Using array.sort

Heinz billgates at microsoft.com
Wed Jan 24 21:03:01 PST 2007


Chris Nicholson-Sauls Wrote:

> 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


Thanks chris, i finally understood. This example is much clearer than the other one.
We could also use in this case std.string.icmp(), as it returns the same values.

Thanks man, good luck.


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