Extending STL when more is needed Algorithms extending

Extending STL when more is needed

Algorithms • • extending STL with specialized algorithms is relatively straightforward algorithm find_all – implemented as template – takes an iterator range and a predicate and returns a vector of iterators pointing to values that match predicate • internally uses find_if 2

Container Example: Hashmap • hashmap a variant of unordered_map developed from scratch • we start with a basic hashmap that does not implement iterators and cannot be used with STL algorithms • elements – key, value pairs • hashes, or maps, a key to a bucket • implements a vector of buckets with a list of <key, values> at each bucket (chained hashing) 3

Hashmap: Hash • hash – hashing function • clients may provide their own, specify number of buckets • Default. Hash a separate class - default implementation, accepts num. Buckets computes a sum of bytes in a key, modulo num. Buckets – basic hash, key uniformity is not achieved 4

Hashmap: Interface & Implementation • • supports three basic functions: insertion, deletion and lookup constructor, destructor, copy constructor, (basic) copy assignment, (c++11) move assignment template parameters – key – value – compare – by default an equal_to functor for comparison – to detect attempts to insert elements with duplicate keys – hash – by default Default. Hash implementation – fixed size (number of buckets) vector of pointers to lists containing <key, value> pairs – comparison object – hash object – number of elements in the container 5

Hashmap: Element Lookup • helper method find. Element() –, then looks into bucket to find element, using functor for comparison, if not found returns end of bucket – note the use of typename in return value – needed because template parameters are used • find() - a wrapper around find. Element() • operator[] – uses find() to determine if element present – if yes – returns it – if not – use insert() to insert it, find() to find and return it • insert() – finds using find. Element() if not found - insert 6