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Iterators

CS253 Iterators

Array Traversal

You have an array, and you want to traverse (walk through) it.

int a[] = {3141, 5926, 5358, 9793, 2384, 6264};
for (int i=0; i!=6; ++i)
    cout << a[i] << ' ';

3141 5926 5358 9793 2384 6264 

• a[i] is the same as *(a+i).
  • It requires addition and indirection.
• I usually say i<6, rather than i!=6.
• Similarly, I usually say i++ rather than ++i.
• Patience.

Array Traversal via Pointers

Let’s do it with a pointer:

int a[] = {3141, 5926, 5358, 9793, 2384, 6264};
for (int *p = &a[0]; p != &a[6]; ++p)
    cout << *p << ' ';

3141 5926 5358 9793 2384 6264 

• That’s a bit faster.
• Instead of a[i] (addition+indirection) we have just indirection.
• Not really a radical improvement, however.

vector traversal

You can traverse a vector in the same way:

vector<int> a = {3141, 5926, 5358, 9793, 2384, 6264};
for (int *p = &a[0]; p != &a[6]; ++p)
    cout << *p << ' ';

3141 5926 5358 9793 2384 6264 

or, avoiding the magic number 6:

vector<int> a = {3141, 5926, 5358, 9793, 2384, 6264};
for (int *p = &a[0]; p != &a[a.size()]; ++p)
    cout << *p << ' ';

3141 5926 5358 9793 2384 6264 

Methods

vector<int> a = {3141, 5926, 5358, 9793, 2384, 6264};
for (vector<int>::iterator it = a.begin(); it != a.end(); ++it)
    cout << *it << ' ';

3141 5926 5358 9793 2384 6264 

• iterator is a type provided by the templated vector class.
• What type is it?
  • might be int *, might not
• You don’t care what type it is. You know that:
  • * works on it
  • ++ works on it
  • you can assign .begin() to it
  • you can compare it to .end()

auto is your friend

This is prettier:

vector<int> a = {3141, 5926, 5358, 9793, 2384, 6264};
for (auto it = a.begin(); it != a.end(); ++it)
    cout << *it << ' ';

3141 5926 5358 9793 2384 6264 

for loop

This is exactly the same:

vector<int> a = {3141, 5926, 5358, 9793, 2384, 6264};
for (auto v : a)
    cout << v << ' ';

3141 5926 5358 9793 2384 6264 

The for loop is defined to use .begin() and .end(), just as the previous code.

Other containers

The same iterator code works for all STL containers.

forward_list<char> l = {'a', 'c', 'k', 'J'};
for (auto it = l.begin(); it != l.end(); ++it)
    cout << *it << ' ';

a c k J 

unordered_set<char> u = {'a', 'c', 'k', 'J'};
for (auto it = u.begin(); it != u.end(); ++it)
    cout << *it << ' ';

J c k a 

set<char> s = {'a', 'c', 'k', 'J'};
for (auto it = s.begin(); it != s.end(); ++it)
    cout << *it << ' ';

J a c k 

iterator type

• What type is set<int>::iterator?
  • Who cares?
  • It’s not an int *, that’s for sure!
• Same for list<int>::iterator and unordered_set<int>::iterator.
• Most iterator types cannot be a native pointer.
  • What happens when you increment a plain pointer?
  • It goes to the next memory location.
  • Is that what you want for a linked list, a tree, or a hash?
  • It must be that operator++ is overloaded.
  • Can’t do that for a plain pointer.
    • C++ is extensible, not mutable.
  • Therefore, most iterators cannot be plain pointers.

Iterator classifications

• BidirectionalIterator (list, set, unordered_set)
  • ++, --, ==, !=, *, ->, =
• RandomAccessIterator (std::array, vector, string)
  • ++, --, +=int, -=int, +int, -int, iter-iter, ==, !=, <, <=, >, >=, *, ->, =
• ForwardIterator (forward_list)
  • ++, ==, !=, *, ->, =

begin() & end()

• .begin() is, conceptually, a pointer to the first element of the container.
• .end() is, conceptually, a pointer one past the end of the container.
• It’s a half-open interval!
  
  

string s = "bonehead";
cout << "First character: " << *s.begin() << '\n';
cout << "This is wrong: " << *s.end() << '\n';

First character: b
This is wrong: ␀

string s = "genius";
cout << "First character: " << *s.begin() << '\n';
cout << "Last character:  " << *(s.end()-1) << '\n';

First character: g
Last character:  s

.front() & .back()

Some containers have .front() and .back(), which return references to the first and last elements.

list<double> c = {1.2, 3.4, 5.6};
cout << "First: " << c.front() << '\n';
cout << "Last:  " << c.back() << '\n';

First: 1.2
Last:  5.6

These are not iterators.

Comparisons

list<string> l = {"kappa", "alpha", "gamma"};
for (auto it = l.begin(); it < l.end(); ++it)
    cout << *it << ' ';

c.cc:2: error: no match for 'operator<' in 'it < 
   l.std::__cxx11::list<std::__cxx11::basic_string<char> >::end()' (operand 
   types are 'std::_List_iterator<std::__cxx11::basic_string<char> >' and 
   'std::__cxx11::list<std::__cxx11::basic_string<char> >::iterator' {aka 
   'std::_List_iterator<std::__cxx11::basic_string<char> >'})

list<string> l = {"kappa", "alpha", "gamma"};
for (auto it = l.begin(); it != l.end(); ++it)
    cout << *it << ' ';

kappa alpha gamma 

set<>::iterator is a BidirectionalIterator, not a RandomAccessIterator, and so < isn’t defined. What would it compare? The addresses of the linked list nodes? That’s not useful.

Constructors

All containers accept a pair of iterators as ctor arguments. These do not have to be iterators for the same type of container.

string today = "2024-04-27";
cout << today << '\n';
multiset<char> ms(today.begin(), today.end());
for (char c : ms)
    cout << c;

2024-04-27
--00222447

string today = "2024-04-27";
cout << today << '\n';
string year(today.begin(), today.begin()+4);
for (char c : year)
    cout << c;

2024-04-27
2024