WolframLang: Position of an Element

Position Spec

A position is a list of integers that uniquely specify a part of expression.

For example, if we have

{a, {{b,c}, d}, e}

• position {1} is a
• position {2} is {{b,c}, d}
• position {2,1} is {b,c}
• position {2,2} is d
• position {2,1,1} is b
• position {2,1,2} is c
• position {3} is e

Index counting start at 1. Index 0 means the Head of Expression.

Get Element by Position Spec: Extract

Extract[expr, position]

return the part of expr at a given position. Extract

xx = {a, {{b,c}, d}, e};

Extract[ xx, {1} ] === a

Extract[ xx, {2} ] === {{b, c}, d}

Extract[ xx, {3} ] === e

Extract[ xx, {2, 1} ] === {b, c}

Extract[ xx, {2, 1, 1} ] === b