This is documentation for Mathematica 6, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.2)
At the core of Mathematica is the foundational idea that everything—data, programs, formulas, graphics, documents—can be represented as symbolic expressions. And it is this unifying concept that underlies Mathematica's symbolic programming paradigm, and makes possible much of the unique power of the Mathematica language and system.
FullForm the full form of an expression, without shortened syntax
TreeForm  ▪ Head  ▪ Length  ▪ Depth  ▪ Symbol  ▪ ...
expr/.rules make replacements for any occurrence of a pattern in an expression
Map, Apply map, apply a function at any level in any expression
SameQ (SameQ)  ▪ FreeQ  ▪ MemberQ  ▪ NumberQ  ▪ OrderedQ  ▪ ...
Part (..[[..]]) — numbered parts of an expression, reset using =
Position  ▪ ReplacePart  ▪ MapAt  ▪ Delete  ▪ ...
Cases find occurrences of a pattern in an expression
Flatten  ▪ Thread  ▪ Distribute  ▪ FlattenAt  ▪ Append  ▪ ...
Hold  ▪ Evaluate  ▪ HoldFirst  ▪ HoldAll  ▪ ...