Iterators

Any time you have a container of things, sooner or later you are probably going to loop through the container and do something with each thing. An iterator is an object that provides indirect access to each element in a container. It is specifically designed to be used in a loop.

Qt 4 supports the following styles of iteration:

  1. Qt 4 style foreach loops, similar to Perl and Python

  2. Java 1.2 style Iterator

  3. Standard Library style ContainerType::iterator

  4. Hand-made while or for loops that use getters of the container

The next section demonstrates the various styles of iteration available in C++ with Qt 4.

4.2.1. QStringList and Iteration

For text processing, it is very useful to work with lists of strings. QStringList is derived from QList so it inherits all of QList's behavior (see Chapter 6). In addition, QStringList has some string-specific convenience functions such as indexOf(), join(), and replaceInStrings().

Converting between lists and individual strings is quite easy with perl-like split() and join() functions. Example 4.1 demonstrates lists, iterations, split(), and join().

Example 4.1. src/collections/lists/lists-examples.cpp

#include #include #include /* Some simple examples using QStringList, split and join */ int main() { QString winter = "December, January, February"; QString spring = "March, April, May"; QString summer = "June, July, August"; QString fall = "September, October, November"; QStringList list; list << winter; <-- 1 list += spring; <-- 2 list.append(summer); <-- 3 list << fall; qDebug() << "The Spring months are: " << list[1] ; QString allmonths = list.join(", "); /* from list to string - join with a ", " delimiter */ qDebug() << allmonths; QStringList list2 = allmonths.split(", "); /* split is the opposite of join. Each month will have its own element. */ assert(list2.size() == 12); <-- 4 foreach (QString str, list) { <-- 5 qDebug() << QString(" [%1] ").arg(str); } for (QStringList::iterator it = list.begin(); it != list.end(); ++it) { <-- 6 QString current = *it; <-- 7 qDebug() << "[[" << current << "]]"; } QListIterator itr (list2); <-- 8 while (itr.hasNext()) { <-- 9 QString current = itr.next(); qDebug() << "{" << current << "}"; } return 0; }  

(1)append operator 1

(2)append operator 2

(3)append member function

(4)Assertions abort the program if the condition is not satisfied.

(5)Qt 4 foreach loop, similar to Perl/Python and Java 1.5 style for loops.

(6)C++ STL-style iteration

(7)pointer-style dereference

(8)Java 1.2 style iterator

(9)Java iterators point in between elements.

src/collections> ./collections The Spring months are: March, April, May December, January, February, March, April, May, June, July, August, September, October, November [December] [January] [February] [March] [April] [May] [June] [July] [August] [September] [October] [November] src/collections>

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