std::list<*> sorting

This is a discussion on std::list<*> sorting within the C++ Programming forums, part of the General Programming Boards category; please could some one explain to me how i implement sorting code for the following: #include <iostream> #include <list> void ...

  1. #1
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    Angry std::list<*> sorting

    please could some one explain to me how i implement sorting code for the following:

    #include <iostream>
    #include <list>

    void main(void)
    {
    list<int *> intPtrList;
    int a = 3, b = 7, c = 5;

    intPtrList.push_back(&a);
    intPtrList.push_back(&b);
    intPtrList.push_back(&c);

    intPtrList.sort(); // doesnt work (it will sort the pointers)
    }

    as you see in the code intPtrList.sort() wount work,
    but list has an overloded version of sort that takes an parameter
    but i dont under stand how to use it. so could some one tell me what that parameter should me to allow me to sort the data correct...

    \\dummy

  2. #2
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    One solution is a predicate.

    Code:
    // Predicate dereferences the elements and compare them.
    std::sort(list.begin(), list.end, Predicate);
    
    // Ttry this solution.  I am not completely sure it works as it is a 
    // feature of the STL.
    
    std::sort(listl.begin(), list.end, std::greater<int *>());
    One solution for a predicate is a function object.

    Kuphryn

  3. #3
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    Angry

    sorry, your solutions didnt work =(
    they didnt even compile =(

    .ziruz

  4. #4
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    What is the error message?

    Add #include <algorithm>

  5. #5
    S Sang-drax's Avatar
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    Here's the solution

    Code:
    bool myCompare(int* a, int* b)
    {
        return *a < *b;
    }
    
    
    int main()
    {
      //...
      myIntList.sort(myCompare);
      //...
    }
    Or perhaps more general:

    Code:
    template<typename T>
    class myPtrCompare
    {
        public:
        bool operator ()(T* a, T* b)
        {
            return *a < *b;
        }
    };
    
    int main()
    {
      //...
      myIntList.sort(myPtrCompare<int>());
      //...
    }
    Last edited by Sang-drax; 10-04-2002 at 11:26 AM.
    Last edited by Sang-drax : Tomorrow at 02:21 AM. Reason: Time travelling

  6. #6
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    wrong thread
    Last edited by Jan79; 06-23-2003 at 10:59 AM.

  7. #7
    Toaster Zach L.'s Avatar
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    Probably need to include <functional> in there to for kuphryn's (for greate<T>).
    The word rap as it applies to music is the result of a peculiar phonological rule which has stripped the word of its initial voiceless velar stop.

  8. #8
    C++ Developer XSquared's Avatar
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    You're using new-style headers, but you forgot about namespaces.

    >>void main(void)
    No. Bad. Very bad. Use int main( void ).
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

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  9. #9
    Cat
    Cat is offline
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    Originally posted by Sang-drax
    Here's the solution

    Code:
    bool myCompare(int* a, int* b)
    {
        return *a < *b;
    }
    
    
    int main()
    {
      //...
      myIntList.sort(myCompare);
      //...
    }
    That won't work without ptr_fun() around it; myCompare is not a functor here.

    The other code works much better, but a few changes to make it const-correct (you can get a lot of bizarre error messages if it's not):

    Code:
    template<typename T>
    class myPtrCompare
    {
        public:
        bool operator ()(const T* a, const T* b) const
        {
            return *a < *b;
        }
    };
    
    int main()
    {
      //...
      myIntList.sort(myPtrCompare<int>());
      //...
    }

  10. #10
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    Originally posted by Cat
    That won't work without ptr_fun() around it; myCompare is not a functor here.
    No it is correct (try it).
    You only need to 'wrap' the pointer in ptr_fun,if you need binder (bind2nd for example).

  11. #11
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    sorting a list is also painfully slow, If I may suggest vector and std::sort(begin,end,cmp) or a sorted container like set/multiset.

  12. #12
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    Originally posted by grib
    sorting a list is also painfully slow,
    Why?
    Both sorts (list.sort and sort) run in O(n*log(n)).
    list.sort is a mergesort,and sort is introsort.

  13. #13
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    Hmm... Maybe I am remembering incorrectly here, but doesn't the sort function not work on pointers?

    I was required to write 5 or 6 sort algorithms and we had to re-write the swap function (part of the sort function) to work with pointers.

    Of course, that was a while ago...
    /*When all else fails, Immortality can be achieved through Massive Failure*/

  14. #14
    Toaster Zach L.'s Avatar
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    >> Hmm... Maybe I am remembering incorrectly here, but doesn't the sort function not work on pointers?

    It will work. If you are using pointers though, the default behavior of the sorting algorithms will be to compare the pointers (i.e. the memory addresses), and sort those, rather than sorting the data by value.

    >>you can get a lot of bizarre error messages if it's not

    I ran into some of those last week... They drove me insane for about half an hour.

    >>No it is correct (try it).

    Probably a default template argument.
    The word rap as it applies to music is the result of a peculiar phonological rule which has stripped the word of its initial voiceless velar stop.

  15. #15
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    It will work. If you are using pointers though, the default behavior of the sorting algorithms will be to compare the pointers (i.e. the memory addresses), and sort those, rather than sorting the data by value.
    Exactly. how does one fix that? Like I mentioned, we rewrote the swap function, but we also had to change the operator<

    - SB
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