Containers & Iterators

Transcription

1 Runtime Error? Topics Containers & Iterators 1) What is the best way to store a group of items? 2) How can we step through all the items? 3) What Qt classes store items? Ch 4 03/02/12 CMPT 212 Slides #8 Dr. B. Fraser 1 03/02/12 2 Generic Containers QList Generic Containers QList Templates: work with any (one) type of data. QList<int> studentnumbers; QList<Pizza> order; Benefit of generic containers Manage dynamic memory management: based on # items. studentnumbers.push_back( ); Provide safe access:.. (Arrays provide no bounds checking) cout << studentnumbers[42]; Declare to hold a specific type: QList<double> laptimes; Add elements using a number of methods: laptimes. (104.3); laptimes. (102.0); Check size: Q_ASSERT(lapTimes. == 2); Access elements: cout << laptimes ; 03/02/ /02/12 4

2 QList example int main(int argc, char *argv[]) QList<QString> heros; heros.push_back("superman"); heros.append("he-man"); heros << "She-Ra"; heros += "Dr. Evil"; Q_ASSERT(heros.size() == 4); if (heros.contains("dr. Evil")) { cout << "Who put Dr. Evil in!?!\n"; if (!heros.contains("mighty Mouse")) { cout << "No mouse? What a massive omission!\n"; cout << "\nfull list:\n"; for (int i = 0; i < heros.size(); i++) { cout << i << ": " << heros[i] << endl; Iterators return 0; 03/02/ /02/12 6 Direct Access Iterator Pattern Direct Access Use a loop to access elements "directly". for (int i = 0; i < heros.size(); i++) { cout << heros[i] << endl; Iterator Pattern Definition: The iterator pattern provides a way to access the elements of an aggregate object sequentially.. Iterators work for all collections (trees, maps, list,...) Iterators for (QList<QString>::iterator itr = heros.begin(); itr!= heros.end(); itr++) { cout << *itr << endl; 03/02/ styles: C++ std iterator, Java Iterator, Qt's foreach Collection of Items: Iterator Path 03/02/12 8

3 C++ std Style Iterator C++ Standard Library Style Iterator Define: Start: pizzas.begin() End: pizzas.end() Advance: Get element: *itr or itr->method() //... and more... // Use a C++ Standard Library style iterator // to output the list. for (QList<Pizza>::iterator itr = pizzas.begin(); itr!= pizzas.end(); itr++) { cout << itr->getname() << endl; 03/02/12 9 Java Style Iterator Java Style Iterator (for Qt in C++) Define & Init: End:!itr.hasNext() Get & Advance: Pizza lunch = //... and more... // Use a Java style iterator to output the list. QListIterator<Pizza> itr(pizzas); while(itr.hasnext()) { Pizza p = itr.next(); cout << p.getname() >> endl; 03/02/12 10 Qt's For Each Qt's foreach loop Iterator is.. Implemented at compile time by the Meta Object Compiler (MOC) //... and more... foreach (const Pizza &p, pizzas) { cout << p.getname() << endl; foreach (Pizza p, pizzas) { cout << p.getname() << endl; Must be a const & because cannot This actually makes a (not a reference) QStringList and QMap 03/02/ /02/12 12

4 QStringList Special list for QStrings: QList<QString> is quite common, so made a custom class: QStringList Need #include <QStringList> Adds other operations: to change a QString into a QStringList to merge a QStringList into a QString Example: QString str = "one, two, three"; // Mwahahahah QStringList lst = str.split(", "); // Has 3 elements QString together = lst.join(" ---> "); // One string. Q_ASSERT(together == "one ---> two ---> three"); 03/02/12 13 QMap QMap is an Ex: Matching student numbers (key) to names (value). #include <QMap> // Create map from int to QString QMap<int, QString> students; // Populate with data students[ ] = "Dr. Evil"; students[ ] = "Ms. Binary"; students[ ] = "Mr. Achiever"; Q_ASSERT(students.size() == 3); Q_ASSERT(students[ ] == "Ms. Binary"); Q_ASSERT(students.contains( ) == true); Q_ASSERT(students.contains(1) == false); // Change the value at a key. students[ ] = "I. R. Best"; 03/02/12 14 Checking items Be careful using [] with QMap Use damap.contains(key) to.. Using [] to check the value at a key will.. // Ends up creating item at index 0! if (students[0]!= "Hamlet") cout << "Alas, I knew him well" << endl; // Checks for item, does not create it! if (students.contains(0) && students[0]!= "Romeo") cout << "Where for ought thou?" << endl; 03/02/12 15 QMap and Iterators Data is stored.. // Create & populate map: QMap<QString, int> numbers; numbers["zero"] = 0; numbers["one"] = 1; numbers["two"] = 2; numbers["three"] = 3; // Java style iterator QMapIterator<QString, int> itr1(numbers); while (itr1.hasnext()) { itr1.next(); cout << itr1.key() << ": " << itr1.value() << endl; // C++ STD style iterator QMap<QString, int>::const_iterator itr2 = numbers.constbegin(); while (itr2!= numbers.constend()) { cout << itr2.key() << ": " << itr2.value() << endl; ++itr2; // foreach iterator: // Accessing just the values, not the keys. foreach (int value, numbers) { cout << value << endl; 03/02/12 16

5 QMap Details Summary Some details on challenges with QMap: key type must work with.. Classes can overload < (ex: QString) Value must be assignable (or won't compile): Generic containers use templates to store any one type of value. QList used to store a series of elements. default constructor copy constructor Iterators abstract the notion of iteration into a class. There are 3 types of iterators supported by Qt. assignment operator Work-around is to use a pointer. Ex: If Pizza is not assignable, will get error with: QMap<int, Pizza> so use:.. QStringList useful for working with strings. QMap stores (key, value) pairs. 03/02/ /02/12 18