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
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
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.