diff --git a/FDTD/engine.cpp b/FDTD/engine.cpp
index 540e0f7..09c011a 100644
--- a/FDTD/engine.cpp
+++ b/FDTD/engine.cpp
@@ -20,14 +20,14 @@
 
 //! \brief construct an Engine instance
 //! it's the responsibility of the caller to free the returned pointer
-Engine* Engine::createEngine(Operator* op)
+Engine* Engine::createEngine(const Operator* op)
 {
 	Engine* e = new Engine(op);
 	e->Init();
 	return e;
 }
 
-Engine::Engine(Operator* op)
+Engine::Engine(const Operator* op)
 {
 	Op = op;
 }
diff --git a/FDTD/engine.h b/FDTD/engine.h
index a917686..7068ac2 100644
--- a/FDTD/engine.h
+++ b/FDTD/engine.h
@@ -23,7 +23,7 @@
 class Engine
 {
 public:
-	static Engine* createEngine(Operator* op);
+	static Engine* createEngine(const Operator* op);
 	virtual ~Engine();
 
 	virtual void Init();
@@ -38,8 +38,8 @@ public:
 	virtual FDTD_FLOAT**** GetCurrents() {return curr;};
 
 protected:
-	Engine(Operator* op);
-	Operator* Op;
+	Engine(const Operator* op);
+	const Operator* Op;
 
 	FDTD_FLOAT**** volt;
 	FDTD_FLOAT**** curr;
diff --git a/FDTD/operator.cpp b/FDTD/operator.cpp
index 4cd1484..bff3584 100644
--- a/FDTD/operator.cpp
+++ b/FDTD/operator.cpp
@@ -205,14 +205,14 @@ struct Operator::Grid_Path Operator::FindPath(double start[], double stop[])
 	return path;
 }
 
-double Operator::GetNumberCells()
+double Operator::GetNumberCells() const
 {
 	if (numLines)
 		return (numLines[0])*(numLines[1])*(numLines[2]); //it's more like number of nodes???
 	return 0;
 }
 
-void Operator::ShowStat()
+void Operator::ShowStat() const
 {
 	unsigned int OpSize = 12*numLines[0]*numLines[1]*numLines[2]*sizeof(FDTD_FLOAT);
 	unsigned int FieldSize = 6*numLines[0]*numLines[1]*numLines[2]*sizeof(FDTD_FLOAT);
diff --git a/FDTD/operator.h b/FDTD/operator.h
index f17197c..45b8db4 100644
--- a/FDTD/operator.h
+++ b/FDTD/operator.h
@@ -46,11 +46,11 @@ public:
 	virtual void ApplyElectricBC(bool* dirs); //applied by default to all boundaries
 	virtual void ApplyMagneticBC(bool* dirs);
 
-	double GetTimestep() {return dT;};
+	double GetTimestep() const {return dT;};
 	unsigned int GetNyquistNum(double fmax);
-	double GetNumberCells();
+	double GetNumberCells() const;
 
-	void ShowStat();
+	void ShowStat() const;
 
 	void DumpOperator2File(string filename);
 	void DumpMaterial2File(string filename);
diff --git a/tools/array_ops.cpp b/tools/array_ops.cpp
index 42c010d..d80a231 100644
--- a/tools/array_ops.cpp
+++ b/tools/array_ops.cpp
@@ -18,7 +18,7 @@
 #include "array_ops.h"
 #include <ostream>
 
-FDTD_FLOAT*** Create3DArray(unsigned int* numLines)
+FDTD_FLOAT*** Create3DArray(const unsigned int* numLines)
 {
 	FDTD_FLOAT*** array=NULL;
 	unsigned int pos[3];
@@ -38,7 +38,7 @@ FDTD_FLOAT*** Create3DArray(unsigned int* numLines)
 	return array;
 }
 
-void Delete3DArray(FDTD_FLOAT*** array, unsigned int* numLines)
+void Delete3DArray(FDTD_FLOAT*** array, const unsigned int* numLines)
 {
 	if (array==NULL) return;
 	unsigned int pos[3];
@@ -53,7 +53,7 @@ void Delete3DArray(FDTD_FLOAT*** array, unsigned int* numLines)
 	delete[] array;
 }
 
-FDTD_FLOAT**** Create_N_3DArray(unsigned int* numLines)
+FDTD_FLOAT**** Create_N_3DArray(const unsigned int* numLines)
 {
 	FDTD_FLOAT**** array=NULL;
 	array = new FDTD_FLOAT***[3];
@@ -64,7 +64,7 @@ FDTD_FLOAT**** Create_N_3DArray(unsigned int* numLines)
 	return array;
 }
 
-void Delete_N_3DArray(FDTD_FLOAT**** array, unsigned int* numLines)
+void Delete_N_3DArray(FDTD_FLOAT**** array, const unsigned int* numLines)
 {
 	if (array==NULL) return;
 	unsigned int pos[3];
diff --git a/tools/array_ops.h b/tools/array_ops.h
index b04c821..5424e05 100644
--- a/tools/array_ops.h
+++ b/tools/array_ops.h
@@ -20,12 +20,12 @@
 
 #include "../FDTD/operator.h"
 
-FDTD_FLOAT*** Create3DArray(unsigned int* numLines);
-void Delete3DArray(FDTD_FLOAT*** array, unsigned int* numLines);
+FDTD_FLOAT*** Create3DArray(const unsigned int* numLines);
+void Delete3DArray(FDTD_FLOAT*** array, const unsigned int* numLines);
 
-FDTD_FLOAT**** Create_N_3DArray(unsigned int* numLines);
-void Delete_N_3DArray(FDTD_FLOAT**** array, unsigned int* numLines);
+FDTD_FLOAT**** Create_N_3DArray(const unsigned int* numLines);
+void Delete_N_3DArray(FDTD_FLOAT**** array, const unsigned int* numLines);
 
-void Dump_N_3DArray2File(ostream &file, FDTD_FLOAT**** array, unsigned int* numLines);
+void Dump_N_3DArray2File(ostream &file, FDTD_FLOAT**** array, const unsigned int* numLines);
 
 #endif // ARRAY_OPS_H