add thomas alg in C++

Author: Yordrar

contents/thomas_algorithm/code/c++/thomas.cpp

@@ -0,0 +1,43 @@
+#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, const size_t size) {
+
+    std::vector<double> y(size);
+    memset(y.data(), 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];
+    }
+}
+
+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};
+
+    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;
+
+    thomas(a, b, c, x, 3);
+
+    for (size_t i = 0; i < 3; ++i) {
+        std::cout << "[" << x[i] << "]" << std::endl;
+    }
+
+    return 0;
+}

contents/thomas_algorithm/thomas_algorithm.md

@@ -113,8 +113,10 @@ You will find this algorithm implemented [in this project](https://scratch.mit.e
 [import, lang:"haskell"](code/haskell/thomas.hs)
 {% sample lang="swift" %}
 [import, lang:"swift"](code/swift/thomas.swift)
+{% sample lang="cpp" %}
+[import, lang:"c_cpp"](code/c++/thomas.cpp)
 {% endmethod %}
 
 <script>
 MathJax.Hub.Queue(["Typeset",MathJax.Hub]);
-</script>
+</script>