prepare (1/3): 
finished

make (2/3): 
g++ -g -Wall -O2 main.cpp lab3.cpp -o main 
main.cpp: In function 'int main()':
main.cpp:31:55: warning: 'var_selected' may be used uninitialized in this function [-Wmaybe-uninitialized]
         cout << "The order-" << idx << " item is " << var_selected << endl;
                                                       ^~~~~~~~~~~~
main.cpp:22:27: warning: 'a' may be used uninitialized in this function [-Wmaybe-uninitialized]
         case 4: quick_sort(a, 0, n-1); break;
                 ~~~~~~~~~~^~~~~~~~~~~
main.cpp:23:30: warning: 'idx' may be used uninitialized in this function [-Wmaybe-uninitialized]
         case 5: var_selected = random_selection(a, idx, 0, n-1); break;
                 ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
main.cpp:17:5: warning: 'method' may be used uninitialized in this function [-Wmaybe-uninitialized]
     switch(method){
     ^~~~~~
main.cpp:18:28: warning: 'n' may be used uninitialized in this function [-Wmaybe-uninitialized]
         case 0: bubble_sort(a, n); break;
                 ~~~~~~~~~~~^~~~~~
finished

clean (3/3): 
finished

all task finished, build successfully