iterators & lists

This is a discussion on iterators & lists within the C++ Programming forums, part of the General Programming Boards category; if I am running through a list list<Class A*> lclassA; list<Class A*>::iterator = iter; for(iter = lclassA.begin(); iter!=lclassA.end(); ++iter) { ...

  1. #1
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    iterators & lists

    if I am running through a list

    list<Class A*> lclassA;
    list<Class A*>::iterator = iter;

    for(iter = lclassA.begin(); iter!=lclassA.end(); ++iter)
    {
    var1 = (*iter)->Getabba();
    var2 = (*iter+1)->Getabba()

    }

    how can I increment iter to look at iter+1 but maintain the position of iter. as in if my list has 4 vars, I want to compare 1 to 2, the 2 to 3, the 3 to 4, then stop.

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Have two iterators?

  3. #3
    The larch
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    I have also used these functions:

    Code:
    template <class ListIterator>
    ListIterator previous(ListIterator it)
    {
        return --it;
    }
    
    template <class ListIterator>
    ListIterator next(ListIterator it)
    {
        return ++it;
    }
    
    ...
    if (*it == sth && *next(it) == sth)
    In your case having two might be simpler (and you would be checking the wrong one for the end).
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  4. #4
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    Quote Originally Posted by lawrenced View Post
    if I am running through a list

    list<Class A*> lclassA;
    list<Class A*>::iterator = iter;

    for(iter = lclassA.begin(); iter!=lclassA.end(); ++iter)
    {
    var1 = (*iter)->Getabba();
    var2 = (*iter+1)->Getabba()

    }

    how can I increment iter to look at iter+1 but maintain the position of iter. as in if my list has 4 vars, I want to compare 1 to 2, the 2 to 3, the 3 to 4, then stop.
    try this:

    Code:
    list<Class A*> lclassA;
    list<Class A*>::iterator = iter;
    
    for(iter = lclassA.begin(); iter!=lclassA.end(); ++iter)
    {
    var1 = (*iter)->Getabba();
    var2 = (*(iter+1))->Getabba()
    
    }

  5. #5
    C++ Witch laserlight's Avatar
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    Elkvis' proposed code will not work (aside from the errors repeated from the original code snippet) because std::list iterators do not have operator+ overloaded since they are not random access iterators.
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  6. #6
    "Why use dynamic memory?"
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    Quote Originally Posted by laserlight View Post
    Elkvis' proposed code will not work (aside from the errors repeated from the original code snippet) because std::list iterators do not have operator+ overloaded since they are not random access iterators.
    yep.... i don't think many people know that. I have seen a lot of topics started in some forum with the same problem
    you can't use iteratros on lists
    "C makes it easy to shoot yourself in the foot; C++ makes it harder, but when you do, it blows away your whole leg."-Bjarne Stroustrup
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  7. #7
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by Hussain Hani
    you can't use iteratros on lists
    That is false, as tabstop implied and anon outlined in a code example.
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  8. #8
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    Quote Originally Posted by laserlight View Post
    Elkvis' proposed code will not work (aside from the errors repeated from the original code snippet) because std::list iterators do not have operator+ overloaded since they are not random access iterators.
    Which brings me a question of my own: are random access iterators required to have operator+? Anywhere I tried they were present, but I couldn't find anything about them being required in the standard. Or maybe I just didn't look properly.

  9. #9
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by EVOEx
    Which brings me a question of my own: are random access iterators required to have operator+?
    Yes.
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  10. #10
    Guest Sebastiani's Avatar
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    >> Which brings me a question of my own: are random access iterators required to have operator+? Anywhere I tried they were present, but I couldn't find anything about them being required in the standard. Or maybe I just didn't look properly.

    I believe so.
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  11. #11
    The larch
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    Code:
    Table 7--Random access iterator requirements (in addition to bidirectional iterator)
    
      +------------------------------------------------------------------------------------+
      |expression       return type         operational             assertion/note         |
      |                                      semantics            pre/post-condition       |
      +------------------------------------------------------------------------------------+
      |r += n       X&                    { Distance m =                                   |
      |                                   n;                                               |
      |                                     if (m >= 0)                                    |
      |                                       while (m--)                                  |
      |                                   ++r;                                             |
      |                                     else                                           |
      |                                       while (m++)                                  |
      |                                   --r;                                             |
      |                                     return r; }                                    |
      +------------------------------------------------------------------------------------+
      |a + n                              { X tmp = a;                                     |
      |             X                       return tmp +=   a + n == n + a.                |
      |                                   n; }                                             |
      |n + a                                                                               |
      +------------------------------------------------------------------------------------+
      |r -= n       X&                    return r += -n;                                  |
      +------------------------------------------------------------------------------------+
      |a - n        X                     { X tmp = a;                                     |
      |                                     return tmp -=                                  |
      |                                   n; }                                             |
      +------------------------------------------------------------------------------------+
      |b - a        Distance              (a<b)? dis-       pre: there exists a value n of |
      |                                   tance(a,b):       Distance such that a + n == b. |
      |                                   -distance(b,a)    b == a + (b - a).              |
      +------------------------------------------------------------------------------------+
      |a[n]         convertible to T      *(a + n)                                         |
      +------------------------------------------------------------------------------------+
      |a < b        convertible to bool   b - a > 0         < is a total ordering relation |
      +------------------------------------------------------------------------------------+
      |a > b        convertible to bool   b < a             > is a total ordering relation |
      |                                                     opposite to <.                 |
      +------------------------------------------------------------------------------------+
      |a >= b       convertible to bool   !(a < b)                                         |
      +------------------------------------------------------------------------------------+
      |a <= b       convertible to bool   !(a > b)                                         |
      +------------------------------------------------------------------------------------+
    Hey, there are things I didn't know:

    Code:
    #include <vector>
    
    int main()
    {
        std::vector<int> vec(10);
        std::vector<int>::iterator it = vec.begin();
        it[1] = 3;
    }
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

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