"Rank" is used in APL/J to denote the number of atoms in the shape of a noun. It made its first published appearance in Ken Iverson's "A Programming Language", 1962. Section 1.20, Levels of Structure, says:
Vectors and matrices are arrays which exhibit one level and two levels of structure, respectively. Although in certain fields, such as tensor analysis, it is convenient to define more general arrays whose _rank_ specifies the number of levels of structure (i.e. zero for a scalar, one for a vector of scalars, two for a vector of vectors (matrix), three for a vector of matrices, etc.), the notation will be here limited to the two levels provided by the matrix. The present section will, however, indicate methods for removing this limitation. ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
