Method for PositionsProperty on non-dense Lists

lib/list.gd

@@ -955,7 +955,8 @@ DeclareGlobalFunction( "PositionMinimum" );
 ##  <Oper Name="PositionsProperty" Arg='list, func'/>
 ##
 ##  <Description>
-##  returns the list of all those positions in the dense list <A>list</A>
+##  returns the list of all those positions in the list <A>list</A>
+##  which are bound and
 ##  for which the property tester function <A>func</A> returns <K>true</K>.
 ##  <P/>
 ##  <Example><![CDATA[
@@ -972,7 +973,7 @@ DeclareGlobalFunction( "PositionMinimum" );
 ##  </ManSection>
 ##  <#/GAPDoc>
 ##
-DeclareOperation( "PositionsProperty", [ IsDenseList, IsFunction ] );
+DeclareOperation( "PositionsProperty", [ IsList, IsFunction ] );
 
 
 #############################################################################

lib/list.gi

@@ -1641,6 +1641,22 @@ InstallGlobalFunction( PositionMinimum,
 ##
 #M  PositionsProperty(<list>,<func>)  . positions of elements with a property
 ##
+InstallMethod( PositionsProperty,
+    "for list and function",
+    [ IsList, IsFunction ],
+    function( list, func )
+    local result, i;
+
+    result:= [];
+    for i in [ 1 .. Length( list ) ] do
+      if IsBound( list[ i ] ) and func( list[i] ) then
+        Add( result, i );
+      fi;
+    od;
+
+    return result;
+    end );
+
 InstallMethod( PositionsProperty,
     "for dense list and function",
     [ IsDenseList, IsFunction ],

tst/testinstall/list.tst

@@ -197,5 +197,17 @@ gap> l := [40, 39 .. 10];;
 gap> RepresentativeSmallest(l);
 10
 
+# PositionsProperty
+gap> ll := [ 1, , "s" ];;
+gap> PositionsProperty( ll, ReturnTrue );
+[ 1, 3 ]
+gap> PositionsProperty( ll, IsInt );
+[ 1 ]
+gap> ll := [ 1, 2, 3 ];;
+gap> PositionsProperty( ll, ReturnTrue );
+[ 1, 2, 3 ]
+gap> PositionsProperty( ll, IsInt );
+[ 1, 2, 3 ]
+
 #
 gap> STOP_TEST("list.tst");