using push_back on an iterator

This is a discussion on using push_back on an iterator within the C++ Programming forums, part of the General Programming Boards category; hi all i want to be able scan text moving from word to word with an iterator and which ever ...

  1. #1
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    using push_back on an iterator

    hi all
    i want to be able scan text moving from word to word with an iterator and which ever that word the iterator is on push it into a vector

    this is what i have but i get an error
    Code:
    string format_paragraph(vector<string> words, int text_width)
    
    vector<string>::iterator pos;
    for(pos=words.end();pos != words.begin(); pos++)
    words.push_back(pos);

  2. #2
    Registered User Sake's Avatar
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    push_back takes a value_type. Try dereferencing the iterator.
    Code:
    words.push_back(*pos);
    Kampai!

  3. #3
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    nice thanx a lot it works but the only problem is it segfaults.
    any other ideas?

  4. #4
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    Quote Originally Posted by twigboy
    Code:
    for(pos=words.end();pos != words.begin(); pos++)
    What is wrong with that code?

  5. #5
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    well i should proly decrement pos so it should be pos-- but other than that i dont know

  6. #6
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    Good start.

    Look into reverse iterators if you really want to iterate in reverse.

    Also, always remember that you shouldn't use the iterator at foo.end() - it is bad to use it by dereferencing, you might just get a seg fault - the only reason it is there is for comparison to see if you are done.

  7. #7
    Registered User Sake's Avatar
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    Quote Originally Posted by Daved
    What is wrong with that code?
    Ah, tunnel vision sucks.
    Code:
    for (pos = words.end() - 1; pos != words.begin(); --pos)
      words.push_back(*pos);
    words.push_back(*pos);
    However, notice that the first item in the vector is ignored by the loop since it's a stopping condition. That's why I added another push_back after the loop. A better solution is what Daved suggested.
    Code:
    vector<string>::reverse_iterator pos;
    for (pos = words.rbegin(); pos != words.rend(); ++pos)
      words.push_back(*pos);
    Kampai!

  8. #8
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    thanx guys thats awsome

  9. #9
    Registered User hk_mp5kpdw's Avatar
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    Isn't the push_back operation going to invalidate the iterator? Once you start pushing the strings back onto the very same vector you are iterating over it is going to invalidate those iterators you are using in the loop. Am I the only one that sees this? You shouldn't iterate over the same container that you are simultaneously modifying. You probably need to use a second vector to push onto.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
    -Christopher Hitchens

  10. #10
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    >Am I the only one that sees this?
    I was thinking the same thing, but thought maybe I was missing something. It seems you would need a second vector.

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