diff --git a/histogram.cpp b/histogram.cpp
index 864bd97..cf4a912 100644
--- a/histogram.cpp
+++ b/histogram.cpp
@@ -1,17 +1,17 @@
 #include "histogram.h"
 
-void find_minmax(const std::vector<double>& vec, double& min, double& max) {
+void find_minmax(std::vector<double>& vec, double& min, double& max) {
 
 
     min = vec[0];
     max = vec[0];
     for (double x : vec) {
         if (x < min) min = x;
-        else if (x > max) max = x;
+        if (x > max) max = x;
     }
 }
 
-std::vector<size_t> make_histogram(size_t number, const std::vector<double>& vec) {
+std::vector<size_t> make_histogram(size_t number, std::vector<double>& vec) {
     std::vector<size_t> bins(number);
     if (vec.empty()) return bins;
 
diff --git a/histogram.h b/histogram.h
index 02727a2..8d2cfd4 100644
--- a/histogram.h
+++ b/histogram.h
@@ -4,6 +4,6 @@
 #include <vector>
 #include <iostream>
 
-std::vector<size_t> make_histogram(size_t number, const std::vector<double>& vec);
+std::vector<size_t> make_histogram(size_t number, std::vector<double>& vec);
 
 #endif // HISTOGRAM_H_INCLUDED
diff --git a/histogram_internal.h b/histogram_internal.h
new file mode 100644
index 0000000..5bbe4ff
--- /dev/null
+++ b/histogram_internal.h
@@ -0,0 +1,6 @@
+#ifndef HISTOGRAM_INTERNAL_H_INCLUDED
+#define HISTOGRAM_INTERNAL_H_INCLUDED
+
+void find_minmax(std::vector<double>& vec, double& min, double& max);
+
+#endif // HISTOGRAM_INTERNAL_H_INCLUDED