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Length

WMA link

Length[expr]

returns the number of elements in expr.

Length of a list:

• • Length[{1, 2, 3}]

Length operates on the FullForm of expressions:

• • Length[Exp[x]]

• • FullForm[Exp[x]]

The length of atoms is 0:

• • Length[a]

Note that rational and complex numbers are atoms, although their
FullForm might suggest the opposite:

• • Length[1/3]

• • FullForm[1/3]