This is documentation for Mathematica 6, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.2)
Parts of Expressions
Mathematica's unified symbolic architecture allows immediate generalization of part-oriented list operations to arbitrary expressions—supporting operations both on individual parts, and on collections of parts at specified levels in expression trees.
Part (..[[..]]) get any part of an expression (reset with =)
Length  ▪ First  ▪ Last  ▪ Most  ▪ Rest  ▪ Take  ▪ Drop  ▪ Extract
ReplacePart replace numbered parts of an expression
Append  ▪ Prepend  ▪ Insert  ▪ Delete  ▪ DeleteCases
Position find the positions of any pattern in an expression
Cases find cases of any pattern in an expression
MapAt apply a function at particular positions in an expression
Map (/@) — apply a function at specified levels in an expression
MapAll (//@) — apply a function to all parts of an expression
Heads option for specifying whether to include heads
Sow, Reap collect parts "sown" inside of an expression