[FEATURE] array_slice expression function

qgis · Aug 6, 2017 · 8978469 · 8978469
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diff --git a/resources/function_help/json/array_slice b/resources/function_help/json/array_slice
@@ -0,0 +1,51 @@
+{
+        "name": "array_get",
+        "type": "function",
+        "description": "Returns a portion of the array. The slice is defined by the startPos and endPos arguments. A slice that ",
+        "arguments": [{
+                        "arg": "array",
+                        "description": "an array"
+                },
+                {
+                        "arg": "startPos",
+                        "description": "the index of the start position of the slice (0 based). The startPos index is included in the slice. If you use a negative startPos, the index is counted from the end of the list (-1 based)."
+                },
+                {
+                        "arg": "endPos",
+                        "description": "the index of the start position of the slice (0 based). The endPos index is included in the slice. If you use a negative endPos, the index is counted from the end of the list (-1 based)."
+                }
+        ],
+        "examples": [{
+                        "expression": "array_slice(array(1,2,3,4,5),0,3)",
+                        "returns": "array: 1,2,3,4"
+                },
+                {
+                        "expression": "array_slice(array(1,2,3,4,5),0,-1)",
+                        "returns": "array: 1,2,3,4,5"
+                },
+                {
+                        "expression": "array_slice(array(1,2,3,4,5),-5,-1)",
+                        "returns": "array: 1,2,3,4,5"
+                },
+                {
+                        "expression": "array_slice(array(1,2,3,4,5),0,0)",
+                        "returns": "array: 1"
+                },
+                {
+                        "expression": "array_slice(array(1,2,3,4,5),-2,-1)",
+                        "returns": "array: 4,5"
+                },
+                {
+                        "expression": "array_slice(array(1,2,3,4,5),-1,-1)",
+                        "returns": "array: 5"
+                },
+                {
+                        "expression": "array_slice(array('Dufour','Valmiera','Chugiak','Brighton'),1,2)",
+                        "returns": "array: 'Valmiera','Chugiak'"
+                },
+                {
+                        "expression": "array_slice(array_slice(array('Dufour','Valmiera','Chugiak','Brighton'),-2,-1)",
+                        "returns": "array: 'Chugiak','Brighton'"
+                }
+        ]
+}
diff --git a/src/core/expression/qgsexpressionfunction.cpp b/src/core/expression/qgsexpressionfunction.cpp
@@ -3602,6 +3602,30 @@ static QVariant fcnArrayCat( const QVariantList &values, const QgsExpressionCont
   return convertToSameType( list, values.at( 0 ).type() );
 }
 
+static QVariant fcnArraySlice( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent )
+{
+  QVariantList list = QgsExpressionUtils::getListValue( values.at( 0 ), parent );
+  qlonglong startPos = QgsExpressionUtils::getIntValue( values.at( 1 ), parent );
+  const qlonglong endPos = QgsExpressionUtils::getIntValue( values.at( 2 ), parent );
+  qlonglong sliceLength = 0;
+  // negative positions means positions taken relative to the end of the array
+  if( startPos < 0 ) {
+      startPos = list.length() + startPos;
+  }
+  if( endPos >= 0 ) {
+      sliceLength = endPos - startPos + 1;
+  }
+  else {
+      sliceLength = list.length() + endPos - startPos + 1;
+  }
+  //avoid negative lengths in QList.mid function
+  if (sliceLength < 0) {
+      sliceLength = 0;
+  }
+  list = list.mid(startPos,sliceLength);
+  return list;
+}
+
 static QVariant fcnArrayReverse( const QVariantList &values, const QgsExpressionContext *, QgsExpression *parent )
 {
   QVariantList list = QgsExpressionUtils::getListValue( values.at( 0 ), parent );
@@ -4263,6 +4287,7 @@ const QList<QgsExpressionFunction *> &QgsExpression::Functions()
         << new QgsStaticExpressionFunction( QStringLiteral( "array_remove_at" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "pos" ) ), fcnArrayRemoveAt, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_remove_all" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "value" ) ), fcnArrayRemoveAll, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_cat" ), -1, fcnArrayCat, QStringLiteral( "Arrays" ) )
+        << new QgsStaticExpressionFunction( QStringLiteral( "array_slice" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "startPos" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "endPos" ) ), fcnArraySlice, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_reverse" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array" ) ), fcnArrayReverse, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_intersect" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "array1" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "array2" ) ), fcnArrayIntersect, QStringLiteral( "Arrays" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "array_distinct" ), 1, fcnArrayDistinct, QStringLiteral( "Arrays" ) )

diff --git a/tests/src/core/testqgsexpression.cpp b/tests/src/core/testqgsexpression.cpp
@@ -2422,6 +2422,9 @@ class TestQgsExpression: public QObject
       QCOMPARE( QgsExpression( "array_intersect(array('1', '2', '3', '4'), array('0', '5'))" ).evaluate( &context ), QVariant( false ) );
 
       QCOMPARE( QgsExpression( "array_reverse(array('Dufour','Valmiera','Chugiak','Wien','Pisa','Lyon','Essen','Nødebo','Las Palmas')) = array('Las Palmas','Nødebo','Essen','Lyon','Pisa','Wien','Chugiak','Valmiera','Dufour')" ).evaluate( &context ), QVariant( true ) );
+
+      QCOMPARE( QgsExpression( "array_slice(array('Dufour','Valmiera','Chugiak','Brighton'),1,2) = array('Valmiera','Chugiak')" ).evaluate( &context ), QVariant( true ) );
+      QCOMPARE( QgsExpression( "array_slice(array('Dufour','Valmiera','Chugiak','Brighton'),-2,-1) = array('Chugiak','Brighton')" ).evaluate( &context ), QVariant( true ) );
     }
 
     void eval_int_array()
@@ -2486,6 +2489,13 @@ class TestQgsExpression: public QObject
 
       QCOMPARE( QgsExpression( "array_reverse(array(2,4,0,10)) = array(10,0,4,2)" ).evaluate( &context ), QVariant( true ) );
 
+      QCOMPARE( QgsExpression( "array_slice(array(1,2,3,4,5),0,3) = array(1,2,3,4)" ).evaluate( &context ), QVariant( true ) );
+      QCOMPARE( QgsExpression( "array_slice(array(1,2,3,4,5),0,-1) = array(1,2,3,4,5)" ).evaluate( &context ), QVariant( true ) );
+      QCOMPARE( QgsExpression( "array_slice(array(1,2,3,4,5),-5,-1) = array(1,2,3,4,5)" ).evaluate( &context ), QVariant( true ) );
+      QCOMPARE( QgsExpression( "array_slice(array(1,2,3,4,5),0,0) = array(1)" ).evaluate( &context ), QVariant( true ) );
+      QCOMPARE( QgsExpression( "array_slice(array(1,2,3,4,5),-2,-1) = array(4,5)" ).evaluate( &context ), QVariant( true ) );
+      QCOMPARE( QgsExpression( "array_slice(array(1,2,3,4,5),-1,-1) = array(5)" ).evaluate( &context ), QVariant( true ) );
+
       QgsExpression badArray( QStringLiteral( "array_get('not an array', 0)" ) );
       QCOMPARE( badArray.evaluate( &context ), QVariant() );
       QVERIFY( badArray.hasEvalError() );