• kdb+

  Missing some items

  Posted by sjt on April 25, 2022 at 12:00 am

  We encounter the enlist keyword early in learning q; a reliable way to make a one-item list where 1# wont do. But there is a bit more to enlist than that.

  In the first place, it is not only variadic taking various numbers of arguments it will take any number of arguments, even more than eight.

   

  q)enlist[`one;`two;`three;`four;`five;`six;`seven;`eight;`nine] `one`two`three`four`five`six`seven`eight`nine

   

  Of course, what it returns is a list, with each argument an item.

   

  q)enlist[`one;`two;`three`and`a`bit;`four;`five;`six;`seven;`eight;`nine]

  `one `two `three`and`a`bit `four `five `six `seven `eight `nine

   

  What is less obvious is that enlist is implicated in lists with missing items.

   

  q)type (`one;`two;`three) / symbol vector 
  11h 
  q)type (`one;::;`three) / mixed list 
  0h 
  q)type (`one;;`three) / a -- projection? 
  104h 
  q)(`one;;`three) ~ enlist[`one;;`three] 
  1b

   

  Is the missing item a generic null? No, a list with one or more missing items is a projection of enlist, and its rank is the number of missing arguments. We can apply and iterate it as any other projection.

   

  q)(`one;;`three;;`five)[`two;`four] 
  `one`two`three`four`five 
  q)(`one;;`three;;`five) . `two`four 
  `one`two`three`four`five 
  q)raze `quick`crafty`cunning(`the;;`brown;;`jumps)/::`fox`cat`dog 
  the quick brown fox jumps 
  the quick brown cat jumps 
  the quick brown dog jumps 
  the crafty brown fox jumps 
  the crafty brown cat jumps 
  the crafty brown dog jumps 
  the cunning brown fox jumps 
  the cunning brown cat jumps 
  the cunning brown dog jumps

   

  Which gives us a few tricks up our sleeves when generating test datasets, or preparing data for loading and ingestion.

  sjt replied 1 month, 1 week ago 1 Member · 0 Replies
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