Hash Array R←{X}1500⌶Y

This function creates a hashed array, returns an unhashed copy of an array or reports the state of hashing of an array.

Y may be any array.

If X is omitted, the result R is a copy of Y that has been invisibly marked as hashed. R behaves the same as Y in all respects. The only difference is that dyadic and related functions are expected to run faster when applied to a hashed array. The hash will be created the first time the array is used as an argument to or other set functions. The hashed property is preserved across assignments and argument passing, but in general is not preserved by any primitive functions.

If X is 1, the result R returns an indication of whether Y has been marked for hashing or whether the hash has been created:

R State of Y
0 Y has not been marked for hashing
1 Y has been marked for hashing, but the hash tables has not yet been created
2 Y has a hash table

If X is 2, the result R is the unhashed form of Y.


      R←1500⌶1 2 3    ⍝ R is marked for hashing
      1 (1500⌶)R
      S←R             ⍝ S is marked for hashing
      {⍵⍳2 3 5}R      ⍝ R is now hashed
      1 (1500⌶)R
      U←(⍴R)⍴R        ⍝ U is not hashed
      U←⊃⊂R           ⍝   ditto
      1 (1500⌶)U

If R is a hashed array then certain forms of modified assignment will preserve and efficiently update the hash table:

      R,←Y    ⍝ only for scalar or vector R
      R↓⍨←Y   ⍝ only for negative singleton Y


      R←1500⌶1 2 3 ⍝ R is hashed

      R,←5    ⍝ ,← preserves and updates
              ⍝ the hash table
1 2 3 5
      R⍳2 4 6
2 5 5

      R↓⍨←¯2  ⍝ ↓⍨← preserves and updates
              ⍝ the hash table
1 2
      R⍳2 4 6
2 3 3

The hashed property survives )SAVE/)LOAD and )SAVE/)COPY. It does not currently survive writing to a component file and reading back again.