为什么迭代器不返回到集合的开头？

Question

I wrote a program in C++ to help me study for the GMAT. I use it to practice the times tables up to 25 in randomized order. It works by generating a random number and asking the user to multiply that number by the times table that they want to work on. The program adds that number to a set so that it is not repeated. Please find the code below:

int randumb(int a){
  random_device rd;
  mt19937 gen(rd());
  uniform_real_distribution <> dis(1,a+1);
  int u=dis(gen);
  return u;
}

void mult_tables(){
  cout<<"Which times table would you like to practice?"<<endl;
  int a, input;
  set<int> vect;
  cin>>a;
  int mult;
  for(int i=0; i<a; ++i){
    mult=randumb(a);
    if(i==0)
      vect.insert(mult);
    if(i!=0){
      for(set<int>::iterator it=vect.begin(); it!=vect.end(); ++it){
        if(mult==*it){
          mult=randumb(a);
          it=vect.begin();
        }
      }
      vect.insert(mult);
    }
    cout<<endl;
    cout<<a<<" x "<<mult<<" = ";
    cin>>input;
    if(input==a*mult)
      cout<<"Correct!"<<endl;
    else
      cout<<"Wrong."<<endl;
  }
} 

As you can see, the code compares each multiplier with the elements already in the set. If a number is already in the set, it generates and new number and returns the iterator to the beginning of the set so that it can compare the new multiplier with ALL of the previously used numbers. I stepped through the program in GDB and every time it hits it=vect.begin(); it returns the iterator to the SECOND element in the set. It happens every time and causes numbers to repeat. Does anyone know why this is happening?

Answer 1

The logic error is that you are incrementing the iterator after the call to vect.begin() in the loop.

What you need is:

  for(set<int>::iterator it=vect.begin(); it!=vect.end(); /** ++it **/){
    if(mult==*it){
      mult=randumb(a);

      // Reset the iterator to the start.
      // Don't increment it.
      it=vect.begin();
    }
    else {
     // Increment the iterator.
     ++it;
    }
  }

Answer 2

Not a fix for your implementation but rather a cleaner way of doing the same thing.

A set container ensures each value inside it is unique, so instead of checking if the number already exists it is easier to just add numbers as long as the size of the set is smaller than required.

Here is a code snippet:

std::set<int> nums;
while(nums.size() < REQUIRED_SIZE)
{
    nums.insert(randumb(a));
}

At this point you have a set of unique numbers, use it as you wish.