Since J allow you to write short code, here are three example in J. The first use iota1000, the second a random permutation, and the third use matrix notation to create a little guessing game.

Example 1: Find the missing number

  xor =: (16 + 2b0110) b.
  iota1000 =: (i. 1000) 
  missingNumber =: (xor/ iota1000) xor (xor/ iota1000 -. 129) 
  echo 'The missing number is ' , ": missingNumber

This print 'The missing number is 129'

Example 2: Using a random permutation, find the missing number.

   permuted =: (1000 ? 1000)
   missingNumber = (xor/ permuted) xor (xor/ permuted -. ? 1000)

 

Example 3: find the missing number in this matrix.

  _ (< 2 2) } 5 5 $ (25 ? 25) 

   12  9  1 20 19
    6 18  3  4  8
   24  7  _ 15 23
   11 21 10  2  5
    0 16 17 22 14

Final test: repeat 10 times the example 3 (random matrices) and collect the time it takes you to solve it in a list of times, then compute the linear regression best fit by

  times %. (1 ,. i. 10)

Did you get better at solving it by playing more times?

I am not affiliated with J, but in case you want to try some J code there is a playground: https://jsoftware.github.io/j-playground/bin/html2/

Edited: It seems I am procrastinating a lot about something I have to do but don't want to.

the top comment trick [ n, 1, n + 1, 0 ][n % 4] can be implemented in J as following :

   f =: ]`1:`>:`0:@.(4&|)"0

Then:

   (,. ; #: ; [: #: f) i.16

 0    0 0 0 0    0 0 0 0    
 1    0 0 0 1    0 0 0 1    
 2    0 0 1 0    0 0 1 1    
 3    0 0 1 1    0 0 0 0    
 4    0 1 0 0    0 1 0 0    
 5    0 1 0 1    0 0 0 1    
 6    0 1 1 0    0 1 1 1    
 7    0 1 1 1    0 0 0 0    
 8    1 0 0 0    1 0 0 0    
 9    1 0 0 1    0 0 0 1    

....

Hello fellow J programmer. In statistic you can estimate the population size by coloring fishes that you put in a lake and some time later you fish in that lake, the proportion of colored fishes allow you to estimate the size of the population. Four month ago I posted some J identities [1] and you were the only one which commented, that means that in this capture I only got one fish and it was the colored fish. This imply that there must be very few J programmers HN, or more precisely, very few of them that post J related material.

The parent's comment (also mine) has a style that was designed not to scare non J programmers. One should also consider that some people dislike J code so downvotes are the usual result except when the post provides some additional insight.

Finally, thank you for this small J lesson, is a pleasure to find here fellow J programmers.

[1] https://news.ycombinator.com/item?id=42859077