subtract the previous element. In other words, these are exactly the entries we want to keep when lazydropsorting.

# Wonder, 27 bytes

-> ':1.!> 'sS#<=.cns2.++[0]


Usage example:

(-> ':1.!> 'sS#<=.cns2.++[0])[8 6 9 9 7 2 3 8 1 3]


# Explanation

Ungolfed version:

(map get 1).(fltr sS <=).(cns 2).(++ [0])


Prepend 0, get list of consecutive pairs, keep list items where first number <= second number, get second number of each pair.

# Wolfram Language (Mathematica), 20 bytes

#&@@@Split[#,##>0&]&
(* or *)
Max/@Split[#,##>0&]&


Try it online!

### Explanation

Input = {8, 6, 9, 9, 7, 2, 3, 8, 1, 3}

Split[#,##>0&]


Group consecutive elements that are strictly decresasing: {{8, 6}, {9}, {9, 7, 2}, {3}, {8, 1}, {3}}

#&@@@


Take the first element of each: {8, 9, 9, 3, 8, 3}

• ##>0 is fancy and everything, but it doesn't really save anything over #>#2 here ;) (which would make your program work with arbitrary integers, not that that's required though). Mar 25, 2018 at 21:49

# Python 2, 524645 42 bytes

lambda a:[v for v,w in zip(a,[1]+a)if v/w]


Try it online!

Saved:

• -3 bytes, thanks to Rod

# SWI-Prolog, 44 bytes

[A,B|C]-[A|E]:-B<A,[B|C]-[B|E];[B|C]-E. L-L.


Usage: Call "List-X" where List is a bracket-enclosed, comma-separated list e.g. [1,4,5,1,11,6,7].

• Welcome to the site! :) Mar 28, 2018 at 12:56

# APL+WIN, 14 bytes

Prompts for screen input of a vector of integers.

(1,1>2-/v)/v←⎕


# 05AB1E, 6 bytes

ĆÁü›_Ï


Try it online!

Explanation

Ć        # append the head of the list
Á       # rotate right
ü›     # apply pair-wise greater-than
_    # logical negate each
Ï   # keep elements of input that are true in this list


# Kotlin, 39 bytes

a->a.filterIndexed{i,v->i<1||v>=a[i-1]}


Try it online!

Filter items that are either the first item (index==0, or even shorter index<1) OR the current Value is greater than or equal to the previous item (a[i-1]).

# APL (Dyalog Unicode), 11 bytes

{⍵⌿⍨1⍪2≤⌿⍵}


Try it online!

This is actually pretty similar to Graham's answer, but in Dyalog, and independently developed. Also, more symmetric.

# K4, 10 bytes

Solution:

x_/|&<':x:


Example:

q)k)x_/|&<':x:8 6 9 9 7 2 3 8 1 3
8 9 9 3 8 3


Explanation:

Find indices where element is less than preceding, remove these indices from the input

x_/|&<':x: / the solution
x: / store input as x
<':   / less-than each-previous
&      / indices where true
|       / reverse
_/        / drop-over
x          / the input


# Attache, 24 bytes

{Mask[1'(Delta!_>=0),_]}


Try it online!

## Explanation

Mask selects all elements from its second argument which correspond to truthy elements in its first argument. 1'(Delta!_>=0) calculates the indices which correspond to elements that are supposed to be in the final array.

## Other attempts

28 bytes (pointfree): ~Mask#(1&'##Delta#>=#C[0])

32 bytes: {Mask[1'(&<= =>Slices[_,2]),_]}

# C# (.NET Core), 33 + 18 = 51bytes

x=>x.Where((a,n)=>n<1||x[n-1]<=a)


Try it online!

basically the statement is where x is the first int in the array, or is greater than or equal to the previous number, keep it. Else drop it.

• You can return an IEnumerable. No ToArray() needed. Mar 22, 2018 at 17:21
• @Pavel I would need to add an extra reference System.Collections, and that would negate all the bytes saved for removing the ToArray(). Mar 22, 2018 at 22:12
• No, since you won't be referencing IEnumerable in the answer, just using it as the return type. Mar 22, 2018 at 22:27
• @Pavel okay thanks, sometimes I'm a bit unsure when to count the bytes or not... sorry Mar 23, 2018 at 7:39

# Swift 4, 56 55 bytes

{var l=0;print($0.filter{($0>=l,l=$0).0})}as([Int])->()  Try it online! ## Explanation {var l=0; // Declare variable l print($0.filter{(   // Print every element e in the input
$0>=l, // where e >= l l=$0).0})         //   And set l to e
}as([Int])->()      // Set the input type to integer array


# Jelly, 9 bytes

0;>ƝżµḢÐṂ


Try it online!

This feels pretty bulky, wouldn't be that surprised if there is a better way.

0;>ƝżµḢÐṂ
Ɲ       For each adjacent pair in the input...
>        ...is the first element greater than the second? (yields a list of 0/1)
0;         prepend a zero to this list (always keep the first element)
ż      zip with the input
ÐṂ  keep elements of the list with minimal value of... (Ðḟ would have worked here and been slightly more clear but I'll leave it as it is)
Ḣ    ...their first element


# Brain-Flak, 136, 120 bytes

((())){{}([{}]({}))([({}<(())>)](<>)){({}())<>}{}{((<{}>))<>{}}{}<>{}{{}(({})<>)(())(<>)}{}([][()])}{}{}<>{{}({}<>)<>}<>


Here it is formatted and "readable".

((()))
{
{}

([{}]({}))

([({}<(())>)](<>)){({}())<>}{}{((<{}>))<>{}}{}<>{}

{
{}(({})<>)(())(<>)
}

{}

([][()])

}{}{}<>

{
{}
({}<>)<>
}<>
`

Try it online!