digitalmars.D.learn - Search for, o/w create element for AA
- Q. Schroll (25/25) Jun 19 2017 Trying to implement some random function I encountered this:
- =?UTF-8?Q?Ali_=c3=87ehreli?= (4/8) Jun 19 2017 aa.get(key, defaultValue)
- Q. Schroll (4/13) Jun 19 2017 aa.get returns the defaultValue if the key is not in the AA, but
- Moritz Maxeiner (9/14) Jun 19 2017 AFAIK the builtin associate array implementation exposes neither
Trying to implement some random function I encountered this: uint randFunc(uint x) { static uint[uint] vals; if (auto r = x in vals) return *r; return vals[x] = uniform!uint(); } I have to lookup x twice and it seems that there is no way around it. Can't I tell the AA to set a value for a given key if it doesn't already have one (1) with only one lookup, and (2) in a safe way? For the semantics, that should behave like this: V set(K, V)(ref const(V)[const(K)] aa, auto ref const(K) key, lazy const(V) value) { if (auto valPtr = key in aa) return *valPtr; // cast away const as initialization is okay: return (cast() aa[key]) = value(); } The function is dual to AA's get. It would make it possible for an AA with const value type to have values added which is perfectly fine for arrays. All I have seen so far, one cannot safely add values to them as initialization and assignment cannot be distinguished.
Jun 19 2017
On 06/19/2017 08:19 AM, Q. Schroll wrote:Can't I tell the AA to set a value for a given key if it doesn't already have one (1) with only one lookup, and (2) in a safe way?aa.get(key, defaultValue) https://dlang.org/spec/hash-map.html#properties Ali
Jun 19 2017
On Monday, 19 June 2017 at 16:54:46 UTC, Ali Çehreli wrote:On 06/19/2017 08:19 AM, Q. Schroll wrote:aa.get returns the defaultValue if the key is not in the AA, but does not add the key-value pair (x, defaultValue) to the AA. I'd find it rather surprising if it did.Can't I tell the AA to set a value for a given key if it doesn't already have one (1) with only one lookup, and (2) in a safe way?aa.get(key, defaultValue) https://dlang.org/spec/hash-map.html#properties Ali
Jun 19 2017
On Monday, 19 June 2017 at 15:19:19 UTC, Q. Schroll wrote:I have to lookup x twice and it seems that there is no way around it. Can't I tell the AA to set a value for a given key if it doesn't already have one (1) with only one lookup, and (2) in a safe way?AFAIK the builtin associate array implementation exposes neither such a function, nor other functions which you can use to assemble this, so no, you cannot do this with only one lookup (currently). Though I can't see why it couldn't be added, seems fairly straight forward. The best you can do with the builtin associative arrays right now is make sure that the `toHash` of your key type is cached.
Jun 19 2017