<QtAlgorithms> - Generic Algorithms

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 member function, provided for convenience.

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 member function, provided for convenience.

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 member function, provided for convenience.

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.

See also forward iterators.

void qDeleteAll ( const Container & c )

This is an overloaded member function, provided for convenience.

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 rec2 = 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 member function, provided for convenience.

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 member function, provided for convenience.

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: [ 68, 33, 12, 12, 6 ]

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 member function, provided for convenience.

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 member function, provided for convenience.

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

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 of two items neither 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 member function, provided for convenience.

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 of two items neither 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 member function, provided for convenience.

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 of two items neither 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 member function, provided for convenience.

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 of two items neither 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 member function, provided for convenience.

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 member function, provided for convenience.

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 member function, provided for convenience.

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