Behaviour and limitations

What VecFor does at the edges, and what it does not do. Every statement of this page is checked by a program of the docs or by the sources.

Comparisons

• <, <=, >, >= between two vectors compare their lengths: two vectors of equal length and different direction are both <= and >= each other. Between a vector and a number, all six compare the length with the number.
• == between two vectors compares the lengths and then the components of the unit vectors; /= is its negation.
• All are exact: no tolerance. For computed vectors, compare normL2(a - b) with a tolerance of your own.

Tolerances of the predicates

is_collinear and is_concyclic take an optional tolerance, 0 by default (the smallest positive number, for is_concyclic). Without it the tests are exact, and points that are collinear or concyclic only up to rounding, as computed points are, pass or fail by chance: pass a tolerance scaled to your geometry. is_concyclic also needs the points in their order around the circle.

Zero vectors and degenerate geometry

Case	Result
normalize, normalized of the zero vector	the zero vector
angle with a zero vector	0
a .paral. b, a .ortho. b with b zero	NaN
distance_to_line with pt1 = pt2	NaN
distance_to_plane and the projections, with three collinear points	0, and the point itself: silently
face_normal3, face_normal4 of a degenerate face	the zero vector, also with norm
a / b with a zero component of b; a / 0	IEEE infinities or NaN, as for numbers

Input and output

• save_into_file, load_from_file and printf never stop the program on an I/O error: without iostat the error is silently ignored, and a vector that fails to load keeps its old value. Always pass iostat.
• fmt is a format string: '*' is not list-directed I/O.
• There is no rec= for direct access files: write the components.
• The I/O procedures and iolen are not pure.

Types and kinds

• Vectors of two kinds do not mix: no a4 + a8, no a8 = a4. Convert by components.
• vector and vector_R8P are two different types, although of the same kind; ex and ex_R8P likewise.
• vector_R16P is quadruple precision only when PENF and VecFor are compiled with -DPENF_R16P, which the FoBiS modes and the makefile do not do: by default it is double precision (see Precision and kinds).
• The scalar arguments of the methods (angle=, tolerance=) and the matrices must have the kind of the vector; the numbers of the operators may have any kind.

Fortran itself

• A method needs a variable or a named constant: (a - b)%normL2() is not Fortran; use the free function, normL2(a - b).
• The user-defined operators bind more loosely than any intrinsic one: a .dot. b + c is a .dot. (b + c); parenthesise (see Precedence).
• The intrinsics sum, dot_product, matmul, maxval, ... do not apply to arrays of vectors.

Not elemental

rotate, mirror, rotation_matrix, mirror_matrix, .matrix. and the I/O procedures work on one vector at a time; loop over arrays. Every other operator and function, and normalize, are elemental.

Not on the device

Only the seven routines of the device-callable API run inside an OpenACC kernel.

Not available

Vectors of two or of more than three components; non-Cartesian frames; a type for points distinct from the type for directions (VecFor uses one type for both); quaternions.