Iterators - C++

ITERATORS

ⁿ    An iterator points to (refers to, denotes) an element of a sequence

ⁿ    The end of the sequence is “one past the last element”

ⁿ    not “the last element”

ⁿ    That’s necessary to elegantly represent an empty sequence

ⁿ    One-past-the-last-element isn’t an element

ⁿ    You can compare an iterator pointing to it

ⁿ    You can’t dereference it (read its value)

ⁿ    Returning the end of the sequence is the standard idiom for “not found” or “unsuccessful”

Simple algorithm: find_if()

ⁿ   Find the first element that matches a criterion (predicate)

ⁿ Here, a predicate takes one argument and returns a bool template<class In, class Pred>

In find_if(In first, In last, Pred pred)

{

while (first!=last && !pred(*first)) ++first; return first;

}

void f(vector<int>& v)

{

vector<int>::iterator p = find_if(v.begin(),v.end,Odd());

 if (p!=v.end()) { /* we found an odd number */ }

// …

}

Iterator Operators

* dereferencing operator

ⁿ   Produces a reference to the object to which the iterator p points

*p

++ point to next element in list

ⁿ    Iterator p now points to the element that followed the previous element to which p points

++p

-- point to previous element in list

ⁿ    Iterator p now points to the element that preceded the previous element to which p points

--p

 viiterator p = C.begin(), q = C.end(); for(;p!=q; p++) {

cout << *p << endl;

}

for(int i=0; i < 10; i++) { cout << C[i] << endl;}

int A[10];

for(int * p = A, i =0; i < 10; i++, p++) { cout << *p << endl;}