QgsAggregateCalculator class

Utility class for calculating aggregates for a field (or expression) over the features from a vector layer.

Reference

It is recommended that QgsVectorLayer::aggregate() is used rather then directly using this class, as the QgsVectorLayer method can handle delegating aggregate calculation to a data provider for remote calculation.

Public types

struct AggregateInfo: Structured information about the available aggregates.
struct AggregateParameters: A bundle of parameters controlling aggregate calculation.
enum Aggregate { Count, CountDistinct, CountMissing, Min, Max, Sum, Mean, Median, StDev, StDevSample, Range, Minority, Majority, FirstQuartile, ThirdQuartile, InterQuartileRange, StringMinimumLength, StringMaximumLength, StringConcatenate, GeometryCollect, ArrayAggregate }: Available aggregates to calculate.

Public static functions

static auto aggregates() -> QList<QgsAggregateCalculator::AggregateInfo>: Structured information for available aggregates.
static auto stringToAggregate(const QString& string, bool* ok = nullptr) -> Aggregate: Converts a string to a aggregate type.

Constructors, destructors, conversion operators

QgsAggregateCalculator(const QgsVectorLayer* layer): Constructor for QgsAggregateCalculator.

Public functions

auto calculate(Aggregate aggregate, const QString& fieldOrExpression, QgsExpressionContext* context = nullptr, bool* ok = nullptr) const -> QVariant: Calculates the value of an aggregate.
auto delimiter() const -> QString: Returns the delimiter used for joining values with the StringConcatenate aggregate.
auto filter() const -> QString: Returns the filter which limits the features used during the aggregate calculation.
auto layer() const -> const QgsVectorLayer*: Returns the associated vector layer.
void setDelimiter(const QString& delimiter): Sets the delimiter to use for joining values with the StringConcatenate aggregate.
void setFilter(const QString& filterExpression): Sets a filter to limit the features used during the aggregate calculation.
void setParameters(const AggregateParameters& parameters): Sets all aggregate parameters from a parameter bundle.

Enum documentation

enum QgsAggregateCalculator::Aggregate

Available aggregates to calculate.

Not all aggregates are available for all field types.

Enumerators
Count	Count.
CountDistinct	Number of distinct values.
CountMissing	Number of missing (null) values.
Min	Min of values.
Max	Max of values.
Sum	Sum of values.
Mean	Mean of values (numeric fields only)
Median	Median of values (numeric fields only)
StDev	Standard deviation of values (numeric fields only)
StDevSample	Sample standard deviation of values (numeric fields only)
Range	Range of values (max - min) (numeric and datetime fields only)
Minority	Minority of values (numeric fields only)
Majority	Majority of values (numeric fields only)
FirstQuartile	First quartile (numeric fields only)
ThirdQuartile	Third quartile (numeric fields only)
InterQuartileRange	Inter quartile range (IQR) (numeric fields only)
StringMinimumLength	Minimum length of string (string fields only)
StringMaximumLength	Maximum length of string (string fields only)
StringConcatenate
GeometryCollect	Concatenate values with a joining string (string fields only). Specify the delimiter using setDelimiter().
ArrayAggregate	Create a multipart geometry from aggregated geometries. Create an array of values

Function documentation

static QList<QgsAggregateCalculator::AggregateInfo> QgsAggregateCalculator::aggregates()

Structured information for available aggregates.

static Aggregate QgsAggregateCalculator::stringToAggregate(const QString& string, bool* ok = nullptr)

Converts a string to a aggregate type.

Parameters
string	string to convert
ok	if specified, will be set to true if conversion was successful
Returns	aggregate type

QgsAggregateCalculator::QgsAggregateCalculator(const QgsVectorLayer* layer)

Constructor for QgsAggregateCalculator.

Parameters
layer	vector layer to calculate aggregate from

QVariant QgsAggregateCalculator::calculate(Aggregate aggregate, const QString& fieldOrExpression, QgsExpressionContext* context = nullptr, bool* ok = nullptr) const

Calculates the value of an aggregate.

Parameters
aggregate	aggregate to calculate
fieldOrExpression	source field or expression to use as basis for aggregated values. If an expression is used, then the context parameter must be set.
context	expression context for evaluating expressions
ok	if specified, will be set to true if aggregate calculation was successful
Returns	calculated aggregate value

QString QgsAggregateCalculator::delimiter() const

Returns the delimiter used for joining values with the StringConcatenate aggregate.

QString QgsAggregateCalculator::filter() const

Returns the filter which limits the features used during the aggregate calculation.

void QgsAggregateCalculator::setDelimiter(const QString& delimiter)

Sets the delimiter to use for joining values with the StringConcatenate aggregate.

Parameters
delimiter	string delimiter

void QgsAggregateCalculator::setFilter(const QString& filterExpression)

Sets a filter to limit the features used during the aggregate calculation.

Parameters
filterExpression	expression for filtering features, or empty string to remove filter

void QgsAggregateCalculator::setParameters(const AggregateParameters& parameters)

Sets all aggregate parameters from a parameter bundle.

Parameters
parameters	aggregate parameters