Collections Intro What - PowerPoint Presentation

Collections Intro What is the STL? Templates, collections, & iterators Adapted from Dr. Mary Eberlein

Data Structures Collection or data structure: an object that stores data elements: objects that are stored collections can be ordered or unordered some collections allow duplicate elements Common operations: insert, size, isEmpty... Good to know how to build collections (implementation) Often use an available collection rather than implementing it yourself (and learn to use it effectively)

Type Parameters (templates) Collection<type> name; specify type of elements in triangle brackets Called a type parameter – you get a parameterized class, or template class type can be any data type: int , char, string, user-defined type... Example: vector<int> v;

STL: Standard Template Library STL: library of collections and algorithms for C++ Many use templates container classes (set, list, stack, vector, queue...) algorithms iterators

VECTOR

Won't Arrays Do? Arrays: fixed size – not easily resized array doesn't "know" its own size array index out of bounds errors not caught don't support useful operations like: inserting/deleting elements at front/middle/back of array reversing order of elements searching or sorting preventing duplicate elements from being added

Vector vector (aka list): collection of elements with 0-based indexing add elements anywhere in vector automatic resizing as needed vector has a size (current # of elements) template class – create vectors of different types No need to allocate/delete memory for vector Header: #include<vector> Recall: all of standard library in namespace std vector< int > myVector ; // vector of ints vector<string> strVec; // vector of strings

Example C++ Array Example: int size = 10; int myArr [10]; int * arrPtr = new int [size]; for( int i = 0; i < 10; i++){ myArr[i] = i; arrPtr[i] = i;}delete [] arrPtr;Vector member function at(): similar to indexing with [], but checks index validityResizing is handled for you when necessary C++ Vector Example: #include <vector> using namespace std ; ... int size = 10; // 10 ints in array, init to 0 vector< int > myArr (size); for( int i = 0; i < size; i ++) myArr [ i ] = i ; // Don't need to delete!

STL vector vector<int> v; // empty vector Member functions: push_back(value) : insert element at the end of the vector v.push_back(3); // 3 added at end of v v.push_back(5); // 5 added at end of v at(index) : return element at specified index int x = v.at(0); // 3 size() : returns number of elements in vectorint numElts = v.size(); // 2front(): returns reference to first element of vectorint& num = v.front(); // 3back(): returns reference to last element of vector

STL iterator iterator: object that iterates over a data structure pointer-like object that can be incremented (++), dereferenced (*), compared to another iterator (!=) stores current position within data structure used to traverse over the elements in a collection iterator generated by STL member functions like begin() and end() begin() : returns iterator pointing to 1 st element of ordered collection (e.g., vector) end() : returns iterator which is past the end of the collection

Iterator A collection has a begin and end iterator to front and back Advance one element: iter++ Go back one element: iter-- Access element the iterator is next to: *iter vector<int> v; ... // loop over vector elements and print them for(vector<int>::iterator it = v.begin(); it != v.end(); it++) { cout << *it << endl; }

Iterator Example // for-each loop – implicit iterator (C++11) vector<int> v; ... for(int k : v) { cout << k << endl; }

Iterator Example Some container member functions take an iterator argument to indicate position Example for vector: insert, erase vector<int> v; ... for(vector<int>::iterator it = v.end(); it != v.begin(); it--) { if(*it % 2 == 0) { it = v.erase(it); // delete element at this position } }

STL Algorithms Many algorithms Example: sorting vectors #include<algorithm> vector<string> vec; vec.push_back("hello"); vec.push_back("world"); vec.push_back("bye"); sort(v.begin(), v.end()); http://www.cplusplus.com/reference/algorithm/