passing a vector so it will be iterated in reverse

This is a discussion on passing a vector so it will be iterated in reverse within the C++ Programming forums, part of the General Programming Boards category; Hi, this is what I'm trying to dor: Code: #include <iostream> #include <vector> #include <algorithm> #include <boost/lambda/lambda.hpp> using namespace std; ...

  1. #1
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    passing a vector so it will be iterated in reverse

    Hi,

    this is what I'm trying to dor:

    Code:
    #include <iostream>
    #include <vector>
    #include <algorithm>
    #include <boost/lambda/lambda.hpp>
    
    using namespace std;
    
    void func(vector<char>::const_iterator start, vector<char>::const_iterator end)
    {
    	for_each(start, end, cout << boost::lambda::_1 << " ");
    }
    
    int main()
    {
    	vector<char> v;
    	v.push_back('A');
    	v.push_back('B');
    	v.push_back('C');
    
    	func(v.begin(), v.end());  // prints "A B C"
    	cout << endl;
    
    	func(v.rbegin(), v.rend()); // error C2664: 'func' : cannot convert
               //parameter 1 from 'std::reverse_iterator<_RanIt>' to 'std::_Vector_const_iterator<_Ty,_Alloc>'
    	cout << endl;
    }
    I've never used reverse iterating so far. How would I have to change the example so the vector gets printed out in reverse (without changing func) like "C B A"?

    Thank you in advance!

  2. #2
    C++まいる!Cをこわせ! Elysia's Avatar
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    Why not
    func(v.end(), v.begin());
    ?
    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.

  3. #3
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    Because func will do start++ in the hope to get to the next element. The minimal example doesn't work then, too. An assertion is thrown

  4. #4
    The larch
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    Since forward and reverse iterators are a different type, why not use templates?

    Code:
    template <class Iterator>
    void func(Iterator start, Iterator end)
    {
    	for_each(start, end, cout << boost::lambda::_1 << " ");
    }
    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).

  5. #5
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    Wow, thanks, that seems to do the job.
    I've no experience with creating templates. Is there a way to restrict the compiler to generate the code for my func-template only if Iterator is of type

    • vector<char>::const_iterator
    • vector<char>::iterator
    • vector<char>::const_reverse_iterator
    • vector<char>::reverse_iterator


    ?

    Else it would be pretty insecure to let clients call that code (with arbitrary types), wouldn't it?

    Thank you!

  6. #6
    C++まいる!Cをこわせ! Elysia's Avatar
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    Well, if they don't have the operators or functions that your code uses, it won't compile, so it's still relatively safe.
    Otherwise they could send their own iterators to your function and it should work fine as long as they follow how the STL iterators were made.
    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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    Thanks, that sounds reasonable.

  8. #8
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by Elysia View Post
    Why not
    func(v.end(), v.begin());
    ?
    Are you nuts?!
    'end' is not dererencable, and
    'begin' is not one past the last real element, and
    those are still forward iterators, that will of course attempt to iterate forwards.

    It's posts like this where I'm reminded of how much you really know about C++, contrary to that way you've got half the beginners here thinking you're an expert.
    My homepage
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  9. #9
    C++まいる!Cをこわせ! Elysia's Avatar
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    I am not knowledgeable about the standard library, iterators, etc.
    Sorry, it was a stab in the dark.
    It just made sense to pass them in reverse. I don't know how for_each works.
    But I do know how std::vector.end() works. I know that it points to one element beyond the array.
    Last edited by Elysia; 02-27-2008 at 11:39 AM.
    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.

  10. #10
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by pheres View Post
    Wow, thanks, that seems to do the job.
    I've no experience with creating templates. Is there a way to restrict the compiler to generate the code for my func-template only if Iterator is of type
    Look at boost::enable_if. This is what it's for.

    Else it would be pretty insecure to let clients call that code (with arbitrary types), wouldn't it?
    I wouldn't chalk it up as a security problem, just a design problem. If a type should only be an iterator, then you should enforce that. But ultimately it will be enforced at compile time if you pass a type that is not suitable. You'll get an error message either way, it's just a question of which error it is.

    EDIT: Also, I'd allow pointer types as well as iterators. Iterators are basically generalized pointers, so a pointer can be seen as a kind of iterator (and the standard algorithms accept them as iterators, so you should probably follow that).
    Last edited by brewbuck; 02-27-2008 at 11:27 AM.

  11. #11
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    Quote Originally Posted by iMalc View Post
    Are you nuts?!
    'end' is not dererencable, and
    'begin' is not one past the last real element, and
    those are still forward iterators, that will of course attempt to iterate forwards.

    It's posts like this where I'm reminded of how much you really know about C++, contrary to that way you've got half the beginners here thinking you're an expert.
    Oh oh, time to decrement that "Quoted 4 times" line.

  12. #12
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by cpjust View Post
    Oh oh, time to decrement that "Quoted 4 times" line.
    Heh. No, I don't think so. It has a great effect when I help someone!
    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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