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Error when trying to display processed data

  • Hello, I am making a program, which basically takes values which would represent points, and will compute the maximum sub-array of these points. The layout is simple -> a QLabel which will eventually show the result, a QLineEdit where the user will enter his text and a QPushButton which will trigger the slot.

    The Slot will do 3 things:

    1. It will read the user input. Since the LineEdit returns a string, and the numbers are originally seperated by a string, we will think up of a way to deal with that.
      We will store this information in a vector.

    2. In order for us to be able to implement the Max_Subarray algorithm, we need to compute the data. We Will use a new vector, where we will store the data. The new vector will have 1 less element than the old one and the way we will compute it is the following. We will start from 1 until the end of the vector and we will substract the i-1'th element from the ith element.

    3. Now that we can process the data we will run a version of the maximum subarray algorithm (there are several, I have implemented an iterative linear time one)

    4. Set the value of the label of the sum.

    All should be well, but I have a mistake somewhere. I am getting wierd output. Sometimes 0, sometimes very long numbers... I am aplying the function.
    One final thing:
    QVector<int> vec;
    QVector<int> comp;
    These guys are declared in the header file.
    Oh also, here is an example user input: 10, 5, 6, 3

    @void MainWindow :: compute()
    // Read user input
    QString numbers = ui->lineEdit->text();
    QString tmp; // create temp string which will be used to store the temp substrings of each int
    numbers+=" ";
    for(int i = 0; i < numbers.length()-1; i++)
    tmp +=numbers[i];
    bool ok = true;
    int iTime = tmp.toInt(&ok, 10);
    //compute the real vector that we will search the max subarray from
    for(int i = 1; i < vec.size(); i++)
    comp.push_back(vec[i] - vec[i-1]);

    //compute the max subarray
       int  max_sum = -9999;
       int sum = 0;
       for(int i = 0; i < comp.size(); i++)
           sum += comp[i];
           if(sum > max_sum)
               max_sum = sum;
           if(sum < 0)
               sum = 0;
      // print the answer (sum);


  • Nothing to do with Qt.

    You are expecting your example input to get sum = 1 but you will be getting sum = 0
    because you have incorrectly implemented "Kadane's algorithm":http://en.wikipedia.org/wiki/Maximum_subarray_problem#Kadane.27s_algorithm It's important to return the maximum sum, not the last sum.
    int kadanes(const QVector<int> &comp)
    int max_ending_here(0);
    int max_so_far(0);
    foreach(int x, comp) {
    max_ending_here = qMax(0, max_ending_here + x);
    max_so_far = qMax(max_so_far, max_ending_here);
    return max_so_far;

    BTW: Are you emptying comp and vec between runs? Is there a reason these vectors are not local variables?

  • I found the reason for the bug- > there was one extra zero at the end of the vector after the reading of the input is over. Just pop_back the vector and the code will work.

    And then there is the mistake which you pointed out.

    The problem is solved, thank you for the help!

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