linalg : apply_givens_rotationに例を追加 (#1233)

Signed-off-by: Yuya Asano <64895419+sukeya@users.noreply.github.com>

Author: sukeya

reference/linalg/apply_givens_rotation.md

@@ -98,9 +98,107 @@ $$
 
 
 ## 例
+**[注意] 処理系にあるコンパイラで確認していないため、間違っているかもしれません。**
+
+```cpp example
+#include <cmath>
+#include <complex>
+#include <initializer_list>
+#include <execution>
+#include <iostream>
+#include <linalg>
+#include <mdspan>
+#include <vector>
+
+template <class Vector>
+void print(Vector v) {
+  for (int i = 0; i < v.extent(0) - 1; ++i) {
+    std::cout << v[i] << ", ";
+  }
+  std::cout << v[v.extent(0) - 1] << std::endl;
+}
+
+int main()
+{
+  constexpr size_t N = 2;
+
+  using Complex = std::complex<double>;
+
+  {
+    std::initializer_list<double> a_coeff = {1, std::sqrt(3.0)};
+    std::initializer_list<double> b_coeff = {std::sqrt(3.0), -0.5};
+
+    std::vector<double> a_vec(a_coeff);
+    std::mdspan a(a_vec.data(), N);
+
+    std::vector<double> b_vec(b_coeff);
+    std::mdspan b(b_vec.data(), N);
+
+    double c = 0.5;
+    double s = -std::sqrt(3.0) / 2;
+
+    // (1)
+    std::linalg::apply_givens_rotation(a, b, c, s);
+    print(a);
+    print(b);
+
+    // (2)
+    // aとbを初期化
+    a_vec = a_coeff;
+    b_vec = b_coeff;
+    std::linalg::apply_givens_rotation(std::execution::par, a, b, c, s);
+    print(a);
+    print(b);
+  }
+
+  {
+    std::initializer_list<Complex> a_coeff = {Complex{1, 0}, Complex{std::sqrt(3.0), 0}};
+    std::initializer_list<Complex> b_coeff = {Complex{0, std::sqrt(3.0)}, Complex{0, -0.5}};
+
+    std::vector<Complex> a_vec(a_coeff);
+    std::mdspan a(a_vec.data(), N);
+
+    std::vector<Complex> b_vec(b_coeff);
+    std::mdspan b(b_vec.data(), N);
+
+    double  c = 0.5;
+    Complex s = Complex{0, -std::sqrt(3.0) / 2};
+
+    // (3)
+    std::linalg::apply_givens_rotation(a, b, c, s);
+    print(a);
+    print(b);
+
+    // (4)
+    // aとbを初期化
+    a_vec = a_coeff;
+    b_vec = b_coeff;
+    std::linalg::apply_givens_rotation(std::execution::par, a, b, c, s);
+    print(a);
+    print(b);
+  }
+
+  return 0;
+}
+```
+* std::linalg::apply_givens_rotation[color ff0000]
 
 
 ### 出力
+```
+(1)
+1, 0
+0, -1
+(2)
+1, 0
+0, -1
+(3)
+(1,0), (0,0)
+(0,0), (0,-1)
+(4)
+(1,0), (0,0)
+(0,0), (0,-1)
+```
 
 
 ## バージョン