"Mike" <ha*****@impkin.com> wrote in message
news:200402181253253492%haladie@impkincom...
Hi there,
I'm very new to the STL and am struggling a bit with a sort, and would
love some help if possible!
I have a simple array V of objects of class OBJ that I want to rank in
order of the float OBJ.t and put the ranks into a list of ints, call it
RANK, so I can then access the OBJ with the i-th smallest value of t by
V[RANK[i]].
This is hideous and does not take advantage of STL's sort algorithm.
I want to use a sort(random_access_iterator first, random_access_iterator
last, compare comp) for this but I don't know how.
I'd like to sent up with something *like*
#include <algorithm>
#include <functional> // I'm assuming I need this included
class OBJ {
public:
OBJ() { t = 0.0; }
~OBJ();
float
t;
};
class compOBJ : public binary_function<OBJ &o1, OBJ &o2, bool>
{
public:
bool operator()(OBJ &o1, OBJ &o2)
{ return (o1.t < o2.t); }
};
int main()
{
vector<OBJ>
V(10);
vector<OBJ*>
S(10); // vector of pointers to OBJ
int i;
for (i = 0; i < 10; i++)
V.t = (float) rand(); // put in some random vals
S.sort(V.first, V.last, compObj); // sort the objects in V and put their
pointers in order into S
OBJ *ob;
for (i = 0; i < 10; i++)
cout << i << "-ranked OBJ pointer is " << S[i] << endl;
}
------------
Is that at all possible or am I going about it entirely the wrong way?
Both, its possible, and you are going about it the wrong way.
All code is untested.
To start with you need to be clear what you are sorting. In your description
you talk about a list (or maybe a vector) of ints called RANK, in your code
you use a vector of pointers called S. I'm going to assume you want a vector
of ints. Since you are sorting a vector of ints, your sort criterion has to
compare two integers. Like this
class RankCmp : public binary_function<int, int, bool>
{
public:
bool operator()(int i1, int i2) const
{
...
And you do the sort of RANK something like this
sort(RANK.begin(), RANK.end(), ...);
Now for the two bits that say ...
The RankCmp::operator() is clearly going to have to refer back to the
original object vector, so the RankCmp class needs a reference to that
vector, i.e.
class RankCmp : public binary_function<int, int, bool>
{
public:
RankCmp(const vector<OBJ>& o) : obj(o) {}
bool operator()(int i1, int i2) const
{
...
}
private:
const vector<OBJ>& obj;
};
The constructor has been added so the obj member variable can be
initialised.
Now the operator() is easy since we have a reference to the original vector,
it's just
bool operator()(int i1, int i2) const
{
return obj[i1].t < obj[i2].t;
}
And finally to fill in the last blank start the sort off with
sort(RANK.begin(), RANK.end(), RankCmp(V));
I guess you were struggling with the idea that a functor (like RankCmp) can
have member variables. It's a very powerful idea which is the great
advantage of functors over traditional function pointers.
john
