count how many lines, allocate vector, and read again

This is a discussion on count how many lines, allocate vector, and read again within the C++ Programming forums, part of the General Programming Boards category; dear there, I need to read a large file line by line into a vector, and it is inefficient to ...

  1. #1
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    count how many lines, allocate vector, and read again

    dear there,
    I need to read a large file line by line into a vector, and it is inefficient to let vector resize itself as needed. so i would like to count how many lines are there, reserve the vector size, and read from the beginning of the file line by line again.
    can anyone give me an idea how to count, and how to move point to beginning of the file again? Thanks.

  2. #2
    Cat without Hat CornedBee's Avatar
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    The rewind() function resets the file position to the start.

    Although, with a smart standard library I'll contend that reading the file twice is not necessarily faster than letting the vector resize, especially if you give it a good initial size. For example, Microsoft's STL in VS 2008 is smart, and will not create copies of all the strings during a resize. So is GCC's STL in 4.3 and higher, if you enable C++0x features (rvalue references).
    All the buzzt!
    CornedBee

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  3. #3
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    If the file is very large, it is extremely likely that it will take longer to read it twice.

    Also, if you do your own resizing (e.g. reserve double the size when you run out of current size), you can reduce the number of times the vector is resized.

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    Mats
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  4. #4
    Cat without Hat CornedBee's Avatar
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    Vector usually doubles its size anyway.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  5. #5
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    >> it is inefficient to let vector resize itself as needed
    As indicated by the other posters, this may not be true. Did you profile it or are you just making that assumption?

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