diff --git a/regression.hh b/regression.hh
index 5d9dece..ccd298d 100644
--- a/regression.hh
+++ b/regression.hh
@@ -14,7 +14,7 @@ class SimpleLinearRegression
 	TYPE xint_, yint_, slope_;
 public:
 	SimpleLinearRegression() : xint_(0), yint_(0), slope_(0) {}
-	SimpleLinearRegression(TYPE *x, TYPE *y, int LEN)
+	SimpleLinearRegression(const TYPE *x, const TYPE *y, int LEN)
 	{
 		TYPE avgX(0), avgY(0);
 		for (int i = 0; i < LEN; ++i) {
@@ -32,7 +32,7 @@ public:
 		slope_ = covXY / varX;
 		yint_ = avgY - slope_ * avgX;
 	}
-	SimpleLinearRegression(TYPE *y, int l, TYPE x0 = 0, TYPE dx = 1)
+	SimpleLinearRegression(const TYPE *y, int l, TYPE x0 = 0, TYPE dx = 1)
 	{
 		TYPE midL((l-1) / TYPE(2));
 		TYPE avgX(x0 + dx*midL);
diff --git a/repeated_median.hh b/repeated_median.hh
index 2ba25b7..eded617 100644
--- a/repeated_median.hh
+++ b/repeated_median.hh
@@ -17,7 +17,7 @@ class RepeatedMedianEstimator
 	TYPE xint_, yint_, slope_;
 public:
 	RepeatedMedianEstimator() : xint_(0), yint_(0), slope_(0) {}
-	void compute(TYPE *x, TYPE *y, int LEN)
+	void compute(const TYPE *x, const TYPE *y, int LEN)
 	{
 		if (LEN > SIZE)
 			LEN = SIZE;
@@ -68,7 +68,7 @@ class RepeatedMedianEstimator2
 	TYPE xint_, yint_, slope_;
 public:
 	RepeatedMedianEstimator2() : xint_(0), yint_(0), slope_(0) {}
-	void compute(TYPE *x, TYPE *y, int LEN)
+	void compute(const TYPE *x, const TYPE *y, int LEN)
 	{
 		if (LEN > SIZE)
 			LEN = SIZE;
diff --git a/spline.hh b/spline.hh
index 8849e80..25114e8 100644
--- a/spline.hh
+++ b/spline.hh
@@ -15,7 +15,7 @@ class UniformNaturalCubicSpline
 	ITYPE x0, dx;
 public:
 	UniformNaturalCubicSpline() = default;
-	UniformNaturalCubicSpline(OTYPE *Y, ITYPE x0 = 0, ITYPE dx = 1, int STRIDE = 1) : x0(x0), dx(dx)
+	UniformNaturalCubicSpline(const OTYPE *Y, ITYPE x0 = 0, ITYPE dx = 1, int STRIDE = 1) : x0(x0), dx(dx)
 	{
 		ITYPE U[KNOTS-1];
 		U[0] = ITYPE(0);
diff --git a/theil_sen.hh b/theil_sen.hh
index c565153..86a343c 100644
--- a/theil_sen.hh
+++ b/theil_sen.hh
@@ -18,7 +18,7 @@ class TheilSenEstimator
 	TYPE xint_, yint_, slope_;
 public:
 	TheilSenEstimator() : xint_(0), yint_(0), slope_(0) {}
-	void compute(TYPE *x, TYPE *y, int LEN)
+	void compute(const TYPE *x, const TYPE *y, int LEN)
 	{
 		int count = 0;
 		for (int i = 0; count < size_ && i < LEN; ++i)