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<QtAlgorithms> - Generic Algorithms

The <QtAlgorithms> header includes the generic, template-based algorithms. More...

Functions

Qt provides a number of global template functions in <QtAlgorithms> that work on containers and perform well-know algorithms. You can use these algorithms with any container class that provides STL-style iterators, including Qt's QList, QLinkedList, QVector, QMap, and QHash classes.

These functions have taken their inspiration from similar functions available in the STL <algorithm> header. Most of them have a direct STL equivalent; for example, qCopyBackward() is the same as STL's copy_backward() algorithm.

If STL is available on all your target platforms, you can use the STL algorithms instead of their Qt counterparts. One reason why you might want to use the STL algorithms is that STL provides dozens and dozens of algorithms, whereas Qt only provides the most important ones, making no attempt to duplicate functionality that is already provided by the C++ standard.

Most algorithms take STL-style iterators as parameters. The algorithms are generic in the sense that they aren't bound to a specific iterator class; you can use them with any iterators that meet a certain set of requirements.

Let's take the qFill() algorithm as an example. Unlike QVector, QList has no fill() function that can be used to fill a list with a particular value. If you need that functionality, you can use qFill():

 QStringList list;
 list << "one" << "two" << "three";

 qFill(list.begin(), list.end(), "eleven");
 // list: [ "eleven", "eleven", "eleven" ]

qFill() takes a begin iterator, an end iterator, and a value. In the example above, we pass list.begin() and list.end() as the begin and end iterators, but this doesn't have to be the case:

 qFill(list.begin() + 1, list.end(), "six");
 // list: [ "eleven", "six", "six" ]

Different algorithms can have different requirements for the iterators they accept. For example, qFill() accepts two forward iterators. The iterator types required are specified for each algorithm. If an iterator of the wrong type is passed (for example, if QList::ConstIterator is passed as an output iterator), you will always get a compiler error, although not necessarily a very informative one.

Some algorithms have special requirements on the value type stored in the containers. For example, qEqual() requires that the value type supports operator==(), which it uses to compare items. Similarly, qDeleteAll() requires that the value type is a non-const pointer type (for example, QWidget *). The value type requirements are specified for each algorithm, and the compiler will produce an error if a requirement isn't met.

The generic algorithms can be used on other container classes than those provided by Qt and STL. The syntax of STL-style iterators is modeled after C++ pointers, so it's possible to use plain arrays as containers and plain pointers as iterators. A common idiom is to use qBinaryFind() together with two static arrays: one that contains a list of keys, and another that contains a list of associated values. For example, the following code will look up an HTML entity (e.g., &amp;) in the name_table array and return the corresponding Unicode value from the value_table if the entity is recognized:

 QChar resolveEntity(const QString &entity)
 {
     static const QLatin1String name_table[] = {
         "AElig", "Aacute", ..., "zwnj"
     };
     static const ushort value_table[] = {
         0x0061, 0x00c1, ..., 0x200c
     };
     int N = sizeof(name_table) / sizeof(name_table[0]);

     const QLatin1String *name = qBinaryFind(name_table, name_table + N,
                                             entity);
     int index = name - name_table;
     if (index == N)
         return QChar();

     return QChar(value_table[index]);
 }

This kind of code is for advanced users only; for most applications, a QMap- or QHash-based approach would work just as well:

 QChar resolveEntity(const QString &entity)
 {
     static QMap<QString, int> entityMap;

     if (!entityMap) {
         entityMap.insert("AElig", 0x0061);
         entityMap.insert("Aacute", 0x00c1);
         ...
         entityMap.insert("zwnj", 0x200c);
     }
     return QChar(entityMap.value(entity));
 }

Types of Iterators

The algorithms have certain requirements on the iterator types they accept, and these are specified individually for each function. The compiler will produce an error if a requirement isn't met.

Input Iterators

An input iterator is an iterator that can be used for reading data sequentially from a container. It must provide the following operators: == and != for comparing two iterators, unary * for retrieving the value stored in the item, and prefix ++ for advancing to the next item.

The Qt containers' iterator types (const and non-const) are all input iterators.

Output Iterators

An output iterator is an iterator that can be used for writing data sequentially to a container or to some output stream. It must provide the following operators: unary * for writing a value (i.e., *it = val) and prefix ++ for advancing to the next item.

The Qt containers' non-const iterator types are all output iterators.

Forward Iterators

A forward iterator is an iterator that meets the requirements of both input iterators and output iterators.

The Qt containers' non-const iterator types are all forward iterators.

Bidirectional Iterators

A bidirectional iterator is an iterator that meets the requirements of forward iterators but that in addition supports prefix -- for iterating backward.

The Qt containers' non-const iterator types are all bidirectional iterators.

Random Access Iterators

The last category, random access iterators, is the most powerful type of iterator. It supports all the requirements of a bidirectional iterator, and supports the following operations:

QList and QVector's non-const iterator types are random access iterators.

See also container classes and <QtGlobal>.

Function Documentation

RandomAccessIterator qBinaryFind ( RandomAccessIterator begin, RandomAccessIterator end, const T & value )

Performs a binary search of the range [begin, end) and returns the position of an occurrence of value. If there are no occurrences of value, returns end.

The items in the range [begin, end) must be sorted in ascending order; see qSort().

If there are many occurrences of the same value, any one of them could be returned. Use qLowerBound() or qUpperBound() if you need finer control.

Example:

 QVector<int> vect;
 vect << 3 << 3 << 6 << 6 << 6 << 8;

 QVector<int>::iterator i =
         qBinaryFind(vect.begin(), vect.end(), 6);
 // i == vect.begin() + 2 (or 3 or 4)

This function requires the item type (in the example above, QString) to implement operator<().

See the detailed description for an example usage.

See also qLowerBound(), qUpperBound(), and random access iterators.

RandomAccessIterator qBinaryFind ( RandomAccessIterator begin, RandomAccessIterator end, const T & value, LessThan lessThan )

This is an overloaded function.

Uses the lessThan function instead of operator<() to compare the items.

Note that the items in the range must be sorted according to the order specified by the lessThan object.

Container::const_iterator qBinaryFind ( const Container & container, const T & value )

This is an overloaded function.

This is the same as qBinaryFind(container.begin(), container.end(), value);

OutputIterator qCopy ( InputIterator begin1, InputIterator end1, OutputIterator begin2 )

Copies the items from range [begin1, end1) to range [begin2, ...), in the order in which they appear.

The item at position begin1 is assigned to that at position begin2; the item at position begin1 + 1 is assigned to that at position begin2 + 1; and so on.

Example:

 QStringList list;
 list << "one" << "two" << "three";

 QVector<QString> vect1(3);
 qCopy(list.begin(), list.end(), vect1.begin());
 // vect: [ "one", "two", "three" ]

 QVector<QString> vect2(8);
 qCopy(list.begin(), list.end(), vect2.begin() + 2);
 // vect: [ "", "", "one", "two", "three", "", "", "" ]

See also qCopyBackward(), input iterators, and output iterators.

BiIterator2 qCopyBackward ( BiIterator1 begin1, BiIterator1 end1, BiIterator2 end2 )

Copies the items from range [begin1, end1) to range [..., end2).

The item at position end1 - 1 is assigned to that at position end2 - 1; the item at position end1 - 2 is assigned to that at position end2 - 2; and so on.

Example:

 QStringList list;
 list << "one" << "two" << "three";

 QVector<QString> vect(5);
 qCopyBackward(list.begin(), list.end(), vect.end());
 // vect: [ "", "", "one", "two", "three" ]

See also qCopy() and bidirectional iterators.

void qCount ( InputIterator begin, InputIterator end, const T & value, Size & n )

Returns the number of occurrences of value in the range [begin, end), which is returned in n. n is never initialized, the count is added to n. It is the caller's responsibility to initialize n.

Example:

 QList<int> list;
 list << 3 << 3 << 6 << 6 << 6 << 8;

 int countOf6 = 0;
 qCount(list.begin(), list.end(), 6, countOf6);
 // countOf6 == 3

 int countOf7 = 0;
 qCount(list.begin(), list.end(), 7, countOf7);
 // countOf7 == 0

This function requires the item type (in the example above, int) to implement operator==().

See also input iterators.

void qCount ( const Container & container, const T & value, Size & n )

This is an overloaded function.

Instead of operating on iterators, as in the other overload, this function operates on the specified container to obtain the number of instances of value in the variable passed as a reference in argument n.

void qDeleteAll ( ForwardIterator begin, ForwardIterator end )

Deletes all the items in the range [begin, end) using the C++ delete operator. The item type must be a pointer type (for example, QWidget *).

Example:

 QList<Employee *> list;
 list.append(new Employee("Blackpool", "Stephen"));
 list.append(new Employee("Twist", "Oliver"));

 qDeleteAll(list.begin(), list.end());
 list.clear();

Notice that qDeleteAll() doesn't remove the items from the container; it merely calls delete on them. In the example above, we call clear() on the container to remove the items.

This function can also be used to delete items stored in associative containers, such as QMap and QHash. Only the objects stored in each container will be deleted by this function; objects used as keys will not be deleted.

See also forward iterators.

void qDeleteAll ( const Container & c )

This is an overloaded function.

This is the same as qDeleteAll(c.begin(), c.end()).

bool qEqual ( InputIterator1 begin1, InputIterator1 end1, InputIterator2 begin2 )

Compares the items in the range [begin1, end1) with the items in the range [begin2, ...). Returns true if all the items compare equal; otherwise returns false.

Example:

 QStringList list;
 list << "one" << "two" << "three";

 QVector<QString> vect(3);
 vect[0] = "one";
 vect[1] = "two";
 vect[2] = "three";

 bool ret1 = qEqual(list.begin(), list.end(), vect.begin());
 // ret1 == true

 vect[2] = "seven";
 bool ret2 = qEqual(list.begin(), list.end(), vect.begin());
 // ret2 == false

This function requires the item type (in the example above, QString) to implement operator==().

See also input iterators.

void qFill ( ForwardIterator begin, ForwardIterator end, const T & value )

Fills the range [begin, end) with value.

Example:

 QStringList list;
 list << "one" << "two" << "three";

 qFill(list.begin(), list.end(), "eleven");
 // list: [ "eleven", "eleven", "eleven" ]

 qFill(list.begin() + 1, list.end(), "six");
 // list: [ "eleven", "six", "six" ]

See also qCopy() and forward iterators.

void qFill ( Container & container, const T & value )

This is an overloaded function.

This is the same as qFill(container.begin(), container.end(), value);

InputIterator qFind ( InputIterator begin, InputIterator end, const T & value )

Returns an iterator to the first occurrence of value in a container in the range [begin, end). Returns end if value isn't found.

Example:

 QStringList list;
 list << "one" << "two" << "three";

 QStringList::iterator i1 = qFind(list.begin(), list.end(), "two");
 // i1 == list.begin() + 1

 QStringList::iterator i2 = qFind(list.begin(), list.end(), "seventy");
 // i2 == list.end()

This function requires the item type (in the example above, QString) to implement operator==().

If the items in the range are in ascending order, you can get faster results by using qLowerBound() or qBinaryFind() instead of qFind().

See also qBinaryFind() and input iterators.

Container::const_iterator qFind ( const Container & container, const T & value )

This is an overloaded function.

This is the same as qFind(container.begin(), container.end(), value);

LessThan qGreater ()

Returns a functional object, or functor, that can be passed to qSort() or qStableSort().

Example:

 QList<int> list;
 list << 33 << 12 << 68 << 6 << 12;
 qSort(list.begin(), list.end(), qGreater<int>());
 // list: [ 68, 33, 12, 12, 6 ]

See also qLess<T>().

LessThan qLess ()

Returns a functional object, or functor, that can be passed to qSort() or qStableSort().

Example:

 QList<int> list;
 list << 33 << 12 << 68 << 6 << 12;
 qSort(list.begin(), list.end(), qLess<int>());
 // list: [ 6, 12, 12, 33, 68 ]

See also qGreater<T>().

RandomAccessIterator qLowerBound ( RandomAccessIterator begin, RandomAccessIterator end, const T & value )

Performs a binary search of the range [begin, end) and returns the position of the first ocurrence of value. If no such item is found, returns the position where it should be inserted.

The items in the range [begin, end) must be sorted in ascending order; see qSort().

Example:

 QList<int> list;
 list << 3 << 3 << 6 << 6 << 6 << 8;

 QList<int>::iterator i = qLowerBound(list.begin(), list.end(), 5);
 list.insert(i, 5);
 // list: [ 3, 3, 5, 6, 6, 6, 8 ]

 i = qLowerBound(list.begin(), list.end(), 12);
 list.insert(i, 12);
 // list: [ 3, 3, 5, 6, 6, 6, 8, 12 ]

This function requires the item type (in the example above, int) to implement operator<().

qLowerBound() can be used in conjunction with qUpperBound() to iterate over all occurrences of the same value:

 QVector<int> vect;
 vect << 3 << 3 << 6 << 6 << 6 << 8;
 QVector<int>::iterator begin6 =
         qLowerBound(vect.begin(), vect.end(), 6);
 QVector<int>::iterator end6 =
         qUpperBound(begin6, vect.end(), 6);

 QVector<int>::iterator i = begin6;
 while (i != end6) {
     *i = 7;
     ++i;
 }
 // vect: [ 3, 3, 7, 7, 7, 8 ]

See also qUpperBound() and qBinaryFind().

RandomAccessIterator qLowerBound ( RandomAccessIterator begin, RandomAccessIterator end, const T & value, LessThan lessThan )

This is an overloaded function.

Uses the lessThan function instead of operator<() to compare the items.

Note that the items in the range must be sorted according to the order specified by the lessThan object.

Container::const_iterator qLowerBound ( const Container & container, const T & value )

This is an overloaded function.

For read-only iteration over containers, this function is broadly equivalent to qLowerBound(container.begin(), container.end(), value). However, since it returns a const iterator, you cannot use it to modify the container; for example, to insert items.

void qSort ( RandomAccessIterator begin, RandomAccessIterator end )

Sorts the items in range [begin, end) in ascending order using the quicksort algorithm.

Example:

 QList<int> list;
 list << 33 << 12 << 68 << 6 << 12;
 qSort(list.begin(), list.end());
 // list: [ 6, 12, 12, 33, 68 ]

The sort algorithm is efficient on large data sets. It operates in linear-logarithmic time, O(n log n).

This function requires the item type (in the example above, int) to implement operator<().

If neither of the two items is "less than" the other, the items are taken to be equal. It is then undefined which one of the two items will appear before the other after the sort.

See also qStableSort() and random access iterators.

void qSort ( RandomAccessIterator begin, RandomAccessIterator end, LessThan lessThan )

This is an overloaded function.

Uses the lessThan function instead of operator<() to compare the items.

For example, here's how to sort the strings in a QStringList in case-insensitive alphabetical order:

 bool caseInsensitiveLessThan(const QString &s1, const QString &s2)
 {
     return s1.toLower() < s2.toLower();
 }

 int doSomething()
 {
     QStringList list;
     list << "AlPha" << "beTA" << "gamma" << "DELTA";
     qSort(list.begin(), list.end(), caseInsensitiveLessThan);
     // list: [ "AlPha", "beTA", "DELTA", "gamma" ]
 }

To sort values in reverse order, pass qGreater<T>() as the lessThan parameter. For example:

 QList<int> list;
 list << 33 << 12 << 68 << 6 << 12;
 qSort(list.begin(), list.end(), qGreater<int>());
 // list: [ 68, 33, 12, 12, 6 ]

If neither of the two items is "less than" the other, the items are taken to be equal. It is then undefined which one of the two items will appear before the other after the sort.

An alternative to using qSort() is to put the items to sort in a QMap, using the sort key as the QMap key. This is often more convenient than defining a lessThan function. For example, the following code shows how to sort a list of strings case insensitively using QMap:

 QStringList list;
 list << "AlPha" << "beTA" << "gamma" << "DELTA";

 QMap<QString, QString> map;
 foreach (QString str, list)
     map.insert(str.toLower(), str);

 list = map.values();

See also QMap.

void qSort ( Container & container )

This is an overloaded function.

This is the same as qSort(container.begin(), container.end());

void qStableSort ( RandomAccessIterator begin, RandomAccessIterator end )

Sorts the items in range [begin, end) in ascending order using a stable sorting algorithm.

If neither of the two items is "less than" the other, the items are taken to be equal. The item that appeared before the other in the original container will still appear first after the sort. This property is often useful when sorting user-visible data.

Example:

 QList<int> list;
 list << 33 << 12 << 68 << 6 << 12;
 qStableSort(list.begin(), list.end());
 // list: [ 6, 12, 12, 33, 68 ]

The sort algorithm is efficient on large data sets. It operates in linear-logarithmic time, O(n log n).

This function requires the item type (in the example above, int) to implement operator<().

See also qSort() and random access iterators.

void qStableSort ( RandomAccessIterator begin, RandomAccessIterator end, LessThan lessThan )

This is an overloaded function.

Uses the lessThan function instead of operator<() to compare the items.

For example, here's how to sort the strings in a QStringList in case-insensitive alphabetical order:

 bool caseInsensitiveLessThan(const QString &s1, const QString &s2)
 {
     return s1.toLower() < s2.toLower();
 }

 int doSomething()
 {
     QStringList list;
     list << "AlPha" << "beTA" << "gamma" << "DELTA";
     qStableSort(list.begin(), list.end(), caseInsensitiveLessThan);
     // list: [ "AlPha", "beTA", "DELTA", "gamma" ]
 }

Note that earlier versions of Qt allowed using a lessThan function that took its arguments by non-const reference. From 4.3 and on this is no longer possible, the arguments has to be passed by const reference or value.

To sort values in reverse order, pass qGreater<T>() as the lessThan parameter. For example:

 QList<int> list;
 list << 33 << 12 << 68 << 6 << 12;
 qStableSort(list.begin(), list.end(), qGreater<int>());
 // list: [ 68, 33, 12, 12, 6 ]

If neither of the two items is "less than" the other, the items are taken to be equal. The item that appeared before the other in the original container will still appear first after the sort. This property is often useful when sorting user-visible data.

void qStableSort ( Container & container )

This is an overloaded function.

This is the same as qStableSort(container.begin(), container.end());

void qSwap ( T & var1, T & var2 )

Exchanges the values of variables var1 and var2.

Example:

 double pi = 3.14;
 double e = 2.71;

 qSwap(pi, e);
 // pi == 2.71, e == 3.14

RandomAccessIterator qUpperBound ( RandomAccessIterator begin, RandomAccessIterator end, const T & value )

Performs a binary search of the range [begin, end) and returns the position of the one-past-the-last occurrence of value. If no such item is found, returns the position where the item should be inserted.

The items in the range [begin, end) must be sorted in ascending order; see qSort().

Example:

 QList<int> list;
 list << 3 << 3 << 6 << 6 << 6 << 8;

 QList<int>::iterator i = qUpperBound(list.begin(), list.end(), 5);
 list.insert(i, 5);
 // list: [ 3, 3, 5, 6, 6, 6, 8 ]

 i = qUpperBound(list.begin(), list.end(), 12);
 list.insert(i, 12);
 // list: [ 3, 3, 5, 6, 6, 6, 8, 12 ]

This function requires the item type (in the example above, int) to implement operator<().

qUpperBound() can be used in conjunction with qLowerBound() to iterate over all occurrences of the same value:

 QVector<int> vect;
 vect << 3 << 3 << 6 << 6 << 6 << 8;
 QVector<int>::iterator begin6 =
         qLowerBound(vect.begin(), vect.end(), 6);
 QVector<int>::iterator end6 =
         qUpperBound(vect.begin(), vect.end(), 6);

 QVector<int>::iterator i = begin6;
 while (i != end6) {
     *i = 7;
     ++i;
 }
 // vect: [ 3, 3, 7, 7, 7, 8 ]

See also qLowerBound() and qBinaryFind().

RandomAccessIterator qUpperBound ( RandomAccessIterator begin, RandomAccessIterator end, const T & value, LessThan lessThan )

This is an overloaded function.

Uses the lessThan function instead of operator<() to compare the items.

Note that the items in the range must be sorted according to the order specified by the lessThan object.

Container::const_iterator qUpperBound ( const Container & container, const T & value )

This is an overloaded function.

This is the same as qUpperBound(container.begin(), container.end(), value);