#include <QList>
#include <QDebug>

int main(int argc, char *argv[]) {

    QList<QList<int>> my_list;

    int result;
    for (int i = 0; i < 2; ++i) {
        for (int j = 0; j < 4; ++j) {
            result = i * j;
            my_list.value(i).push_back(result);
            qDebug() << my_list.size() << "," << my_list.value(i).size() << " : " << my_list.value(i).value(j);
        }
    }

    return 0;
}

Starting C:Users ... buildreleasename_of_the_app.exe...
0 , 0  :  0
0 , 0  :  0
0 , 0  :  0
0 , 0  :  0
0 , 0  :  0
0 , 0  :  0
0 , 0  :  0
0 , 0  :  0
C:Users ... buildreleasename_of_the_app.exe exited with code 0

my_list.value(i).push_back(result);

for (int i = 0; i < 2; ++i) {
    my_list.push_back(QList<int>());   // first appends a new QList at index `i`

    for (int j = 0; j < 4; ++j) {
        result = i * j;
        my_list[i].push_back(result);  // safely retrieves QList at index `i` and adds an element
    }
}