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sorting problems

Started by ekrax Oct 30, 2004 at 6:35 PM 3 replies 1k views

Original Post

Oct 30, 2004 at 6:35 PM

hello, i've come to a point in my project where i need to sort some objects based on their vertical position. im using a vector for the objects, which i'm going to hafta change to a list since i wasnt thinking ahead. but im not sure how to sort using the sort member function. in my book it says i can pass it a comparison function. but it gives no examples and im not clear on the syntax. i also tried using vectors and coverting to a standard array and then use qSort (waste of memory, but i just wanted to try it out) ... i could get that to work either, since the compare function prototype is ... int comp (const void *, const void *) and i coudnlt get it to sort the y variable of my obj class. so i can see the sort in the list class is like this ... void sort(Comp cmpfn); how do i use this function? thanks for any help in advance.

pammatt

Oct 30, 2004 at 6:48 PM

The STL sort method does not work on lists, that's why list has its own sort method. They all work by using the objects less-than operator when comparing.

//Define the less-than operator for the class SomeObjstd::vector<SomeObj> v;//Add elements...//Sort the vector is ascending orderstd::sort(v.begin(), v.end());

Something like the above code. Just google for STL vector and list and you will see examples of how to use the sort methods.

ekrax

Author

ekrax

Oct 30, 2004 at 10:03 PM

so i basically have to overload < operator for my class and tell it to compare it's y variable.

so once i have overloaded < and do this ...
std::sort(v.begin(), v.end());

STL will do the rest?

pammatt

Oct 30, 2004 at 10:24 PM

Yep.

snk_kid

131

snk_kid

Oct 31, 2004 at 2:41 AM

Quote:
Original post by ekrax
so i basically have to overload < operator for my class and tell it to compare it's y variable.

so once i have overloaded < and do this ...
std::sort(v.begin(), v.end());

STL will do the rest?

That is not the only way, you don't have to overload relational operators, you can use member functions/free-functions/functor e.g.

#include <iterator>#include <algorithm>#include <list>#include <iostream>struct foo {  int i;  foo(int j = 0): i(j) {}};std::ostream& operator<<(std::ostream& out, const foo& f) {   return (out << f.i);}bool compare(const foo& f1, const foo& f2) {   return f1.i < f2.i;}int main() {  std::list<foo> foo_list;  foo_list.push_back(3);  foo_list.push_back(1);  foo_list.push_back(8);  foo_list.push_back(6);  std::copy(foo_list.begin(), foo_list.end(),            std::ostream_iterator<foo>(std::cout, "\n"));  std::cout << "\n\nsorting list\n";  foo_list.sort(compare);  std::copy(foo_list.begin(), foo_list.end(),            std::ostream_iterator<foo>(std::cout, "\n"));}

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