<QtAlgorithms> - Generic Algorithms

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.

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.

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.

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.

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.

Most of the times, an application using the deprecated Qt algorithmic functions can be easily ported to use the equivalent STL functions. You need to

1. add the #include <algorithm> preprocessor directive;

2. replace the Qt functions with the STL counterparts, according to the table below.

The only cases in which the port may not be straightforward is if the old code relied on template specializations of the qLess() and/or the qSwap() functions, which were used internally by the implementations of the Qt algorithmic functions, but are instead ignored by the STL ones.

In case the old code relied on the specialization of the qLess() functor, then a workaround is explicitly passing an instance of the qLess() class to the STL function, for instance like this:

std::sort(container.begin(), container.end(), qLess<T>());

Instead, since it's not possible to pass a custom swapper functor to STL functions, the only workaround for a template specialization for qSwap() is providing the same specialization for std::swap().

See also forward iterators

This function is deprecated. We strongly advise against using it in new code.

Use std::binary_search or std::lower_bound instead.

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 Also▲

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::binary_search or std::lower_bound instead.

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.

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::binary_search or std::lower_bound instead.

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

This function is deprecated. We strongly advise against using it in new code.

Use std::copy instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

Use std::copy_backward instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

Use std::count instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::count instead.

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.

This function is deprecated. We strongly advise against using it in new code.

Use std::equal instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

Use std::fill instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::fill instead.

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

This function is deprecated. We strongly advise against using it in new code.

Use std::find instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::find instead.

This is the same as qFind(container.constBegin(), container.constEnd(), value);

This function is deprecated. We strongly advise against using it in new code.

Use std::greater instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

Use std::less instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

Use std::lower_bound instead.

Performs a binary search of the range [begin, end) and returns the position of the first occurrence 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▲

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::lower_bound instead.

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.

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::lower_bound instead.

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.

This function is deprecated. We strongly advise against using it in new code.

Use std::sort instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::sort instead.

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 (const QString &str, list)
    map.insert(str.toLower(), str);

list = map.values();

See Also▲

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::sort instead.

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

This function is deprecated. We strongly advise against using it in new code.

Use std::stable_sort instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::stable_sort instead.

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.

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::stable_sort instead.

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

This function is deprecated. We strongly advise against using it in new code.

Use std::swap instead.

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

This function is deprecated. We strongly advise against using it in new code.

Use std::upper_bound instead.

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▲

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::upper_bound instead.

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.

This function is deprecated. We strongly advise against using it in new code.

This is an overloaded function.

Use std::upper_bound instead.

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