Thread: Is what I think is gonna happen gonna happen

  1. #1
    Registered User
    Join Date
    Oct 2005

    Is what I think is gonna happen gonna happen

    I'm trying to get rid of some inefficiencies in my code. The current (simplified) version is this:

    class matrix {
    double data*; matrix& operator=(const matrix& m) {
    (for each element of { data[i] =[i];}
    } matrix getminor(range_of_minor) {
    matrix m; copy_all_elements_of_this_in_range_to_m; return m;
    Then I would do this:
    matrix big_matrix, smaller_matrix;
    smaller_matrix = big_matrix.getminor(range_of_minor);
    Am I copying matrices twice in the above code? Once when matrix::getminor creates a minor matrix and once when it assigns it to another one? Would it be better if I rewrote the function so that smaller_matrix is passed by reference to matrix::getminor and the values of data are copied directly?

  2. #2
    & the hat of GPL slaying Thantos's Avatar
    Join Date
    Sep 2001
    Maybe. However, I would imagine that any smart compiler would optimize out the second construction.

  3. #3
    Master Apprentice phantomotap's Avatar
    Join Date
    Jan 2008
    Instead of relying on the compiler to do something clever in that context, overload a constructor to create the submatrix, the "minor" of ''matrix matrix::getminor(p1, ..., pn)', directly as the elements of a new matrix.

    If you now have this:

    matrix a;
    matrix b;
    // ...
    b = a.getminor(x, y, cx, cy);
    You'll get thins:

    matrix a;
    // ...
    matrix b(a, x, y, cx, cy);
    (If there are any, you may want to lift the duplicate parts of 'matrix::matrix(const matrix &)', 'matrix & matrix:perator = (const matrix &)', 'matrix matrix::getminor(p1, ..., pn)', and 'void matrix::getminor(matrix &, p1, ..., pn)' out and put them in a shared function.)


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