diff --git a/code/core/rttbUtils.cpp b/code/core/rttbUtils.cpp
index c2340fc..090726a 100644
--- a/code/core/rttbUtils.cpp
+++ b/code/core/rttbUtils.cpp
@@ -1,54 +1,54 @@
 // -----------------------------------------------------------------------
 // RTToolbox - DKFZ radiotherapy quantitative evaluation library
 //
 // Copyright (c) German Cancer Research Center (DKFZ),
 // Software development for Integrated Diagnostics and Therapy (SIDT).
 // ALL RIGHTS RESERVED.
 // See rttbCopyright.txt or
 // http://www.dkfz.de/en/sidt/projects/rttb/copyright.html
 //
 // This software is distributed WITHOUT ANY WARRANTY; without even
 // the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
 // PURPOSE.  See the above copyright notices for more information.
 //
 //------------------------------------------------------------------------
 /*!
 // @file
 // @version $Revision: 1674 $ (last changed revision)
 // @date    $Date: 2017-01-27 10:34:46 +0100 (Fr, 27 Jan 2017) $ (last change date)
 // @author  $Author: hentsch $ (last changed by)
 */
 
 #ifndef __RTTB_UTILS_H
 #define __RTTB_UTILS_H
 
 #include <map>
 #include <rttbBaseType.h>
 
 namespace rttb
 {
 	namespace core
 	{
 
-		static bool isKey(std::map<double, double> values, double value) {
+		static bool isKey(const std::map<double, double>& values, const double value) {
 			for (auto const& collectionElements : values) {
 				if (std::abs(collectionElements.first - value) <= rttb::errorConstant)
 				{
 					return true;
 				}
 			}
 			return false;
 		}
 
-		static bool isKey(std::vector<double> values, double value) {
+		static bool isKey(const std::vector<double>& values, const double value) {
 			for (auto const& collectionElement : values) {
 				if (std::abs(collectionElement - value) <= rttb::errorConstant)
 				{
 					return true;
 				}
 			}
 			return false;
 		}
 	}
 }
 #endif