Fix warnings and remove VLA on C++ Thomas algorithm (#932)

* Fix warnings and remove VLA

* Simplified the reverse for-loop and changed endl to '\n'

Co-authored-by: James Schloss <jrs.schloss@gmail.com>

Author: ShadowMitia

contents/thomas_algorithm/code/cpp/thomas.cpp

@@ -1,43 +1,45 @@
+#include <cstddef>
 #include <iostream>
 #include <vector>
-#include <cstring>
 
-void thomas(std::vector<double> const a, std::vector<double> const b, std::vector<double> const c, std::vector<double>& x) {
-    int size = a.size();
-    double y[size];
-    memset(y, 0, size * sizeof(double));
-
-    y[0] = c[0] / b[0];
-    x[0] = x[0] / b[0];
-
-    for (size_t i = 1; i < size; ++i) {
-        double scale = 1.0 / (b[i] - a[i] * y[i - 1]);
-        y[i] = c[i] * scale;
-        x[i] = (x[i] - a[i] * x[i - 1]) * scale;
-    }
-
-    for (int i = size - 2; i >= 0; --i) {
-        x[i] -= y[i] * x[i + 1];
-    }
+void thomas(
+    std::vector<double> const& a,
+    std::vector<double> const& b,
+    std::vector<double> const& c,
+    std::vector<double>& x) {
+  auto y = std::vector<double>(a.size(), 0.0);
+
+  y[0] = c[0] / b[0];
+  x[0] = x[0] / b[0];
+
+  for (std::size_t i = 1; i < a.size(); ++i) {
+    const auto scale = 1.0 / (b[i] - a[i] * y[i - 1]);
+    y[i] = c[i] * scale;
+    x[i] = (x[i] - a[i] * x[i - 1]) * scale;
+  }
+
+  for (std::size_t i = a.size() - 2; i < a.size(); --i) {
+    x[i] -= y[i] * x[i + 1];
+  }
 }
 
 int main() {
-    std::vector<double> a = {0.0, 2.0, 3.0};
-    std::vector<double> b = {1.0, 3.0, 6.0};
-    std::vector<double> c = {4.0, 5.0, 0.0};
-    std::vector<double> x = {7.0, 5.0, 3.0};
+  const std::vector<double> a = {0.0, 2.0, 3.0};
+  const std::vector<double> b = {1.0, 3.0, 6.0};
+  const std::vector<double> c = {4.0, 5.0, 0.0};
+  std::vector<double> x = {7.0, 5.0, 3.0};
 
-    std::cout << "The system" << std::endl; 
-    std::cout << "[1.0  4.0  0.0][x] = [7.0]" << std::endl;
-    std::cout << "[2.0  3.0  5.0][y] = [5.0]" << std::endl;
-    std::cout << "[0.0  3.0  6.0][z] = [3.0]" << std::endl;
-    std::cout << "has the solution" << std::endl;
+  std::cout << "The system\n";
+  std::cout << "[1.0  4.0  0.0][x] = [7.0]\n";
+  std::cout << "[2.0  3.0  5.0][y] = [5.0]\n";
+  std::cout << "[0.0  3.0  6.0][z] = [3.0]\n";
+  std::cout << "has the solution:\n";
 
-    thomas(a, b, c, x);
+  thomas(a, b, c, x);
 
-    for (size_t i = 0; i < 3; ++i) {
-        std::cout << "[" << x[i] << "]" << std::endl;
-    }
+  for (auto const& val : x) {
+    std::cout << "[" << val << "]\n";
+  }
 
-    return 0;
+  return 0;
 }