如何使用迭代器查找对象在矢量中的位置？

Question

I have method which chec is client objects exists (by its name) and returns its position:

   int Program::checkClient(string userName){
      vector<Client> *ptr = &(impl->clients);
      int i;
      for (i = 0; i < ptr->size(); i++){
         if(ptr->at(i).getName() == userName){
            return i;
         }
      }
      return -1;
   }

The problem is that I get warning, because of i < ptr->size() . I believe I need to use iterator for this. But how to do it correctly, because not only do I need to run through loop, but also return int as a result, of where it was found. Any ideas? i tried doing something like this, but had no luck:

   int Program::checkClient(string userName){
      vector<Client> *ptr = &(impl->clients);
      vector<Client>::iterator it;
      for (it.ptr->begin(); it < it.ptr->end(); it++){
         if(ptr->at(it).getName() == userName){
            return it;
         }
      }
      return -1;
   }

I get an error to similar places like this: it.ptr->begin() .

Answer 1

You don't need to use iterators; you haven't mentioned what warning you're getting, but it's probably because you're comparing an int to vector<Client>::size_type , which is, in all likelihood, an unsigned type. Change the declaration of i to

std::vector<Client>::size_type i;

---

Now, you're getting errors in your iterator code because its syntax is incorrect. The for loop should look like this:

for(it = ptr->begin(), it != ptr->end(); ++it) { ... }

You can find the index of the item the iterator is currently pointing at using std::distance

return std::distance(ptr->begin(), it);

---

Additionally, as mentioned in the comments above, it'd be better to use std::find_if for this, especially if you have a compiler that supports lambda expressions.

auto it = std::find_if(impl->clients.begin(),
                       impl->clients.end(),
                       [&]( Client const& c ) {
                          return c.getName() == userName;
                       } );
return (it != impl->clients.end()) ? 
         std::distance(impl->clients.begin(), it) : -1;

Answer 2

Use this:

int Program::checkClient(string userName)
{
    vector<Client> * ptr = &(impl->clients);
    vector<Client>::iterator it;
    int count=0; //for counting the position
    for (it=ptr->begin(); it!=ptr->end(); ++it)
    {
        ++count;
        if( (*it).getName() == userName )
        {
            return count;
        }
    }
    return -1;
}

Answer 3

For searching/finding I prefer using while , it's easier to understand.
You'd rather use const& for your method parameter.

So it'd give something like that:

int Program::checkClient(string const& userName) const
{
    vector<Client> const& clients = impl->clients;
    vector<Client>::const_iterator it = clients.begin();
    while (it != clients.end() && it->getName() != userName)
        ++it;
    return it != clients.end() ? it - clients.begin() : -1;
}