ElementComparator


#1

Are there any examples on how to use this? Basically, I have an array of objects of a class I defined, and the class contains an int and I want to sort by the value of that int. How easy would this be?


#2

Err…


#3

eh?


#4

Ok, here’s what I’m trying right now that’s not working…
i have a class SampleData with these variables:

	StringArray files;
	Array<int> velLayers;
	String name;
	int loKey;
	int highKey;
	int rootKey;

i have no idea how to use operators but i made a guess and came up with this:

	bool operator>  (const SampleData& other) {
		return rootKey>other.rootKey;
	}

	bool operator<  (const SampleData& other) {
		return rootKey<other.rootKey;
	}


	bool operator==  (const SampleData& other) {
		return rootKey==other.rootKey;
	}

is this the right way to do it?

then i have an OwnedArray of type SampleData:

OwnedArray<SampleData> sampsL;

and i want to sort it so i tried this:
FloatElementComparator<SampleData*> sorter; sampsL.sort(sorter);
but it doesn’t work… i know this is probably my lack of experiance w/ c++ so any help as to where i went wrong would be appreciated… :slight_smile:


#5

I might be wrong (I haven’t used FloatElementComparator myself), but isn’t the problem that FloatElementComparator is meant to only be used with floating point data types? I think what you probably want to do is write your own subclass of ElementComparator to handle your SampleData class.

(the operator stuff looks right to me, by the way)

  • Niall.

#6

well it compiles ok, it just doesn’t do anything. i took a look at the floatcomparator class and all it does is check to see if a < b a==b or a>b so shouldn’t that just use my custom operators?


#7

Oh, I see what you mean :oops:. Could it perhaps be that you’re initialising the element comparator with a pointer instead of the actual data type then? i.e. it’s comparing two addresses instead of the actual class itself.

  • Niall.

#8

that makes sense, so if that’s the case then the easiest way to do it would probably be to subclass elementcomparator… here’s floatcomparator

00224 template <class ElementType>
00225 class FloatElementComparator  : public ElementComparator<ElementType>
00226 {
00227     int compareElements (ElementType first,
00228                          ElementType second)
00229     {
00230         return (first < second) ? -1
00231                                 : ((first == second) ? 0
00232                                                      : 1);
00233     }
00234 };

so i have no idea how to change this to work with my type… in java you could just cast to your class and then perform the checks, is there a way to cast like this in c++?


#9

Yeah, basically you just want to write a simple subclass like this:

template <class ElementType> class NiallsElementComparator : public ElementComparator<ElementType> { int compareElements (ElementType first, ElementType second) { return (*first < *second) ? -1 : ((*first == *second) ? 0 : 1); } };
i.e. you’re comparing the contents of the object rather than the pointer to it (because first and second in this case are pointers to your SampleData objects, held by the sampsL OwnedArray).

  • Niall.

#10

doh, i feel stupid :stuck_out_tongue: … thank you :smiley:


#11

No problem :smiley:

  • Niall.

#12