How can I sort a list when the sorting criterion requires an extra variable? C++

Question

Hi there, this is for an assignment so I will be deliberately general. My question is related to implementation decisions I already made--maybe they weren't good ones.

I have a list of pointers to structs, e.g. list<MyStruct*> bob; At one point I've needed to sort these pointers by one of the data members of their targets and I was able to do that easily with

bool sortbyarrival(const MyStruct* a, const MyStruct* b) {
return a->arrival < b->arrival;
}

And then calling bob.sort(sortbyarrival); Works great.

Now somewhere else I need to sort by a different criterion, which involves a counter in the program. I need something like return counter*a->arrival < counter*b->arrival; But the way I just described is the only way I know how to do a sort, I think, and I don't know how to pass my counter as an additional argument. How can I sort this list of pointers?

ETA: The counter is just a variable in main. So ideally I could call something like bob.sort(sortbyratio, counter); or sort(bob.begin(), bob.end(), sortbyratio, counter);

Answer 1

Just create an function-object, a class/struct with an overload of operator() that does the right thing for you. In this case, taking into consideration the extra variables. Then, you pass an instance of it to the sort method.

struct my_comparison : binary_function<MyStruct const*, MyStruct const*, bool> 
{
  bool operator()(MyStruct const* a, MyStruct const* b) 
  { 
    return (a->counter * a->arrival) < (b->counter * b->arrival);
  }
};

//Use it this way.
my_comparison comp;

//Set the arrival and counter data in instance comp.
/* ... */

//Now, pass it to the list.
bob.sort(comp);

EDIT: I just noticed that you have a list of pointers so I changed a bit the struct.

Answer 2

Similar to ltcmelo's example, but if the objects themselves don't contain the counter:

struct sort_with_counter {
    sort_with_counter(const double d): counter(d) {}

    bool operator()(const MyStruct* a, const MyStruct* b) {
        return(counter*a->arrival < counter*b->arrival);
    }

    const double counter;
};

mylist.sort(sort_with_counter(5.0));

If your counter is an external variable like that though it won't affect the ordering (at least if it's positive - thanks onebyone!) - so this may in fact not be necessary at all (or maybe I misunderstand what you're after?). It's a useful technique in other cases though.

Answer 3

Create a functor, and store the extra value in the functor object:

struct CompareByCounter {
    CompareByCounter(int c) : counter(c) {}
    bool operator()(const MyStruct *lhs, const MyStruct *rhs) {
        return (counter * lhs->arrival) < (counter * rhs->arrival);
    }
private:
    int counter;
};

// sort ascending
bob.sort(CompareByCounter(1));
// sort descending
bob.sort(CompareByCounter(-1));