Construct a Modulo Multiplication Table

Question

Consider a n x n multiplication table and replace each item with its remainder of division by n. For example, here is a 6x6 table and its "modulo 6" structure: (The last column and row are ignored since both are null)

1  2  3  4  5  6   |   1 2 3 4 5  
2  4  6  8 10 12   |   2 4 0 2 4  
3  6  9 12 15 18   |   3 0 3 0 3  
4  8 12 16 20 24   |   4 2 0 4 2  
5 10 15 20 25 30   |   5 4 3 2 1  
6 12 18 24 30 36   |              

Now it is evident that the multiplication table modulo n is symmetric and can be reconstructed by one of its triangular quadrants:

1 2 3 4 5
  4 0 2
    3

Challenge

Given a positive integer N, print the upper quadrant of multiplication table modulo N. Assume that there is no restriction on the width of string in your output environment. The alignment of numbers shall be preserved. This means, the output should look like a part of a uniform product table, where the cells have equal widths. So for example, if we have a two-digit number in the table, all single-digit entries are separated by two spaces.

Rules

Standard code-golf rules apply.

Test cases

N = 1:
// no output is printed

N = 3:
1 2

N = 13:
1  2  3  4  5  6  7  8  9 10 11 12
   4  6  8 10 12  1  3  5  7  9
      9 12  2  5  8 11  1  4
         3  7 11  2  6 10
           12  4  9  1
              10  3

_Sandbox

Answer 1

Charcoal, 32 bytes

Ｅ⊘θ⪫Ｅ⊖θ◧⎇‹›ιλ‹⁺ιλ⊖θＩ﹪×⊕ι⊕λＩθωＬθ 

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

  θ                                 Input `n`
 ⊘                                  Halved
Ｅ                                   Map over implicit range
      θ                             Input `n`
     ⊖                              Decremented
    Ｅ                               Map over implicit range
        ⎇‹›ιλ‹⁺ιλ⊖θ                 If this entry is wanted
                      ⊕ι            Row value
                     ×              Multiplied by
                        ⊕λ          Column value
                    ﹪               Modulo
                          Ｉθ        `n` as an intger
                   Ｉ                Cast to string
                            ω       Otherwise the empty string
       ◧                     Ｌθ     Pad to length of `n`
   ⪫                                Join with spaces
                                    Implicitly print

Answer 2

Jelly, 14 bytes

’×þ%ɓoU¥⁺GµḶ⁶ẋ

Try it online!

Outputs a bunch of trailing whitespace. Used the ’×þ% trick from hyper-neutrino's answer, so be sure to give them an upvote

How it works

’×þ%ɓoU¥⁺GµḶ⁶ẋ - Main link. Takes N on the left
’              - Decrement
 ×þ            - Create a multiplication table for all integers from 1 to N-1 and 1 to N
   %           - Mod each by N. This leaves a trailing row of zeros. 
                 Call this table T

           Ḷ   - Yield [0, 1, 2, ..., N-1]
            ⁶ẋ - Yield that many spaces for each
                 Call this list of spaces S

    ɓ     µ    - New dyadic chain f(S, T):
       ¥       -   Group the previous 2 links into a dyad g(S, T):
     o         -     For each row in S and T, replace the first N elements of the row in T with spaces,
                      where N is the number of spaces in the row in S
      U        -     Reverse each row
        ⁺      -   Do it again
         G     -  Format it as a grid and output

Answer 3

Jelly, 14 bytes

’×þ%µJ’⁶ẋoUµ⁺G

Try it online!

I stole the idea to use logical OR to avoid needing to trim and re-pad the lists from caird. Go upvote their solution too, please.

’×þ%µJ’⁶ẋoUµ⁺G   Main Link; accepts x
’                x - 1
 ×               using multiplication
  þ              create a product table (this does x - 1 by x)
   %             modulo x
----µ---------   start a new chain on this table
     J           range over length (1, 2, ..., N)
      ’          decrement (0, 1, 2, ..., N - 1)
       ⁶ẋ        repeat " " by ^
         o       vectorized logical OR; basically fills the lower left triangle with spaces
          U      reverse each row
     ------µ⁺    duplicate the last chain, so this fills the lower left and flips horizontally twice, thus cutting the list to just the upper triangle
             G   format as grid

Answer 4

J, ⁶⁰ 55 bytes

1}."1,/"2@((' ',.' ',":@,.){~(<|.)@(>/~)*1+#|>:*/>:)@i.

Try it online!

^{-5 thanks to xash}

I thought this would be a good challenge for J, and the essence of the problem is, but the formatting details destroyed me here.

Answer 5

R >= 4.1.0, 165 152 bytes

\(n,`/`=sprintf){z=outer(1:n,1:n,\(x,y)(x*y)%%n[2-(pmin(y,n-y)>=x)]);z[is.na(z)]="";z[]="%%%ds"/nchar(n-1)/z;cat(apply(z,1,paste,collapse=" "),sep="
")}

Try it online!

An anonymous function that takes an integer and prints the triangle to STDOUT. The TIO link replaces \ with function since TIO is on an earlier version of R.

Thanks to @RobinRyder for saving 5 bytes, and @KirillL for saving a further 8!

Answer 6

MATLAB/Octave, 117 bytes

function f(n)
r=1:n;m=num2str(mod(r'*r,n));s=(nnz(m(1,:))+2)/n;for k=1:n
m(k,1:k*s-s)=32;disp(m(k,1:(n-k)*s))
end
end

Try it online! This prints out quite a lot of trailing whitespaces. If we sacrifice 2 bytes to replace k=1:n with k=1:n/2 the empty lines won't be printed though.

Despite my efforts to find more elegant solution using triu function (takes upper triangle of square matrix) this stayed the shortest solution I came up with.

Ungolfed/explained:

function f(n)
r=1:n;
m=num2str(mod(r'*r,n)); % modulo table transformed to char array
s=(length(m(1,:))+2)/n; % how many characters takes up one number
for k=1:n
  m(k,1:k*s-s)=' ';     % replace first k-1 numbers with spaces
  disp(m(k,1:(n-k)*s))  % print first n-k numbers
end
end

Answer 7

JavaScript (ES8), 92 bytes

Expects the input as a string.

n=>(g=k=>++k>n/2?'':(h=i=>++i>n-k?`
`:(i<k?'':i*k%n+'').padStart(n.length+1)+h(i))``+g(k))``

Try it online!

Answer 8

Python 3, 113 96 bytes

def f(n):d=range(1,n);[print(*[f"{['',v*x%n][v>=x<=n-v]:{len(str(n-1))}}"for v in d])for x in d]

Try it online!

Answer 9

APL (Dyalog Unicode), 43 41 35 bytes (SBCS)

Anonymous prefix lambda.

{1↓⍕d⍪' '@{∨∘⌽⍨∘.>⍨⍳d}⍵|∘.×⍨⍳d←⍵-1}

Try it online!

{…} "dfn"; N is ⍵:

 d←⍵-1 let d be N minus one

 ⍳ indices one through N

 ∘.×⍨ multiplication table

 ⍵| that modulus N

 ' '@{…} replace with spaces at locations indicated by:

  ⍳d indices one through d

  ∘.>⍨ greater-than table

  ∨∘⌽⍨ logical OR with its own mirror image

 d⍪ prepend a row of ds (to size columns widths right)

 ⍕ format to character matrix

 1↓ drop the first row

Answer 10

Jelly, 16 15 bytes

’<þa⁶oU$o’×þ%ƊG

Try it online!

Also outputs a ton of trailing whitespace

Answer 11

Stax, 16 bytes

ù▼åU☻┐≈◘E╝ÖBñGRR

Run and debug it

about as short as I could get it. centering the stuff took more bytes than i expected.

Explanation

vc{*x%K{itiT|>Jm|Cm
vc                  decrement and duplicate input n
  {   K             cross-product map ranges (1..n-1) and (1..n-1)
   *                multiply the numbers
    x%              mod n
       {       m    map each row to
        itiT        iteration number trimmed from both sides
            |>      right align each number
              J     join with spaces
                |C  center
                  m print joined with newlines  

Answer 12

Vyxal CM, 35 30 bytes

ƛ⁰ɽƛ⁰ɽ*⁰%;$in(ṪḢ);ƛƛ:L⁰Lεð*p;Ṅ

Try it Online!

ƛ⁰ɽƛ⁰ɽ*⁰%;$in(ṪḢ);ƛƛ:L⁰Lεð*p;Ṅ    
ƛ                ;                For each n in range(0,input):
 ⁰ɽƛ     ;                         Generate the table: For each in range(1,input):
    ⁰ɽ*                            Multiply by range(1,input) to make a table.
       ⁰%                          Mod(input) everything
          $i                       Make a triangle: Get the nth row of the table
            n(  )                  Repeat n times:
              ṪḢ                    Remove the head and tail

                                  Align the numbers:
ƛ                                 For each row of the triangular table:
 ƛ        ;                        For each number in that row:
  :L⁰Lε                             Take difference of length(input) and length(number)
       ð*p                          Prepend that many spaces
           Ṅ                       Join each row on spaces

C centers the output, and M makes implicit ranges start from 0.

Improvements welcome.

Answer 13

R, 109 bytes

n=scan();m=(1:n%o%1:n%%n)[,-n];m[l<-lower.tri(m)]="";m[l[,n-1:n]]="";write.table(format(m,j="r"),,,F,r=F,c=F)

Try it online!

A full program with some hacking around the built-in formatting/printing functionality to achieve the desired result. Here is the printing statement with all the formatting options ungolfed:

write.table(format(m,justify="right"),quote=FALSE,row.names=FALSE,col.names=FALSE)

If printing the numbers left-aligned is allowed after all, then we can save a few extra bytes by removing right-justification.

Answer 14

C (gcc), 106 103 bytes

-3 thanks to @ceilingcat

w,i,j;f(a){for(w=2,i=a;i/=10;w++);for(;i++<a;puts(""))for(j=i;j+i<=a;)printf("%*d",j++-i?w:w*i,i*j%a);}

Try it online!