CSE 332: C++ Algorithms II From Last Time: Search with Generic Iterators Third generalization: separate iterator type parameter We arrive at the find algorithm (Austern pp. 13): template Iterator find (Iterator first, Iterator last, const T & value) { while (first != last && *first != value) ++first; return first; } Which kinds of iterators will work with this algorithm? How can we determine that from the algorithm itself?

CSE 332: C++ Algorithms II Key Ideas: Concepts and Models A concept gives a set of type requirements –Classify/categorize types (e.g., random access iterators) –Tells whether or not a type can or cannot be used with a particular algorithm (get a compiler error if it cannot) E.g., in the examples from last time, we could not use a linked list iterator in find1 or even find2, but we can use one in find Any specific type that meets the requirements is a model of that concept –E.g., list ::iterator vs. char * in find Different abstractions (bi-linked list iterator vs. char array iterator) No inheritance-based relationship between them But both model iterator concept necessary for find

CSE 332: C++ Algorithms II Concepts and Models, Continued What very basic concept does the last statement of find, (the line return first; ) assume? –Asked another way, what must be able to happen to first when it’s returned from function find ? –Same requirement imposed by by-value iterator parameters What other capabilities are required of the Iterator and T type parameters by the STL find algorithm ? template Iterator find (Iterator first, Iterator last, const T & value) { while (first != last && *first != value) ++first; return first; }

CSE 332: C++ Algorithms II Matching an Algorithm to the Iterators it Needs Category/ Operation OutputInputForwardBidirectional Random Access Read =*p (r-value) =*p (r-value) =*p (r-value) =*p (r-value) Access -> [] Write *p= (l-value) *p= (l-value) *p= (l-value) *p= (l-value) Iteration = -= Comparison == != == != = What STL iterator category does find require?

CSE 332: C++ Algorithms II Iterator Concept Hierarchy Input IteratorOutput Iterator Forward Iterator Bidirectional Iterator Random Access Iterator value persists after read/write values have locations can express distance between two iterators read or write a value (one-shot) Singly-inked-list style access ( forward_list ) Bi-linked-list style access ( list ) Array/buffer style access ( vector, deque ) “destructive” read at head of stream ( istream ) “transient” write to stream (ostream) is-a ( refines )

CSE 332: C++ Algorithms II What if an Algorithm Has Alternative Versions? Static dispatching –Implementations provide different signatures –Iterator type is evaluated at compile-time –Links to the best implementation Notice how type tags are used // Based on Austern, pp. 38, 39 template void move (Iter i, Distance d, fwd) { while (d>0) {--d; ++i;} // O(d) } template void move (Iter i, Distance d, rand) { i += d; // O(1) } template void move (Iter i, Distance d) { move (i, d, iterator_traits :: iterator_category() ) } concrete tag (empty struct) type default constructor (call syntax) concrete tag (empty struct) type

CSE 332: C++ Algorithms II Iterator Traits and Category Type Tags Need a few concrete types to use as tags –E.g., empty structs –E.g., input, output, fwd, bidir, and rand Tags provide yet another associated type for iterators –Iterator category –Again, made available by using the traits idiom struct input {}; // empty structs for type tags struct output {}; struct fwd : public input {}; // note inheritance struct bidir : public fwd {}; struct rand : public bidir {}; template struct iterator_traits {... typedef typename I::iterator_category iterator_category; }; template struct iterator_traits {... typedef rand iterator_category; }; template struct iterator_traits {... typedef rand iterator_category; }; (actually, random_access_iterator_tag )

CSE 332: C++ Algorithms II Can Extend STL Algorithms with Callable Objects Make the algorithms even more general Can be used parameterize policy –E.g., the order produced by a sorting algorithm –E.g., the order maintained by an associative containe Each callable object does a single, specific operation –E.g., returns true if first value is less than second value Algorithms often have overloaded versions –E.g., sort that takes two iterators (uses operator< ) –E.g., sort that takes two iterators and a binary predicate, uses the binary predicate to compare elements in range

CSE 332: C++ Algorithms II Callable Objects and Adapters Callable objects support function call syntax –A function or function pointer bool (*PF) (const string &, const string &); // function pointer bool string_func (const string &, const string &); // function –A struct or class providing an overloaded operator() struct strings_ok { bool operator() (const string &s, const string &t) { return (s != “quit”) && (t != “quit”); } }; –A lambda expression (unnamed inline function) [quit_string] (const string &s, const string &t) -> bool {return (s != quit_string) && (t != quit_string);} Adapters further extend callable objects –E.g., bind any argument using bind and _1 _2 _3 etc. –E.g., wrap a member function using mem_fn –E.g., wrap callable object with function (associates types)

CSE 332: C++ Algorithms II Using Functions with an Algorithm #include using namespace std; struct Employee { Employee (const char * n, int i) : name_(n), id_(i) {} string name_; int id_; }; typedef Employee * EmployeePtr; ostream& operator<< (ostream & os, const EmployeePtr & e) { os name_ id_ << " "; return os; } // function for comparing EmployeePtrs bool id_compare (const EmployeePtr & e, const EmployeePtr & f) { return e->id_ id_|| (e->id_ == f->id_ && e->name_ name_); } int main (int, char *[]) { vector v; v.push_back(new Employee("Claire", 23451)); v.push_back(new Employee("Bob", 12345)); v.push_back(new Employee("Alice", 54321)); cout << "v: " ; copy (v.begin(), v.end(), ostream_iterator (cout)); cout << endl; // "v: Claire Bob Alice " sort (v.begin(), v.end(), id_compare); cout << "v: " ; copy (v.begin(), v.end(), ostream_iterator (cout)); cout << endl; // "v: Bob Claire Alice " // clean up: pointers "own" the heap objects for (vector ::iterator i = v.begin(); i != v.end(); ++i) { delete *i; } return 0; } pass function name heap object

CSE 332: C++ Algorithms II Using Function Objects with an Algorithm count_if algorithm –Generalizes the count algorithm –Instead of comparing for equality to a value –Applies a given predicate function object (functor) –If functor’s result is true, increases count #include using namespace std; template // covers many types struct odd { bool operator() (T t) const { return (t % 2) != 0; } }; int main (int, char * []) { vector v; v.push_back(1); v.push_back(2); v.push_back(3); v.push_back(2); cout << "there are " << count_if(v.begin(), v.end(), odd ()) << " odd numbers in v" << endl; return 0; } /* output is there are 2 odd numbers in v */

CSE 332: C++ Algorithms II Concluding Remarks STL algorithms give you useful, generic functions –Combine easily with a variety of containers/iterators –Support many common data structure manipulations Finding and modifying values, re-ordering, numeric operations –Reusing them saves you from writing/debugging code Many STL algorithms can be extended –Especially by plugging callable objects (functors) into them –C++11 lambdas & the bind function give new ways to do that Think about how iterators and algorithms combine –Think about which category of iterator a container provides –Think about the concept each iterator models –Think about the type requirements an algorithm imposes –Think about which combinations are valid, accordingly