Calculate the strange sum of two natural numbers (also known as lunar addition):

Given A=... a2 a1 a0 and B=... b2 b1 b0 two natural numbers written in the decimal base, the strange sum is defined, based on the maximum operation, as: A+B=... max(a2,b2) max(a1,b1) max(a0,b0)

   ...         a2         a1         a0
 + ...         b2         b1         b0
   ...  max(a2,b2) max(a1,b1) max(a0,b0)


Two natural numbers

All the following is allowed:

  • Zero-padded strings (same length)
  • Left-space-padded strings
  • Right-space-padded strings
  • Array of two padded strings
  • 2D space-padded char array


A natural numbers




  • The input and output can be given in any convenient format (choose the most appropriate format for your language/solution).
  • No need to handle negative values or invalid input
  • Either a full program or a function are acceptable. If a function, you can return the output rather than printing it.
  • If possible, please include a link to an online testing environment so other people can try out your code!
  • Standard loopholes are forbidden.
  • This is so all usual golfing rules apply, and the shortest code (in bytes) wins.
  • 3
    also known as lunar addition – TFeld Nov 15 at 14:38
  • 3
    Can we take the input as zero-padded strings (same length)? – TFeld Nov 15 at 14:44
  • 1
    I think this is a bit too trivial. Weird that has not been asked before – Windmill Cookies Nov 15 at 14:45
  • 1
    Can we have the numbers in the same length? Like 17210 00701 instead of 17210 701 ? – Windmill Cookies Nov 15 at 14:57
  • 7
    I fail to see what input formats are allowed exactly. The input format is very important in this challenge, as some formats allow much easier processing. What of the following are allowed? 1) Zero-padded strings (same length) 2) Left-space-padded strings 3) Right-space-padded strings. 4) Array of two padded strings. 5) 2D space-padded char array. Voting to close and downvoting for now; I will happily remove my votes when solved – Luis Mendo Nov 15 at 19:01

27 Answers 27

Python 2, 20 bytes


Try it online!

I/O as 0-pre-padded lists of digits.

Jelly, 1 byte


Try it online!

I/O as 0-pre-padded lists of digits.

R, 68 65 bytes


Try it online!

Input as integers, output as list of digits.

If zero-padding lists of digits was allowed, then simply pmax would suffice.

MATL, 2 bytes


Choose the most appropriate format for your language/solution

The input format is: 2D char array of two rows, each corresponding to a line, with the shorter number left-padded with spaces. For example


which in MATL is defined as

['17210'; '  701']

Try it online!


      % Implicit input: 2D char array with two rows 
X>    % Take maximum of (code points of) each column
      % Implicit display

Python 2, 73 60 56 bytes

lambda a,b:map(max,zip(a.rjust(len(b)),b.rjust(len(a))))

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Takes input as two strings, and returns a list of digits


Takes input as two integers; same output

Python 2, 60 59 bytes

lambda*i:map(max,zip(*['%*d'%(len(`max(i)`),v)for v in i]))

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Java 10, 78 57 bytes

a->b->{for(int i=a.length;i-->0;)if(a[i]<b[i])a[i]=b[i];}

Input as two space-padded character arrays.

Modifies the first input-array instead of returning a new one to save 21 bytes (thanks to @OlivierGrégoire).

Try it online.


a->b->{            // Method with two char-array parameters and String return-type
  for(int i=a.length;i-->0;)
                   //  Loop `i` in the range (length, 0]:
    if(a[i]<b[i])  //   If the `i`'th character in input `a` is smaller than in input `b`:
      a[i]=b[i];}  //    Change the `i`'th character in `a` to the `i`'th character of `b`
  • 1
    a->b->{for(int i=a.length;i-->0;)if(a[i]<b[i])a[i]=b[i];} (57 bytes). Reuse a as output to gain much, much bytes. – Olivier Grégoire Nov 16 at 12:30
  • @OlivierGrégoire Ah, can't believe I hadn't thought of that. Thanks! :D – Kevin Cruijssen Nov 16 at 12:59

J, 14 12 bytes

-2 bytes thanks to Jonah


Try it online!

Input and output as list(s) of digits

Japt, 9 8 7 bytes

Takes input as an array of digit arrays.


Try it

m            :Map
 Ô           :  Reverse
  Õ          :Transpose
   Ô         :Reverse
    Ë        :Map
     r       :  Reduce by
      w      :  Maximum

If taking zero-padded arrays as input is permitted (it would currently fall under a "convenient format" but I suspect that's not the challenger's intent) then this can be 3 bytes.


Try it

í       :Interleave the first input
  V     :With the second
 w      :Reduce each pair by maximum
  • 1
    Here's another 8 byte solution with a different strategy and simpler input format. maybe you can shave a byte from that? – Kamil Drakari Nov 15 at 17:13
  • @KamilDrakari: Uncanny - I was just updating with the exact same solution! – Shaggy Nov 15 at 17:14
  • Interestingly, the version that separates the "transpose" and "reduce" steps is also 8 bytes because of better shortcuts – Kamil Drakari Nov 15 at 17:20
  • @KamilDrakari, oh, we have a shortcut for y now? I did not know that. Here's another way, also 8 bytes. – Shaggy Nov 15 at 17:23
  • Ooh, that's a neat trick with '. I don't know whether that would ever save bytes, but it's definitely cool. – Kamil Drakari Nov 15 at 17:25

Perl 6, 37 bytes

{[R~] roundrobin($_».reverse)».max}

Try it online!

Takes input as a list of lists of digits.

05AB1E, 9 6 5 bytes

-3 thanks to Emigna
-1 thanks to Shaggy


Takes input as a list of lists of digits

í      # Reverse both inputs
 ζ     # Zip
  ۈ   # Keep the bigger digits
    R  # Reverse

Try it online! or Try all test cases

  • €R can be í. Also, I don't think you need €þ as number > space – Emigna Nov 15 at 16:50
  • @Emigna Thanks! I thought I tried it without €þ and it didn't work, but now it does... – Riley Nov 15 at 16:59
  • Do you need the J at the end? – Shaggy Nov 15 at 19:57
  • @Shaggy No, I guess I don't. Thanks! – Riley Nov 15 at 21:00
  • It's lame, but the rules allow a 3 byte solution by taking a 2D space-padded char array.. – Kevin Cruijssen Nov 16 at 8:40

Perl 6, 15 bytes

{[~] [Zmax] $_}

Try it online!

Takes input as a list of space padded arrays of characters, though for this challenge the lax input format makes it rather boring. Alternatively, here's the program that takes a list of two integers instead:

Perl 6, 41 bytes

{+[~] [Zmax] $_>>.fmt("%{.max}d")>>.comb}

Try it online!

If you don't mind a huge amount of whitespace, you can also remove the + from the front.


{                                       }  # Anonymous code block
             $_>>    # Map each integer to 
                 .fmt("%{.max}d") # The number padded by the max of the list spaces
                                 >>.comb   # And split each to list of characters
      [Zmax]  # Get the max of each digit at each index
              # This works because space is coerced to 0
              # Otherwise we would have to add a 0 to the formatting string
  [~]   # Join the list of digits and spaces
 +      # And coerce the string to a number to get rid of leading whitespace

Haskell, 40 bytes

a#b=zipWith max(p b++a)$p a++b
p=(' '<$)

Input/output as strings, try it online!


The function p replaces each character by a space, using p b++a and p a++b are thus the same length. This way we can use zipWith without losing any elements, using max with it works because a (space) has lower codepoint than any of the characters ['0'..'9'].

JavaScript (ES6), 51 49 bytes

NB: This answer was posted before the loose I/O formats were explicitly allowed. With zero-padded arrays of digits, this can be done in 33 bytes, (but is much less interesting, IMHO).

Takes input as two integers. Returns an integer.


Try it online!


f = (                     // f = recursive function taking:
  a,                      //   a = first integer
  b,                      //   b = second integer
  t = 10                  //   t = 10 (which is used 6 times below)
) =>                      //
  a | b                   // bitwise OR between a and b to test whether at least one of
                          // them still has an integer part
  &&                      // if not, stop recursion; otherwise:
  (                       //
    a % t < b % t ? b : a // if a % 10 is less than b % 10: use b; otherwise: use a
  ) % t +                 // isolate the last decimal digit of the selected number
  t *                     // add 10 times the result of
  f(a / t, b / t)         // a recursive call with a / 10 and b / 10
  | 0                     // bitwise OR with 0 to isolate the integer part

Alternate version

Same I/O format.


Try it online!

  • You can get much shorter if you assume your input to be a 2d space padded char array. – kamoroso94 Nov 15 at 22:20
  • Hi! Could you provide some explanation? I ""tried"" to do this challenge in JavaScript but I failed and would like to see how your solution works :) – Neyt Nov 16 at 9:35
  • 1
    @Neyt I've added a commented version. The alternate version is using the same logic; the only difference is that we add the next digit to the left as a string instead of multiplying the result of the recursive call by 10. – Arnauld Nov 16 at 9:53
  • @Arnauld Thank you! :) – Neyt Nov 16 at 9:54

Tcl, 156 bytes

proc S a\ b {join [lmap x [split [format %0[set l [expr max([string le $a],[string le $b])]]d $a] ""] y [split [format %0$l\d $b] ""] {expr max($x,$y)}] ""}

Try it online!

Not very golfy, but I had to give a try on it. Will golf it later!

Batch, 120 bytes

@for /l %%i in (0,1,9)do @set/a"z+=w*((v=y%%10)+(v-=x%%10)*(v>>4)),y/=10,x/=10,w*=10
@echo %z%

Takes input as command-line parameters. 188-byte version works on arbitrary length integers:

@set z=
@if %x:~-1% gtr %y:~-1% (set z=%x:~-1%%z%)else set z=%y:~-1%%z%
@set x=%x:~,-1%
@set y=%y:~,-1%
@if "%x%" neq "" if "%y%" neq "" goto l
@echo %x%%y%%z%

Takes input on STDIN.

Twig, 125 bytes

When I saw this challenge, I though: "let me use a template language! sure is a good fit"

I was wrong ... so wrong .... ... But was fun!

{%macro a(a,b,s='')%}{%for k,x in a|reverse|split('')%}{%set s=max(x,(b|reverse|split('')[k]))~s%}{%endfor%}{{s}}{%endmacro%}

This requires that "strict_variables" is set to false (default value).

To use this macro, you can do like this:

{% import 'file.twig' as my_macro %}

{{ my_macro.a(195,67) }}

Should display 167.

You can try this in
("strict_variables" set to off, it is on by default on the website)

Husk, 5 bytes


Conveniently takes input/output as list of digits, try it online or verify all!


↔¤żY↔  -- example inputs [1,4] [3,2]
 ¤  ↔  -- reverse the arguments of: [4,1] [2,3]
  żY   -- | zipWith (keeping elements of longer) max: [4,3]
↔      -- reverse: [3,4]

Stax, 5 bytes


Run and debug it

This program takes input as an array of strings.

|>  Right align inputs (filling with \0)
E   "Explode" array onto stack separately
:o  "Overlay" Keep the maximum element respective element from two arrays.

Run this one

This is the first time I've seen a use for the overlay instruction "in the wild".

SNOBOL4 (CSNOBOL4), 153 bytes

S	X LEN(1) . A REM . X	:F(O)
	Y LEN(1) . B REM . Y
	O =O GT(A,B) A	:S(S)
	O =O B	:(S)

Try it online!

Pyth, 5 bytes


Takes input as array of two space-padded strings.

meSd       map greatest
    C      on the transpose of input

Try it here.

Japt, 4 bytes

Input is taken as a an array of two 0-padded number arrays.


Try it online!

Ceylon, 55 / 99

With 0- or space-padded strings of same length (returning an iterable of characters):

function t(String a,String b)=>zipPairs(a,b).map(max);

With 0- or space-padded strings (returning a String):

String t(String a,String b)=>String(zipPairs(a,b).map(max));

With strings of possibly different length (returning a String):

String u(String a,String b)=>String(zipPairs(a.padLeading(b.size),b.padLeading(a.size)).map(max));

C# (.NET Core), 57 bytes


-1 bytes by adding currying

Try It Online

Ruby, 25 bytes

->a,b{ &:max}

Try it online!

Pre-padded list blah blah. (Although it feels a little like cheating.)

Retina 0.8.2, 39 bytes


Try it online! Link includes test suite. Previous 45-byte Retina 1 version accepts unpadded strings:


Try it online! Link includes test suite. Explanation:


Pad both values to the same length. (Retina 1 only. There are ways of emulating this in Retina 0.8.2 but they are not very golfy.)


Transpose the values.


Sort each pair into order.


Delete all the low digits and surplus newlines.

Charcoal, 8 bytes


Try it online! Link is to verbose version of code. Explanation:

 θ          First input
⭆           Map over characters and join
  ⌈         Maximum of
   ⟦        List of
    ι       Current character of first input and
      η     Second input
     §      Indexed by
       κ    Current index
            Implicitly print

10-byte version "adds" any number of padded strings:


Try it online! Link is to verbose version of code. Previous 14-byte version accepts unpadded strings:


Try it online! Link is to verbose version of code. Explanation:

  θ             First input
 ◧              Padded to
   L            Length of
    η           Second input
⭆               Map over characters and join
     ⌈          Maximum of
      ⟦         List of
       ι        Current character of first input and
          η     Second input
         ◧      Padded to
           L    Length of
            θ   First input
        §       Indexed by
             κ  Current index
                Implicitly print

17-byte version "adds" any number of strings:


Try it online! Link is to verbose version of code.

  • The question has been changed so the strings can be input as padded – ASCII-only Nov 19 at 6:59
  • Nitpicking, but you missed a close paren in the first verbose version :P – ASCII-only Nov 19 at 23:30
  • @ASCII-only If only TIO matched parens for me ;-) – Neil Nov 20 at 1:47

Mathematica 50 bytes

a = 543; b = 791;

FromDigits@MapThread[Max, IntegerDigits /@ {a, b}]

(* 793 *)

  • 1
    Can you add a TIO? I don't know Mathematica but I suspect this might fail if the 2 inputs don't have an equal number of digits. – Shaggy Nov 15 at 23:22
  • 1
    I think you're taking input through predefined variables, which would make this a snippet, which is not allowed. Submissions should be either a function or a full program – Jo King Nov 16 at 2:09

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