Documentation

Mathematica Teacher's Edition

Built-in Functions

Lists and Matrices

List Testing

Length

Length[expr] gives the number of elements in expr.

See The Mathematica Book on the web: Section 1.4.8, Section 1.8.3 and Section 2.1.5.
Length[expr] returns 0 whenever AtomQ[expr] is True.
See also: LeafCount, ByteCount.

Further Examples