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

Qt provides a number of global template functions in <QtAlgorithms> that work on containers and perform small tasks to make life easier, such as qDeleteAll() , which invokes operator delete on all items in a given container or in a given range. You can use these algorithms with any container class that provides STL-style iterators, including Qt’s QList , QLinkedList , QVector , QMap , and QHash classes.

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.

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 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, 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:

i += n

advances iterator i by n positions

i -= n

moves iterator i back by n positions

i + n or n + i

returns the iterator for the item n positions ahead of iterator i

i - n

returns the iterator for the item n positions behind of iterator i

i - j

returns the number of items between iterators i and j

i[n]

same as *(i + n)

i < j

returns true if iterator j comes after iterator i

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

Qt and the STL Algorithms¶

Historically, Qt used to provide functions which were direct equivalents of many STL algorithmic functions. Starting with Qt 5.0, you are instead encouraged to use directly the implementations available in the STL; most of the Qt ones have been deprecated (although they are still available to keep the old code compiling).

Porting guidelines¶

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

add the #include <algorithm> preprocessor directive;

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

Qt function

STL function

qBinaryFind

std::binary_search or std::lower_bound

qCopy

std::copy

qCopyBackward

std::copy_backward

qEqual

std::equal

qFill

std::fill

qFind

std::find

qCount

std::count

qSort

std::sort

qStableSort

std::stable_sort

qLowerBound

std::lower_bound

qUpperBound

std::upper_bound

qLess

std::less

qGreater

std::greater

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

container classes <QtGlobal>

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

qLowerBound() qUpperBound() random access iterators

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 is an overloaded function.

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

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

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

qCopyBackward() input iterators output iterators

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

qCopy() bidirectional iterators

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

input iterators

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 .

Returns the number of consecutive zero bits in v , when searching from the MSB. For example, (quint8(1)) returns 7 and (quint8(8)) returns 4.

Returns the number of consecutive zero bits in v , when searching from the MSB. For example, qCountLeadingZeroBits (quint16(1)) returns 15 and qCountLeadingZeroBits (quint16(8)) returns 12.

Returns the number of consecutive zero bits in v , when searching from the MSB. For example, qCountLeadingZeroBits (quint64(1)) returns 63 and qCountLeadingZeroBits (quint64(8)) returns 60.

Returns the number of consecutive zero bits in v , when searching from the LSB. For example, (1) returns 0 and (8) returns 3.

This is an overloaded function.

This is an overloaded function.

This is an overloaded function.

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 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

This is an overloaded function.

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

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

input iterators

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

qCopy() forward iterators

This is an overloaded function.

Use std::fill instead.

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

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 .

See also

qBinaryFind() input iterators

This is an overloaded function.

Use std::find instead.

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

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

qLess

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

qGreater

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<() .

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() qBinaryFind()

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 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.

Returns the number of bits set in v . This number is also called the Hamming Weight of v .

This is an overloaded function.

This is an overloaded function.

This is an overloaded function.

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

qStableSort() random access iterators

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 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

QMap

This is an overloaded function.

Use std::sort instead.

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

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

qSort() random access iterators

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 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 is an overloaded function.

Use std::stable_sort instead.

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

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
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<() .

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() qBinaryFind()

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 is an overloaded function.

Use std::upper_bound instead.

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

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