rslt ← (func ##.until cond) argt            ⍝ Conditional function power.

NB: From Dyalog V11, this operator is implemented as primitive ⍣.

Left  operand  [func]  is  applied  cumulatively  to argument [argt] until right
operand  [cond],  applied  to the result, returns 'true'. Notice that, in common
with  the  :Repeat ··· :Until  control  structure,  the  test for termination is
performed  _after_  an  initial application of the left operand. This means that
the  operand  is  applied  once  even  if  the  original  argument satisfies the
termination  conditions.  Compare this with the →while← operator.

Examples:

      {⍵,'.'} until {60=⍴⍵} 'Note'          ⍝ Extend until 60 wide.
Note........................................................

      ↓ until {1=⍴⍴⍵} 2 2 2 2⍴⍳16           ⍝ Split until vector.
┌─────────────────────┬────────────────────────────┐
│┌─────────┬─────────┐│┌────────────┬─────────────┐│
││┌───┬───┐│┌───┬───┐│││┌────┬─────┐│┌─────┬─────┐││
│││1 2│3 4│││5 6│7 8│││││9 10│11 12│││13 14│15 16│││
││└───┴───┘│└───┴───┘│││└────┴─────┘│└─────┴─────┘││
│└─────────┴─────────┘│└────────────┴─────────────┘│
└─────────────────────┴────────────────────────────┘

      {⍵+1} until {1} 0                     ⍝ {⍵+1} applied once.
1

      {1↓⍵} while {' '∊⍵}  'hello world'    ⍝ While & until _concur_ if initial
world
      {1↓⍵} until {~' '∊⍵} 'hello world'    ⍝ ·· argument _escapes_ termination.
world

      {1↓⍵} while {'⎕'∊⍵}  'hello world'    ⍝ While & until _differ_ if initial
hello world
      {1↓⍵} until {~'⎕'∊⍵} 'hello world'    ⍝ ·· argument _incurs_ termination.
ello world

See also: cond while limit pow traj

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