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.

Examples

      R←1500⌶1 2 3    ⍝ R is marked for hashing

      1 (1500⌶)R
1
      S←R             ⍝ S is marked for hashing
      {⍵⍳2 3 5}R      ⍝ R is now hashed
      1 (1500⌶)R
2
      U←(⍴R)⍴R        ⍝ U is not hashed
      U←⊃⊂R           ⍝   ditto
      1 (1500⌶)U
0

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
      R↓⍨←Y   ⍝ only for negative singleton Y

Examples

      R←1500⌶1 2 3 ⍝ R is hashed

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

      R↓⍨←¯2  ⍝ ↓⍨← preserves and updates
              ⍝ the hash table
      R
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.