Matrices have two diagonals: main diagonal and anti-diagonal. The main diagonal runs from top-left to bottom-right. For square matrix A:

// main diagonal:

for (size_t xy=0; xy<A.size(); ++xy)

cin >> A[xy][xy];

// anti-diagonal

for (size_t x = A.size()-1, y=0; y<A.size(); --x; ++y

cin >> A[x][y];

No. A scalar matrix is a diagonal matrix whose main diagonal elements are the same. Only if the diagonal elements are all 1 is it an identity matrix.

The diagonal is 1.414 feet.

= 1/2 diagonal 1 * diagonal 2 * sin of angle between them

= 1/2* diagonal * sum of perpendicular to digonal from opp vertices

Diagonal plier is actually a curious misnaming, as it's main job is cutting wires.