Using array.sort

[Chris Nicholson-Sauls]

Heinz wrote:
> Chris Nicholson-Sauls Wrote:
> 
>> Heinz wrote:
>>> Carlos Santander Wrote:
>>>
>>>> Heinz escribió:
>>>>> torhu Wrote:
>>>>>
>>>>>> Heinz wrote:
>>>>>>> What does this sort property do? how can i use it? how do i implement opCmp. thanks in advance
>>>>>> .sort sorts an array, using some default sort order.  If you want to 
>>>>>> change the order, you implement opCmp.
>>>>>>
>>>>>>
>>>>>> opCmp has got these signatures, think.  It doesn't seem to be documented 
>>>>>> much:
>>>>>>
>>>>>> class C {
>>>>>>      // the argument is of type Object, not C
>>>>>>      int opCmp(Object other);
>>>>>> }
>>>>>>
>>>>>> or:
>>>>>>
>>>>>> struct S {
>>>>>>     int opCmp(S other);
>>>>>> }
>>>>>>
>>>>>>
>>>>>> opCmp() has to return less than zero if it's own object is smaller, more 
>>>>>> than zero if 'other' is smaller, and zero if they are equal.
>>>>>>
>>>>>> You can't define opCmp for any other types, if you want to change the 
>>>>>> sort order of ints, you have to write a separate sort function.
>>>>> So, lets assume we have the following class:
>>>>>
>>>>> class myclass
>>>>> {
>>>>>         char[] cs;
>>>>> }
>>>>>
>>>>> and then we have a dinamic array of myclass, can we sort this array by the cs property?
>>>> Yes, you would have to write an opCmp that does that.
>>>>
>>>> -- 
>>>> Carlos Santander Bernal
>>> Hi,
>>>
>>> Do i have to declare opCmp(Object) and it'll do it automatically? or
>>> Do i have to write the algorithm to sort the classes by the cs property? if so, what's the sense of having a sort property if i have to implement a propietary function, it acts as a link.
>> opCmp needs only return the sort-order of an object in relation to another given object. 
>> Using a simpler example:
>>
>> class Number {
>>    int i;
>>
>>    int opCmp (Object obj) {
>>      if (auto other = cast(Number) obj) {
>>        return this.i - other.i;
>>      }
>>      else {
>>        throw new Exception("Can only compare Number with another Number.");
>>      }
>>    }
>> }
>>
>> Otherwise the .sort property would have no idea how to order the objects (there really is 
>> no generic means to do this).  IIRC, there is/was a default Object.opCmp which compared 
>> the address of objects, but this is really useless in terms of proper sorting.  (Although 
>> it does have the usefulness of making all classes available as associative array keys.)
>>
>> -- Chris Nicholson-Sauls
> 
> 
> Hi Chris, i still don't get it hahaha. Replace the int in your example by a char[] str; How the hell opCmp can compare a string, what value of type int should return? thx

Return any value less than 0 for "less than", 0 for "equal to", and any value greater than 
zero for "greater than".  (Most people default to -1,0,1.)  For a char[] you'd have to 
decide how you want to order characters... but one generic means is just to compare their 
character codes.  So something like:

class myclass {
   char[] cs;

   int opCmp (Object obj) {
     char oc ;

     if (auto other = cast(myclass) obj) {
       foreach (i, c; cs) {
         oc = other.cs[i];
         if (c < oc) {
           return -1;
         }
         else if (c > oc) {
           return 1;
         }
       }
       return 0;
     }
     else {
       throw new Exception("Can only compare myclass with myclass.");
     }
   }
}

-- Chris Nicholson-Sauls