# 28 Numerics library [numerics]

## 28.9 Basic linear algebra algorithms [linalg]

### 28.9.7 Exposition-only helpers [linalg.helpers]

#### 28.9.7.7 Preconditions [linalg.helpers.precond]

[Note 1:
These exposition-only helper functions use the less constraining input concepts even for the output arguments, because the additional constraint for assignability of elements is not necessary, and they are sometimes used in a context where the third argument is an input type too.
— end note]
constexpr bool addable( // exposition only const in-vector auto& in1, const in-vector auto& in2, const in-vector auto& out) { return out.extent(0) == in1.extent(0) && out.extent(0) == in2.extent(0); } constexpr bool addable( // exposition only const in-matrix auto& in1, const in-matrix auto& in2, const in-matrix auto& out) { return out.extent(0) == in1.extent(0) && out.extent(1) == in1.extent(1) && out.extent(0) == in2.extent(0) && out.extent(1) == in2.extent(1); } constexpr bool multipliable( // exposition only const in-matrix auto& in_mat, const in-vector auto& in_vec, const in-vector auto& out_vec) { return out_vec.extent(0) == in_mat.extent(0) && in_mat.extent(1) == in_vec.extent(0); } constexpr bool multipliable( // exposition only const in-vector auto& in_vec, const in-matrix auto& in_mat, const in-vector auto& out_vec) { return out_vec.extent(0) == in_mat.extent(1) && in_mat.extent(0) == in_vec.extent(0); } constexpr bool multipliable( // exposition only const in-matrix auto& in_mat1, const in-matrix auto& in_mat2, const in-matrix auto& out_mat) { return out_mat.extent(0) == in_mat1.extent(0) && out_mat.extent(1) == in_mat2.extent(1) && in1_mat.extent(1) == in_mat2.extent(0); }