What is the preferred method for testing the existence of a key in an associative array?

On Friday, 13 September 2013 at 14:03:48 UTC, Gary Willoughby wrote:
> What is the preferred method for testing the existence of a key in an associative array?

I use the in operator:

if(key in aa) {
  there
}

if(key !in aa) { not there}


You can also fetch the pointer right there too:

if(auto pvalue = key in aa) {
  auto value = *pvalue;
  // go ahead and use it
}

On Friday, 13 September 2013 at 14:03:48 UTC, Gary Willoughby wrote:
> What is the preferred method for testing the existence of a key in an associative array?

Well, I usually do it as:

int[string] someCache;

int getValue(string key)
{
    if (auto val = key in someCache)
        return *val;
    return someCache[key] = -3;
}

On Friday, 13 September 2013 at 14:06:50 UTC, Adam D. Ruppe wrote:
> I use the in operator:

Aha! ta.

On 2013-09-13 16:17, Orvid King wrote:

> Well, I usually do it as:
>
> int[string] someCache;
>
> int getValue(string key)
> {
>      if (auto val = key in someCache)
>          return *val;
>      return someCache[key] = -3;
> }

That doesn't work with generic code. I mean, -3 can be a legal value. There are many types that doesn't have an invalid value, like pointers do.

-- 
/Jacob Carlborg

On Saturday, 14 September 2013 at 10:50:17 UTC, Jacob Carlborg wrote:
> On 2013-09-13 16:17, Orvid King wrote:
>
>> Well, I usually do it as:
>>
>> int[string] someCache;
>>
>> int getValue(string key)
>> {
>>     if (auto val = key in someCache)
>>         return *val;
>>     return someCache[key] = -3;
>> }
>
> That doesn't work with generic code. I mean, -3 can be a legal value. There are many types that doesn't have an invalid value, like pointers do.

I don't understand. What doesn't work? If the key exists, val !is null, if it doesn't, val is null.

On Saturday, 14 September 2013 at 11:32:23 UTC, simendsjo wrote:
> On Saturday, 14 September 2013 at 10:50:17 UTC, Jacob Carlborg wrote:
>> On 2013-09-13 16:17, Orvid King wrote:
>>
>>> Well, I usually do it as:
>>>
>>> int[string] someCache;
>>>
>>> int getValue(string key)
>>> {
>>>    if (auto val = key in someCache)
>>>        return *val;
>>>    return someCache[key] = -3;
>>> }
>>
>> That doesn't work with generic code. I mean, -3 can be a legal value. There are many types that doesn't have an invalid value, like pointers do.
>
> I don't understand. What doesn't work? If the key exists, val !is null, if it doesn't, val is null.

I guess, he means your default value -3 isn't appropriate for all types.