Thread: Removing duplicate values from std::list

  1. 11-03-2010 #1
    Phenom
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    Removing duplicate values from std::list

    I have an std::list<BallPair> list, where BallPair is a custom class that overloads operator==. In the list there are duplicate values, which I want to remove. I tried the following:

    Code:
    typedef std::list<BallPair>::iterator BallPairListItor;
    Code:
    for(BallPairListItor i = l.begin(); i != l.end(); ++i)
        for(BallPairListItor j = l.begin() + 1; j != l.end()  - 1; ++j)
            if(*i == *j)
                j = l.erase(j);
    but unfortunately list iterators do not provide operator + or operator -.

    I also tried list::unique but at unique - C++ Reference says:
    Notice that an element is only removed from the list if it is equal to the element immediately preceding it.
    Any ideas?
  2. 11-03-2010 #2
    laserlight
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    Is it okay for you to sort the linked list? That is one way to get it to work with std::unique, and is certainly more efficient.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way
  3. 11-03-2010 #3
    Phenom
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    The BallPair class just holds two pointers(to Ball objects). If b1, b2 are pointers then
    BallPair(b1, b2) will be equal to BallPair(b2, b1).

    So, no, there isn't a way to sort the list to make unique work.
  4. 11-03-2010 #4
    laserlight
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    In that case, a possible solution is to go through the list of BallPair objects, and then swap the pointers such that the second pointer is not less than the first pointer. Then, you can sort them according to the first and second pointers.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way
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