Allow negative array get

qgis · Nov 9, 2020 · 9543350 · 9543350
1 parent 5b73ae2
commit 9543350
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Showing 3 changed files with 10 additions and 4 deletions.
diff --git a/resources/function_help/json/array_get b/resources/function_help/json/array_get
@@ -1,8 +1,11 @@
 {
   "name": "array_get",
   "type": "function",
-  "description": "Returns the Nth value (0 for the first one) of an array.",
+  "description": "Returns the Nth value (0 for the first one) or the last -Nth value (-1 for the last one) of an array.",
   "arguments": [ {"arg":"array","description":"an array"},
                  {"arg":"index","description":"the index to get (0 based)"}],
-  "examples": [ { "expression":"array_get(array('a','b','c'),1)", "returns":"'b'"}]
+  "examples": [ 
+  { "expression":"array_get(array('a','b','c'),1)", "returns":"'b'"},
+  { "expression":"array_get(array('a','b','c'),-1)", "returns":"'c'"}
+  ]
 }
diff --git a/src/core/expression/qgsexpressionfunction.cpp b/src/core/expression/qgsexpressionfunction.cpp
@@ -5229,7 +5229,8 @@ static QVariant fcnArrayGet( const QVariantList &values, const QgsExpressionCont
 {
   const QVariantList list = QgsExpressionUtils::getListValue( values.at( 0 ), parent );
   const int pos = QgsExpressionUtils::getNativeIntValue( values.at( 1 ), parent );
-  if ( pos < 0 || pos >= list.length() ) return QVariant();
+  if ( pos >= list.length() ) return QVariant();
+  if ( pos < 0 && ( list.size() + pos ) >= 0 ) return list.at( list.size() + pos );
   return list.at( pos );
 }
 

diff --git a/tests/src/core/testqgsexpression.cpp b/tests/src/core/testqgsexpression.cpp
@@ -3326,7 +3326,9 @@ class TestQgsExpression: public QObject
 
       QCOMPARE( QgsExpression( "array_get(\"ints\", 1)" ).evaluate( &context ), QVariant( -2 ) );
       QCOMPARE( QgsExpression( "array_get(\"ints\", 2)" ).evaluate( &context ), QVariant() );
-      QCOMPARE( QgsExpression( "array_get(\"ints\", -1)" ).evaluate( &context ), QVariant() );
+      QCOMPARE( QgsExpression( "array_get(\"ints\", -1)" ).evaluate( &context ), QVariant( -2 ) );
+      QCOMPARE( QgsExpression( "array_get(\"ints\", -2)" ).evaluate( &context ), QVariant( 1 ) );
+      QCOMPARE( QgsExpression( "array_get(\"ints\", -3)" ).evaluate( &context ), QVariant() );
 
       QVariantList appendExpected = array;
       appendExpected << 3;