разделил код

histogram.cpp

@@ -0,0 +1,31 @@
+#include "histogram.h"
+#include <vector>
+#include <cstddef>
+
+using namespace std;
+
+void find_minmax(const vector<double>& numbers, double& min, double& max) {
+    if (numbers.empty()) {
+        return;
+    }
+    min = numbers[0];
+    max = numbers[0];
+
+    for (double x : numbers) {
+        if (x < min) min = x;
+        if (x > max) max = x;
+    }
+}
+
+vector<size_t> make_histogram(const vector<double>& numbers, size_t bin_count) {
+    double min, max;
+    find_minmax(numbers, min, max);
+
+    vector<size_t> bins(bin_count);
+    for (double x : numbers) {
+        size_t bin_index = (x - min) / (max - min) * bin_count;
+        if (bin_index == bin_count) bin_index--;
+        bins[bin_index]++;
+    }
+    return bins;
+}

histogram.h

@@ -0,0 +1,9 @@
+#ifndef HISTOGRAM_H_INCLUDED
+#define HISTOGRAM_H_INCLUDED
+
+#include <vector>
+#include <cstddef>
+
+std::vector<size_t> make_histogram(const std::vector<double>& numbers, size_t bin_count);
+
+#endif

main.cpp

@@ -1,5 +1,6 @@
 #include <iostream>
 #include <vector>
+#include "histogram.h"
 
 using namespace std;
 
@@ -26,32 +27,6 @@ Input input_data()
     return in;
 }
 
-void find_minmax(const vector<double>& numbers, double& min, double& max) {
-    if (numbers.empty()) {
-        return;
-    }
-    min = numbers[0];
-    max = numbers[0];
-
-    for (double x : numbers) {
-        if (x < min) min = x;
-        if (x > max) max = x;
-    }
-}
-
-vector<size_t> make_histogram(const vector<double>& numbers, size_t bin_count) {
-    double min, max;
-    find_minmax(numbers, min, max);
-
-    vector<size_t> bins(bin_count);
-    for (double x : numbers) {
-        size_t bin_index = (x - min) / (max - min) * bin_count;
-        if (bin_index == bin_count) bin_index--;
-        bins[bin_index]++;
-    }
-    return bins;
-}
-
 void show_histogram_text(const vector<size_t>& bins) {
     const size_t SCREEN_WIDTH = 80;
     size_t max_count = 0;