CS106L(6): Arcade Toy Claw with Iterators

Enjoy Arcade Toy Claw Games?

I have tried that game some times ago, here is the evidence:

HAHAHAHAHAHAHA…..The shop owner will bankrupt soon 💸

Summary

  • Understand Iterators Concept: It’s like Arcade Toy Claw Game!
  • Iterator Practice: Example and Exercises
    • Introduce different types of Iterators

std:vector Loop Example

std::vector<int> vector{3, 1, 4, 1, 5, 9};
for (size_t i = 0; i < vector.size(); i++) {
 const auto& elem = vector[i];
 cout << elem << endl;
}

Iterators

Iterators allow iteration over any container whether ordered or unordered.

Iterators (“the claw”) can:

  • move “forward”
    • according to some order…
  • retrieve element
  • check if two claws are in the same place

Containers (“the machine”) provide:

  • the bounds (begin and end)

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Key idea: iterator has ordering over elements, it always knows what the “next” element is.

it (the reference)
*it; (dereference, the item)
(it == c.end())  //until

Examples

std::set<T> s;
auto iter = s.begin();
iter++;                     // increment; prefix operator is faster (why?)
++iter;
*iter;                      // dereference iter to get curr value
(iter != s.end());          // equality comparison
iter = another_iter         // copy construction

STL sets have the following operations:

s.begin(); // an iterator pointing to the first element
s.end()    // one `past` the last element

Why use ++iter and not iter++?

Answer: There’s no need to store the old value of the iterator, so ++iter returns the value after being incremented.

Iterator Practise

What type is it?

std::map<int, int> map {{1, 2}, {3, 4}};
auto it = map.first();      // what type is this?    My ans:  std::map::Iterator<int, int>
auto map_elem = *it;        // how about this?       My ans:  <int, int>

Quiz: Iterator Basics

Example Code:

std::map<int, int> map {{1, 2}, {3, 4}};     // note that dereferencing a std::map::iterator returns a std::pair
auto iter = map.begin(); // what is *iter?                       My ans:  <int, int>
++iter;                  // what is (*iter).second now?          My ans:  4
auto iter2 = iter;
++iter;                  // what does (*iter).first return?      My ans:  undefined

Notes:

  • ++iter: go to the next element
  • *iter: retrieve what’s at iter’s position
  • copy constructor: create another iterator pointing to same thing
  1. iter is a copy of begin iterator.
auto iter = map.begin();
  1. *iter returns {1, 2}

  2. ++iter; : *iter incremented to next element

  3. *iter.second is 4

  4. auto iter2 = iter;: create an independent copy of iter pointing to same thing

  5. last ++iter;: increment iter (iter2 not impacted…)

  6. what does (*iter).first return?

undefined!, because (iter == map.end())

Recall the Loop Example

std::vector<int> vector{3, 1, 4, 1, 5, 9};
for (size_t i = 0; i < vector.size(); i++) {
 const auto& elem = vector[i];
 cout << elem << endl;
}

Iterator Example

std::set<int> set {3, 1, 4, 1, 5, 9};
for (auto iter = set.begin(); iter != set.end(); ++iter) {
 const auto& elem = *iter;
 cout << elem << endl;
}

std::map<int> map {{1, 6}, {1, 8}, {0, 3}, {3, 9}};
for (auto iter = map.begin(); iter != map.end(); ++iter) {
 const auto& [key, value] = *iter; // structured binding!
 cout << key << : << value << endl;
}

Exercise: print all elements in these collections

You discovered For-Each Loop!

std::set<int> set {3, 1, 4, 1, 5, 9};
for (const auto& elem : set) {
 cout << elem << endl;
}
std::map<int> map {{1, 6}, {1, 8}, {0, 3}, {3, 9}};
for (const auto& [key, value] : map) {
 cout << key << ":" << value << endl;
}

Thoughts: auto or auto&?

Seems that if it is value, auto& will be used to represent the dereference/or the content in that address, if it’s pointer/iterator, then auto will be used. Is it correct?

Iterator Shorthand

These are equivalent:

auto key = (*iter).first;
auto key = iter->first;

Types of Iterators

  • All iterators are incrementable (++)
  • Input iterators can be on the right side of =: auto elem = *it;
  • Output iterators can be on the left side of =: *elem = value;
  • Forward iterators can be traversed multiple times:
iterator a;
b = a;
a++; b++;
assert (*a == *b) // true

Q: Can you think of an example of an iterator that should not be a forward iterator?

  • Random access iterators support indexing by integers.
it += 3; // move forward 3
it -= 70; // move backwards by 70
auto elem = it[5]; // offset by 5

Haha, congrats, the 6th lecture finished! Wish you good luck next time when you see this:

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Famous Arcade Game!

References