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.

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

On Monday, 19 June 2017 at 16:54:46 UTC, Ali Çehreli wrote:
> 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

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.

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.