Insert new element into a vector

This is a discussion on Insert new element into a vector within the C++ Programming forums, part of the General Programming Boards category; I am trying to insert a new element into a vector in C++ under Ubuntu Linux. The following is what ...

  1. #1
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    Insert new element into a vector

    I am trying to insert a new element into a vector in C++ under Ubuntu Linux.

    The following is what I did, where v contain some element already. InsertElementNo is the element number the new content "entry" should be inserted.

    For example:

    v contains "Test,Is,Good"
    If entry = "Lord" and InsertElementNo = 2,
    After v.insert v should look like "Test,Lord,Is,Good"


    Code:
        vector <string> v;
        vector <string>::iterator pos = v.begin();
        .......
        v.insert(pos+(InsertElementNo-1), entry);
    However the above code gives me "segmentation error".

    Couldn't figure out why.

  2. #2
    C++ Witch laserlight's Avatar
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    Perhaps the problem lies in the code that you did not show, e.g., we do not know for sure what is v, pos, InsertElementNo and entry at this point:
    Code:
    v.insert(pos+(InsertElementNo-1), entry);
    In fact, if you tried to populate the vector in the "......." part, pos would likely be invalidated, thus the insertion would be based on an invalid iterator, which could explain the error.
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  3. #3
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    Quote Originally Posted by appointment View Post
    I am trying to insert a new element into a vector in C++ under Ubuntu Linux.

    The following is what I did, where v contain some element already. InsertElementNo is the element number the new content "entry" should be inserted.

    For example:

    v contains "Test,Is,Good"
    If entry = "Lord" and InsertElementNo = 2,
    After v.insert v should look like "Test,Lord,Is,Good"


    Code:
        vector <string> v;
        vector <string>::iterator pos = v.begin();
        .......
        v.insert(pos+(InsertElementNo-1), entry);
    However the above code gives me "segmentation error".

    Couldn't figure out why.
    How about this?

    Code:
    void do_insertion( vector<string>& v, const string& s, const unsigned int pos )
    {
        vector<string> buff;
        buffer.reserve( v.size()+1 );
        copy( v.begin(), v.begin()+pos, back_inserter(buff) );
        buff.push_back( s );
        copy( v.begin()+pos, v.end(), back_inserter(buff) );
        v.swap( buff);    
    }

  4. #4
    The larch
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    What's this reinventing the wheel? std::vector has a perfectly good insert method. (From performance view, this means that each insertion needs to allocate all that extra memory - which std::vector can avoid, and it means that the entire vector must be copied - imagine you want to insert something to the back part of a large vector.)

    The only thing looking bad in OP's code is how he stores an iterator to vector. Iterators are generally meant for instant use, rather than keeping around for longer, particularly std::vector::iterators as std::vector is rather prone to invalidate them.

    You can't go wrong with
    Code:
    v.insert(v.begin() + pos - 1, entry);
    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
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by anon
    You can't go wrong with
    Actually, it should be:
    Code:
    v.insert(v.begin() + InsertElementNo - 1, entry);
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  6. #6
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    Quote Originally Posted by anon View Post
    What's this reinventing the wheel? std::vector has a perfectly good insert method. (From performance view, this means that each insertion needs to allocate all that extra memory - which std::vector can avoid, and it means that the entire vector must be copied - imagine you want to insert something to the back part of a large vector.)

    The only thing looking bad in OP's code is how he stores an iterator to vector. Iterators are generally meant for instant use, rather than keeping around for longer, particularly std::vector::iterators as std::vector is rather prone to invalidate them.

    You can't go wrong with
    Code:
    v.insert(v.begin() + pos - 1, entry);
    thanks that worked.

    still dont know what's wrong with original way of doing it.

  7. #7
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    What probably was wrong is that somewhere in the code that you have simply as ..... , you are changing the size of vector v. If you change a vector, any previously created iterators will become invalid.

  8. #8
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    Quote Originally Posted by rossipoo View Post
    What probably was wrong is that somewhere in the code that you have simply as ..... , you are changing the size of vector v. If you change a vector, any previously created iterators will become invalid.
    ahhh i see thanks.

    i was inserting new element into the vector in ..... section by using v.push_back() inside a while loop.

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