Collections Intro What is the STL? Templates, collections, & iterators Focus on vector, for now

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) You use an iphone proficiently, but you don't necessarily understand internal function.

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; If you pass an object in C++ (without the &) it's passed by value – C++ makes a complete copy of its contents, rather than sharing its location. Objects are passed by value by default, not be reference. Making copies of objects is usually slow – you probably don't want this. So: int computeSum(Something<int>& s) { ... Have to pass by reference if you want function tmodify object also.

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

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 Arrays don't have many features. Not very powerful. Go right ahead and exceed bound of array and access random garbage in memory

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 Don't have to worry about resizing yourself.

Example Vector member function at(): 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 validity Resizing 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! Vectors have advantages of static and dynamically allocated array – it takes a non constant size like the dyn allocated array and ia automatically deleted like the static one. Vector uses [] to allow indexing like arrays.

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 vector int numElts = v.size(); // 2 front(): returns reference to first element of vector int& num = v.front(); // 3 back(): 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 1st 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/