support for adding/subtracting constant

Author: evaleev

btas/tensor.h

@@ -585,6 +585,24 @@ namespace btas {
       return y; /* automatically called move semantics */
     }
 
+    /// adds a number to every element
+    Tensor operator+(const value_type& x) const {
+      Tensor y = this->clone();
+      y += x;
+      return y; /* automatically called move semantics */
+    }
+
+    /// adds a number to every element
+    Tensor& operator+=(const value_type& x) {
+      using std::begin;
+      using std::cbegin;
+      using std::cend;
+      std::transform(cbegin(storage_), cend(storage_), begin(storage_), [x](const auto& v) {
+        return v+x;
+      });
+      return *this;
+    }
+
     /// subtraction assignment
     Tensor& operator-=(const Tensor& x) {
       using std::begin;
@@ -603,6 +621,24 @@ namespace btas {
       return y; /* automatically called move semantics */
     }
 
+    /// subtracts a number from every element
+    Tensor operator-(const value_type& x) const {
+      Tensor y = this->clone();
+      y -= x;
+      return y; /* automatically called move semantics */
+    }
+
+    /// subtracts a number from every element
+    Tensor& operator-=(const value_type& x) {
+      using std::begin;
+      using std::cbegin;
+      using std::cend;
+      std::transform(cbegin(storage_), cend(storage_), begin(storage_), [x](const auto& v) {
+        return v-x;
+      });
+      return *this;
+    }
+
     /// \return bare const pointer to the first element of data_
     /// this enables to call BLAS functions
     const_pointer data() const {

unittest/tensor_test.cc

@@ -219,6 +219,14 @@ TEST_CASE("Tensor Operations") {
     CHECK_NOTHROW( static_cast<double>(T0) == 11);
   }
 
+  SECTION("Add/subtract constant") {
+    T3.fill(1.);
+    auto T3_plus_1 = T3 + 1.;
+    for (auto x : T3_plus_1) CHECK(x == 2.);
+    T3_plus_1 -= 1.;
+    for (auto x : T3_plus_1) CHECK(x == 1.);
+  }
+
   SECTION("Generate") {
     std::vector<double> data(T3.size());
     for (auto& x : data) x = rng();