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See this page as a slide show

CS253 Iterators

Array Traversal

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

int a[] = {31, 41, 59, 26, 53, 58};
for (int i=0; i!=6; ++i)
    cout << a[i];

314159265358

• 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[] = {31, 41, 59, 26, 53, 58};
for (int *p = &a[0]; p != &a[6]; ++p)
    cout << *p;

314159265358

• 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 = {31, 41, 59, 26, 53, 58};
for (int *p = &a[0]; p != &a[6]; ++p)
    cout << *p;

314159265358

or, avoiding the magic number 6:

vector<int> a = {31, 41, 59, 26, 53, 58};
for (int *p = &a[0]; p != &a[a.size()]; ++p)
    cout << *p;

314159265358

Methods

vector<int> a = {31, 41, 59, 26, 53, 58};
for (vector<int>::iterator it = a.begin(); it != a.end(); ++it)
    cout << *it;

314159265358

• 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 = {31, 41, 59, 26, 53, 58};
for (auto it = a.begin(); it != a.end(); ++it)
    cout << *it;

314159265358

for loop

This is exactly the same:

vector<int> a = {31, 41, 59, 26, 53, 58};
for (auto v : a)
    cout << v;

314159265358

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;

ackJ

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

Jcka

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

Jack

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() & .end() return iterators. Not necessarily pointers.
• .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, part one

This won’t work:

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> >'})

Comparisons, part two

This will work:

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

kappa alpha gamma 

list<>::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.

char hhmmss[] = "024251"; // ISO 8601!
string now(hhmmss, hhmmss+6);
cout << now << '\n';
string mins(now.begin()+2, now.begin()+4);
cout << mins << '\n';
multiset<int> ms(now.begin(), now.end());
for (auto n : ms)
    cout << n << ' ';
cout << '\n';

024251
42
48 49 50 50 52 53 

begin() and end() functions

There are also free functions begin() and end(), which work on arrays (not pointers) and all standard containers:

char now[] = "02:42am Fri";
string current(now, now+10);
cout << current << '\n';

02:42am Fr

Oh dear, I counted wrong. Why am I counting!?

char now[] = "02:42am Fri";
string current(begin(now), end(now));
cout << current << '\n';

02:42am Fri␀

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