Add in the blanks

Question

In this challenge you will be given a list of positive integers which represents some range of integers which has been truncated for display. Your job is to find the missing bits and insert ellipses to show that that part has been truncated.

The input will always be strictly ascending and if two consecutive values differ by more than 1 then we know some data has been removed in between them.

For example if the input is:

1,2,3,4,20,21,22

Then since 4 and 20 differ by more than 1 we insert an ellipsis between them:

1,2,3,4,...,20,21,22

However if the consecutive values differ by exactly two, then we just reinsert the missing value (their mean), since an ellipsis doesn't actually save any space when it's just covering 1 value.

So for example if the input is:

1,2,3,5,6

Then 3 and 5 differ by more than 1 so data has been removed between them, but they differ by 2 so we just insert the missing 4.

1,2,3,4,5,6

Additionally if the first value is greater than 1 we know that something has been truncated off the front. So we add an ellipsis, or if the first value is 2, a 1 as the first value. We can't know that something has been truncated off the end so we never add anything there.

Task

Given a list of positive strictly-ascending integers as input output the list with the proper insertions. You can represent ellipses in the output as any constant value other than a positive integer. For the convenience of strongly typed programming, you may also output a list of optional values with the null value representing ellipses.

This is code-golf so the goal is to minimize the size of your source code as measured in bytes.

Test cases

[1,2,3,4,20,21,22] -> [1,2,3,4,...,20,21,22]
[1,2,3,5,6] -> [1,2,3,4,5,6]
[2,3,4,9] -> [1,2,3,4,...,9]
[3,5,7,8,10] -> [...,3,4,5,6,7,8,9,10]
[1,20,23] -> [1,...,20,...,23]

Answer 1

JavaScript (ES10),  45  43 bytes

Uses undefined for ellipses.

a=>a.map(p=v=>[[[],v-1][v+~p],p=v]).flat(2)

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Commented

a =>             // a[] = input array, e.g. [ 1, 3, 6 ]
  a.map(p =      // initialize p to a zero'ish value
  v =>           // for each value v in a[]:
    [            //   build an array:
      [          //     according to v - p - 1, append:
        [],      //       an empty array if it's 0
        v - 1    //       v - 1 if it's 1
                 //       implicitly: undefined for anything else
      ][v + ~p], //
      p = v      //     append v and update p to v
    ]            //   end of array
  )              // end of map(); this gives:
                 //   [ [ [], 1 ], [ 2, 3 ], [ undefined, 6 ] ]
  .flat(2)       // apply .flat() at depth 2 to clean this up:
                 //   [ 1, 2, 3, undefined, 6 ]

Answer 2

K (ngn/k), 24 bytes

,/0{((0;();x-1)x-y),x}':

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Uses 0N for ellipses.

Explanation

• 0{...}': pairwise map with placeholder 0 (y is prev elem, x is next elem)...
  • (0;();x-1)x-y get elem of list (0;();x-1) at index x - y
  • ,x append x
• ,/ flatten

Answer 3

R, 69 68 bytes

Edit: -1 byte thanks to pajonk

\(x){F[i<-cumsum(2-!(j=diff(c(0,x))-1))]=x;F[-i]=(x*(j<2))[j>0]-1;F}

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Uses -1 as the ellipsis character.

How?

j=diff(c(0,x))-1    # get the differences between adjacent elements -1
!j                  # zero for consecutive integers, 1 otherwise
i=cumsum(2-!j)      # indices of original elements, leaving gaps for ellipses or new elements-in-gaps
F[i]=x              # fill-in the elements into vector F
F[-i]=              # fill-in the gaps with...
      (x...)[j>0]-1 # one less than the subsequent original value...
        *(j<2)      # ...adjusted to zero if the gap was >2
F                   # finally, return F

Answer 4

05AB1E, 22 19 17 15 bytes

0šüŸε¦¤®s‚‚éн}˜

Uses -1 for ellipsis, but could alternatively use another 1-byte alternative (e.g. 0, "", " ", "\n", "abcdefghijklmnopqrstuvwxyz", etc.) by replacing the ®.

-3 bytes porting @MamaFunRoll's K (ngn/k) answer
-2 bytes porting @DominicVanEssen's Husk answer
-2 bytes thanks to @CommandMaster with yet another different approach

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Explanation:

          #  Example input: [2,3,5,9,13]
0š        # Prepend a 0 to the (implicit) input-list
          #  STACK: [0,2,3,5,9,13]
  ü       # For each overlapping pair {a,b}:
   Ÿ      #  Push a list in the range [a,b]
          #   STACK: [[0,1,2],[2,3],[3,4,5],[5,6,7,8,9],[9,10,11,12,13]]
ε         # Map over each inner list:
          #   STACK1: [0,1,2] ;        STACK2: [5,6,7,8,9]
 ¦        #  Remove the first item
          #   STACK1: [1,2] ;          STACK2: [6,7,8,9]
  ¤       #  Push the last item (without popping the list)
          #   STACK1: [1,2],2 ;        STACK2: [6,7,8,9],9
   ®      #  Push -1, the ellipsis value
          #   STACK1: [1,2],2,-1 ;     STACK2: [6,7,8,9],9,-1
    s     #  Swap the top two values on the stack
          #   STACK1: [1,2],-1,2 ;     STACK2: [6,7,8,9],-1,9
     ‚    #  Pair them together
          #   STACK1: [1,2],[-1,2] ;   STACK2: [6,7,8,9],[-1,9]
      ‚   #  Pair the two lists together
          #   STACK1: [[1,2],[-1,2]] ; STACK2: [[6,7,8,9],[-1,9]]
       é  #  Sort by length (shortest to longest)
          #   STACK1: [[1,2],[-1,2]] ; STACK2: [[-1,9],[6,7,8,9]]
        н #  Pop and leave the first inner list
          #   STACK1: [1,2] ;          STACK2: [-1,9]
}         # After the map
          #  STACK: [[1,2],[3],[4,5],[-1,9],[-1,13]]
 ˜        # Flatten the resulting list of lists
          #  STACK: [1,2,3,4,5,-1,9,-1,13]
          # (after which the result is output implicitly)

Answer 5

Python 3, 62 bytes

lambda a:sum([[~-i*(i-2in[0]+a),i][i-1in[0]+a:]for i in a],[])

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Use 0 for ellipses.

For each value i in input:

• If i - 1 is already included in input, or i - 1 is 0, we need insert nothing before it;
• Otherwise, if i - 2 is already included in input, or i - 2 is 0, we need insert i - 1 before it;
• Otherwise, we insert ... before it.

Answer 6

Wolfram Language (Mathematica), 40 bytes

##&[Pick[#>p+2||++p,p<#],p=#]&/@(p=0;#)&

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Represents ellipses by True.

##&[Pick[#>p+2||++p,p<#],p=#]&/@(p=0;#)&
                                 p=0    starting from 0,
                              /@(   ;#) for each element:
         #>p+2                            True if current-previous>2,
              ||++p                         otherwise previous+1,
    Pick[          ,p<#]                      but omit if =current
##&[                    ,p=#]           prepend and update previous

Answer 7

Jelly, 13 11 bytes

ŻṖ‘ż¹FQµIỊa

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0 for ellipses.

Ż              Prepend a 0,
 Ṗ             trim the last element,
  ‘            increment,
   ż¹F         flat-interleave with the original input,
      Q        and uniquify.
       µ  a    Zero out any elements of the result which
        I      have a difference with the next element
         Ị     greater than 1.

Answer 8

Haskell, 69 bytes

g x y|x+1==y=[y]|x+2==y=[x+1,y]|True=[0,y]
f a=concat$zipWith g(0:a)a

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Answer 9

PARI/GP, 50 bytes

a->b=0;concat([if(c-b<3,[b+1..b=c],[x,b=c])|c<-a])

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Use x for ellipses.

Answer 10

Perl 5 + -p -M5.10.0, 35 bytes

If using a 0 is acceptable this would be 31 bytes, but that feels a bit cheaty...

$;=$_*say$--1?"...":$_-1if$-=$_-1-$

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Answer 11

Python 2, 57 bytes

p=1
for n in input():
 if p<n:print-~p/n*p
 print n;p=n+1

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A program that takes in a list on STDIN and prints the output values on separate lines.

Answer 12

J, ³⁴ 31 bytes

0(}.(+_*_1>])2-/\])@~.@,<:,@,.]

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This was surprisingly difficult to golf in J and the K approach ended up being longer than either of these.

Idea

We take a two pass strategy:

• First we insert the numbers below each number.
• Then we check if there are any gaps greater than 1. If so, we replace those gaps with infinity _.

How

Consider 2 3 4 9:

• <:...,.] Decrement zipped with original input:

  1 2
  2 3
  3 4
  8 9
  

• 0...~.@,...,@ Flatten, prepend 0, and take the unique:

  0 1 2 3 4 8 9
  

• }.(...)2-/\] On the left hand side, kill the 0, on the right hand side take consecutive deltas:

  1 2 3 4 8 9  (...) _1 _1 _1 _1 _4 _1 
  

• _*_1>] Turn every entry less than _1 on the right into infinity:

  0 0 0 0 _ 0
  

• + And add that to the left side:

  1 2 3 4 _ 9
  

J, first approach, 34 bytes

[:;]<@~.@,~"+]+_3>.@%@+3<.2-~/\0,]

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Answer 13

Vyxal, 15 bytes

Gɾ‡?cḊƛḢ[h?c∧;f

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Uses 0 for ellpisis.

Answer 14

Brainfuck, 71 bytes

,[[->+>+<<]>>[-<<+>>]<<<[->-<]+>-[-[<<.>-]<[>>-.+<<-<]>]<[-<]>>[-]>.>,]

Ungolfed program (a port of my Haskell solution)

Script to test it

Answer 15

Rust, 135 130 bytes

-5 bytes thanks to @alpehalpha

|v:Vec<i8>|v.iter().scan(0,|c,&a|Some((match a-*c{1=>vec![a],2=>vec![a-1,a],_=>vec![-1,a]},*c=a).0)).flatten().collect::<Vec<_>>()

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Answer 16

Husk, 16 14 bytes

Edit: -2 bytes by stealing Kevin Cruijssen's improvements in his port of this!

ΣẊö◄LSeoe0→t…Θ

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Uses 0 for ellipsis; could use any other integer by exchanging the 0 at position 10 in the code.

             Θ     # add a zero at the start of the list; 
 Ẋȯ                # now, for every pair of elements:
            …      #  fill gaps with numeric ranges,
           t       #  and discard the first element,
     Se            #  then make a 2-element list of this and
       oe0→        #  just the last element preceded by zero,
   ◄L              #  and select the element with minimum length;
Σ                  # finally, flatten the resulting list-of-lists.  

Answer 17

C (clang), ⁷⁶ 71 bytes

p;f(*a,n){for(p=0;n--;)printf("%d %d "+(++p==*a)*3,p+1<*a?0:p,p=*a++);}

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Saved 5 bytes thanks to att!!!

Inputs a pointer to an array of positive integers and its length (because pointers in C carry no length info).
Prints out the filled in array using \$0\$ for ellipses.

Answer 18

Ruby, 50 45 bytes

->l{a=0;l.map{|x|p x-a<2?a:0if x>a+=1;p a=x}}

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Thanks Jonah for -2 bytes and a different approach which saved 3 more bytes.

Answer 19

AWK, 44 bytes

{d=$0-a;if(d>1)print d==2?$0-1:0;print a=$0}

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Thanks to Dominic van Essen for pointing out that 5 bytes needed to initialize the variable a on TIO aren't needed on GNU awk, and so shouldn't count in the score.

Outputs 0 for ....

Answer 20

Pyth, 17 bytes

Inspired by Arnauld's JavaScript answer

.n,Le<[YtdG)aZ=Zd

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Uses the lowercase alphabet as the ellipsis (could alternatively use 0 or empty string).

.n,Le<[YtdG)aZ=Zd
                    Implicitly initialize Z to 0
   L                Left map over the input with variable d
      [    )        3-element list of...
       Y              ... the empty list
        td            ... (d - 1)
          G           ... the lowercase alphabet
     <              Keep the first N elements of that list, where...
            aZd      ... N = the absolute difference of Z and d
              =Zd   (Set Z to d)
    e               Take the last element of the resulting list
  ,                 Create a two-element list of (result, current element)
.n                  Flatten

Answer 21

Retina 0.8.2, 73 bytes

\d+
$*
^
,
(^|\b1+),(?=11\1\b)
$&1$1,
(^|\b1+),(?=11\1)
$&...,
^,

1+
$.&

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\d+
$*

Convert to unary.

^
,

Prefix 0 (in unary).

(^|\b1+),(?=11\1\b)
$&1$1,

Where two consecutive integers differ by 2, insert the missing integer.

(^|\b1+),(?=11\1)
$&...,

Where two consecutive integers differ by more than 1, insert an ellipsis.

^,

Remove the leading 0.

1+
$.&

Convert to decimal.

Answer 22

R, 75 bytes

\(x,y=c(0,x))for(i in x-1){if(!i%in%y)show(`if`((i-1)%in%y,i,0));show(i+1)}

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Answer 23

Charcoal, 24 bytes

ＩＥ⊕Φ⌈θ№⁺θ⊖θ⊕ι∧№⊞Ｏ⁺θ⊕θ¹ιι

Try it online! Link is to verbose version of code. Uses 0 to represent an ellipsis. Explanation:

     θ                      Input array
    ⌈                       Maximum
   Φ                        Filter on implicit 0-indexed range
        θ                   Input array
       ⁺                    Concatenated with
          θ                 Input array
         ⊖                  Vectorised decrement
      №                     Contains
            ι               0-indexed value
           ⊕                Incremented (i.e. 1-indexed)
  ⊕                         Incremented (back to 1-indexed values)
 Ｅ                          Map over values
                  θ         Input array
                 ⁺          Concatenated with
                    θ       Input array
                   ⊕        Vectorised increment
               ⊞Ｏ           Concatenated with
                     ¹      Literal integer `1`
              №             Contains
                      ι     Current value
             ∧              Logical And
                       ι    Current value
Ｉ                           Cast to string
                            Implicitly print