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  • 1 Post By rcgldr

iterators

This is a discussion on iterators within the C++ Programming forums, part of the General Programming Boards category; I have the following code Code: //p.345 ex1 #include <iostream> #include <cstdlib> #include <ctime> #include <list> #include <vector> #define RND_RANGE ...

  1. #1
    Registered User hex_dump's Avatar
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    iterators

    I have the following code
    Code:
    //p.345 ex1
    
    #include <iostream>
    #include <cstdlib>
    #include <ctime>
    #include <list>
    #include <vector>
    
    #define RND_RANGE 10000
    #define TIMES (RND_RANGE)
    
    using namespace std;
    
    int main(void){
    
        srand(time(NULL));
    
        //generate 10,000 numbers between 0 and 9,999
        list<int> the_list;
        list<int>::iterator iter;
    
        vector<int> freqs;
        vector<int>::iterator p;
    
        //place in list<int> container
        for(int i = 0; i < TIMES; i++){
            int j = (rand() % RND_RANGE);
            the_list.push_back(j);
        }
    
        //iterate over values and display them
        int x = 0;
        for(iter = the_list.begin(); iter < the_list.end(); iter++){
            cout << "item: " << x << " = " << *iter << "..." << endl;
            if(x % 10 == 0)
                cin.get();
            
            x++;
        }
    
        //print the median value
        //computer number freqencies
        
        return 0;
    }
    when I comile it i get an error namely
    Code:
    /usr/include/c++/4.7/bits/stl_list.h:1603:5: note: template<class _Tp, class _Alloc> bool std::operator<(const std::list<_Tp, _Alloc>&, const std::list<_Tp, _Alloc>&)
    /usr/include/c++/4.7/bits/stl_list.h:1603:5: note:   template argument deduction/substitution failed:
    p345no1.cpp:33:51: note:   ‘std::list<int>::iterator {aka std::_List_iterator<int>}’ is not derived from ‘const std::list<_Tp, _Alloc>’
    is what I'm thinking is most significant.

    My question is it not possible to use
    Code:
    for(iter = the_list.begin(); iter < the_list.end(); iter++)
    do you have to use the synatx
    Code:
    for(iter = the_list.begin(); iter != the_list.end(); iter++)
    why is that? I assumed since i was iterating over it like a pointer using the increment operators it was probably stored in contiguous memory. Could have sworn I saw the < and > operators used to test it before.

  2. #2
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    Until one of the resident experts responds, I think you will have to use (at least try this to see if it fixes your compile error):

    Code:
    for(iter = the_list.begin(); iter != the_list.end(); iter++)
    I don't think that LIST overloads < and > operators (I'm not even sure if this would be possible).
    hex_dump likes this.

  3. #3
    Registered User hex_dump's Avatar
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    thanks rcgldr. That's what i used to get it to compile and run. I thought maybe the same as you too however I wanted to find out if that was truly the case. Thanks again.

  4. #4
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    two things that are worth pointing out:

    1. iterators are not pointers
    2. lists are not guaranteed, nor are they likely to have contiguous storage, due to their typical implementation as a doubly linked list.

    there's no way to know, without iterating through the remainder of the list. whether a particular node is less than the last+1 node. that is why the list iterator does not implement > or < operators.

    Edit: it should also be noted that it is never correct to say <= list.end(), even if it was possible to do so. end() returns the iterator that is one past the last element. you're going to run into all sorts of trouble if you try to use end() as the last element.
    Last edited by Elkvis; 05-13-2013 at 06:45 AM. Reason: additional info
    Code:
    namespace life
    {
        const bool change = true;
    }

  5. #5
    C++まいる!Cをこわせ! Elysia's Avatar
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    You should declare variables near first use. The iterator, for example, only needs be in the loop.
    You can also make it easier by using "auto", eg auto it = the_list.begin() (C++11 feature).
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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