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elisp : Cons Cell Type
table of contents
Introduction
Coding Conventions
Lisp Data Types
Numbers
Strings and Characters
Lists
Sequences Arrays Vectors
Hash Tables
Symbols
Evaluation
Control Structures
Variables
Functions
Macros
Customization
Loading
Byte Compilation
Advising Functions
Debugging
Read and Print
Minibuffers
Command Loop
Keymaps
Modes
Documentation
Files
Backups and Auto-Saving
Buffers
Windows
Frames
Positions
Markers
Text
Non-ASCII Characters
Searching and Matching
Syntax Tables
Abbrevs
Processes
Display
Calendar
System Interface
Antinews
GNU Free Documentation License
GPL
Tips
GNU Emacs Internals
Standard Errors
Standard Buffer-Local Variables
Standard Keymaps
Standard Hooks
Index
New Symbols
Caveats
Lisp History
Conventions
Acknowledgements
Some Terms
nil and t
Evaluation Notation
Printing Notation
Error Messages
Buffer Text Notation
Format of Descriptions
Coding Conventions
Compilation Tips
Documentation Tips
Comment Tips
Library Headers
A Sample Function Description
A Sample Variable Description
Printed Representation
Comments
Programming Types
Editing Types
Type Predicates
Equality Predicates
Integer Type
Floating Point Type
Character Type
Sequence Type
Cons Cell Type
Array Type
String Type
Vector Type
Symbol Type
Function Type
Macro Type
Primitive Function Type
Byte-Code Type
Autoload Type
Dotted Pair Notation
Association List Type
Buffer Type
Window Type
Window Configuration Type
Marker Type
Process Type
Stream Type
Keymap Type
Overlay Type
Integer Basics
Float Basics
Predicates on Numbers
Comparison of Numbers
Arithmetic Operations
Bitwise Operations
Numeric Conversions
Math Functions
Random Numbers
String Basics
Predicates for Strings
Creating Strings
Text Comparison
String Conversion
Formatting Strings
Case Conversion
Cons Cells
Lists as Boxes
List-related Predicates
List Elements
Building Lists
Modifying Lists
Sets And Lists
Association Lists
Setcar
Setcdr
Rearrangement
Sequence Functions
Arrays
Array Functions
Vectors
Symbol Components
Definitions
Creating Symbols
Property Lists
Intro Eval
Eval
Forms
Quoting
Self-Evaluating Forms
Symbol Forms
Classifying Lists
Function Forms
Macro Forms
Special Forms
Autoloading
Sequencing
Conditionals
Combining Conditions
Iteration
Nonlocal Exits
Catch and Throw
Examples of Catch
Errors
Cleanups
Signaling Errors
Processing of Errors
Handling Errors
Error Symbols
Global Variables
Constant Variables
Local Variables
Void Variables
Defining Variables
Accessing Variables
Setting Variables
Variable Scoping
Buffer-Local Variables
Scope
Extent
Impl of Scope
Using Scoping
Intro to Buffer-Local
Creating Buffer-Local
Default Value
What Is a Function
Lambda Expressions
Function Names
Defining Functions
Calling Functions
Mapping Functions
Anonymous Functions
Function Cells
Related Topics
Lambda Components
Simple Lambda
Argument List
Function Documentation
Simple Macro
Expansion
Compiling Macros
Defining Macros
Backquote
Problems with Macros
How Programs Do Loading
Autoload
Named Features
Repeated Loading
Compilation Functions
Disassembly
Simple Advice
Defining Advice
Computed Advice
Activation of Advice
Enabling Advice
Preactivation
Argument Access in Advice
Subr Arguments
Combined Definition
Debugger
Syntax Errors
Compilation Errors
Edebug
Error Debugging
Function Debugging
Explicit Debug
Using Debugger
Debugger Commands
Invoking the Debugger
Internals of Debugger
Excess Open
Excess Close
Streams Intro
Input Streams
Input Functions
Output Streams
Output Functions
Intro to Minibuffers
Text from Minibuffer
Object from Minibuffer
Completion
Yes-or-No Queries
Minibuffer Misc
Basic Completion
Minibuffer Completion
Completion Commands
High-Level Completion
Reading File Names
Programmed Completion
Command Overview
Defining Commands
Interactive Call
Command Loop Info
Input Events
Reading Input
Waiting
Quitting
Prefix Command Arguments
Recursive Editing
Disabling Commands
Command History
Keyboard Macros
Using Interactive
Interactive Codes
Interactive Examples
Keymap Terminology
Format of Keymaps
Creating Keymaps
Inheritance and Keymaps
Prefix Keys
Menu Keymaps
Active Keymaps
Key Lookup
Functions for Key Lookup
Changing Key Bindings
Key Binding Commands
Scanning Keymaps
Major Modes
Minor Modes
Mode Line Format
Hooks
Major Mode Conventions
Example Major Modes
Auto Major Mode
Mode Help
Minor Mode Conventions
Keymaps and Minor Modes
Mode Line Data
Mode Line Variables
%-Constructs
Documentation Basics
Accessing Documentation
Keys in Documentation
Describing Characters
Help Functions
Visiting Files
Saving Buffers
Reading from Files
Writing to Files
File Locks
Information about Files
Contents of Directories
Changing Files
File Names
Visiting Functions
Subroutines of Visiting
Testing Accessibility
Kinds of Files
File Attributes
File Name Components
Directory Names
Relative File Names
File Name Expansion
Unique File Names
File Name Completion
Backup Files
Auto-Saving
Reverting
Making Backups
Rename or Copy
Numbered Backups
Backup Names
Buffer Basics
Buffer Names
Buffer File Name
Buffer Modification
Modification Time
Read Only Buffers
The Buffer List
Creating Buffers
Killing Buffers
Current Buffer
Basic Windows
Splitting Windows
Deleting Windows
Selecting Windows
Cyclic Window Ordering
Buffers and Windows
Displaying Buffers
Window Point
Window Start
Vertical Scrolling
Horizontal Scrolling
Size of Window
Resizing Windows
Window Configurations
Creating Frames
Multiple Displays
Frame Parameters
Frame Titles
Deleting Frames
Finding All Frames
Frames and Windows
Minibuffers and Frames
Input Focus
Visibility of Frames
Raising and Lowering
Frame Configurations
Mouse Tracking
Mouse Position
Pop-Up Menus
Dialog Boxes
Pointer Shapes
Window System Selections
Color Names
Resources
Display Feature Testing
Point
Motion
Excursions
Narrowing
Character Motion
Word Motion
Buffer End Motion
Text Lines
Screen Lines
List Motion
Skipping Characters
Overview of Markers
Predicates on Markers
Creating Markers
Information from Markers
Moving Markers
The Mark
The Region
Near Point
Buffer Contents
Insertion
Commands for Insertion
Deletion
User-Level Deletion
The Kill Ring
Undo
Auto Filling
Filling
Margins
Sorting
Indentation
Columns
Case Changes
Text Properties
Substitution
Transposition
Registers
Change Hooks
Kill Ring Concepts
Kill Functions
Yank Commands
Low-Level Kill Ring
Internals of Kill Ring
Primitive Indent
Mode-Specific Indent
Region Indent
Relative Indent
Indent Tabs
Motion by Indent
Examining Properties
Changing Properties
Property Search
Special Properties
Format Properties
Sticky Properties
Saving Properties
Lazy Properties
Clickable Text
Fields
Not Intervals
Text Representations
Converting Representations
Selecting a Representation
Character Codes
Character Sets
Chars and Bytes
Splitting Characters
Scanning Charsets
Translation of Characters
Coding Systems
Input Methods
Locales
String Search
Regular Expressions
Regexp Search
Match Data
Saving Match Data
Standard Regexps
Searching and Case
Syntax of Regexps
Regexp Example
Syntax Descriptors
Syntax Table Functions
Parsing Expressions
Standard Syntax Tables
Syntax Table Internals
Syntax Class Table
Syntax Flags
Abbrev Mode
Abbrev Tables
Defining Abbrevs
Abbrev Files
Abbrev Expansion
Standard Abbrev Tables
Subprocess Creation
Synchronous Processes
Asynchronous Processes
Deleting Processes
Process Information
Input to Processes
Signals to Processes
Output from Processes
Sentinels
Network
Process Buffers
Filter Functions
Accepting Output
Starting Up
Getting Out
System Environment
Terminal Input
Terminal Output
Flow Control
Batch Mode
Startup Summary
Init File
Terminal-Specific
Command-Line Arguments
Killing Emacs
Suspending Emacs
Refresh Screen
Truncation
The Echo Area
Selective Display
Overlay Arrow
Temporary Displays
Waiting
Blinking
Usual Display
Beeping
Window Systems
Building Emacs
Pure Storage
Garbage Collection
Object Internals
Writing Emacs Primitives
Buffer Internals
Window Internals
Process Internals
Cons Cell and List Types
------------------------
A "cons cell" is an object that consists of two slots, called the
CAR slot and the CDR slot. Each slot can "hold" or "refer to" any Lisp
object. We also say that "the CAR of this cons cell is" whatever
object its CAR slot currently holds, and likewise for the CDR.
A note to C programmers: in Lisp, we do not distinguish between
"holding" a value and "pointing to" the value, because pointers in
Lisp are implicit.
A "list" is a series of cons cells, linked together so that the CDR
slot of each cons cell holds either the next cons cell or the empty
list. *Note Lists::, for functions that work on lists. Because most
cons cells are used as part of lists, the phrase "list structure" has
come to refer to any structure made out of cons cells.
The names CAR and CDR derive from the history of Lisp. The original
Lisp implementation ran on an IBM 704 computer which divided words into
two parts, called the "address" part and the "decrement"; CAR was an
instruction to extract the contents of the address part of a register,
and CDR an instruction to extract the contents of the decrement. By
contrast, "cons cells" are named for the function `cons' that creates
them, which in turn was named for its purpose, the construction of
cells.
Because cons cells are so central to Lisp, we also have a word for
"an object which is not a cons cell". These objects are called "atoms".
The read syntax and printed representation for lists are identical,
and consist of a left parenthesis, an arbitrary number of elements, and
a right parenthesis.
Upon reading, each object inside the parentheses becomes an element
of the list. That is, a cons cell is made for each element. The CAR
slot of the cons cell holds the element, and its CDR slot refers to the
next cons cell of the list, which holds the next element in the list.
The CDR slot of the last cons cell is set to hold `nil'.
A list can be illustrated by a diagram in which the cons cells are
shown as pairs of boxes, like dominoes. (The Lisp reader cannot read
such an illustration; unlike the textual notation, which can be
understood by both humans and computers, the box illustrations can be
understood only by humans.) This picture represents the three-element
list `(rose violet buttercup)':
--- --- --- --- --- ---
| | |--> | | |--> | | |--> nil
--- --- --- --- --- ---
| | |
| | |
--> rose --> violet --> buttercup
In this diagram, each box represents a slot that can hold or refer to
any Lisp object. Each pair of boxes represents a cons cell. Each arrow
represents a reference to a Lisp object, either an atom or another cons
cell.
In this example, the first box, which holds the CAR of the first
cons cell, refers to or "holds" `rose' (a symbol). The second box,
holding the CDR of the first cons cell, refers to the next pair of
boxes, the second cons cell. The CAR of the second cons cell is
`violet', and its CDR is the third cons cell. The CDR of the third
(and last) cons cell is `nil'.
Here is another diagram of the same list, `(rose violet buttercup)',
sketched in a different manner:
--------------- ---------------- -------------------
| car | cdr | | car | cdr | | car | cdr |
| rose | o-------->| violet | o-------->| buttercup | nil |
| | | | | | | | |
--------------- ---------------- -------------------
A list with no elements in it is the "empty list"; it is identical
to the symbol `nil'. In other words, `nil' is both a symbol and a list.
Here are examples of lists written in Lisp syntax:
(A 2 "A") ; A list of three elements.
() ; A list of no elements (the empty list).
nil ; A list of no elements (the empty list).
("A ()") ; A list of one element: the string `"A ()"'.
(A ()) ; A list of two elements: `A' and the empty list.
(A nil) ; Equivalent to the previous.
((A B C)) ; A list of one element
; (which is a list of three elements).
Here is the list `(A ())', or equivalently `(A nil)', depicted with
boxes and arrows:
--- --- --- ---
| | |--> | | |--> nil
--- --- --- ---
| |
| |
--> A --> nil
* Menu:
* Dotted Pair Notation:: An alternative syntax for lists.
* Association List Type:: A specially constructed list.
back to top
elisp : Cons Cell Type
table of contents
Introduction
Coding Conventions
Lisp Data Types
Numbers
Strings and Characters
Lists
Sequences Arrays Vectors
Hash Tables
Symbols
Evaluation
Control Structures
Variables
Functions
Macros
Customization
Loading
Byte Compilation
Advising Functions
Debugging
Read and Print
Minibuffers
Command Loop
Keymaps
Modes
Documentation
Files
Backups and Auto-Saving
Buffers
Windows
Frames
Positions
Markers
Text
Non-ASCII Characters
Searching and Matching
Syntax Tables
Abbrevs
Processes
Display
Calendar
System Interface
Antinews
GNU Free Documentation License
GPL
Tips
GNU Emacs Internals
Standard Errors
Standard Buffer-Local Variables
Standard Keymaps
Standard Hooks
Index
New Symbols
Caveats
Lisp History
Conventions
Acknowledgements
Some Terms
nil and t
Evaluation Notation
Printing Notation
Error Messages
Buffer Text Notation
Format of Descriptions
Coding Conventions
Compilation Tips
Documentation Tips
Comment Tips
Library Headers
A Sample Function Description
A Sample Variable Description
Printed Representation
Comments
Programming Types
Editing Types
Type Predicates
Equality Predicates
Integer Type
Floating Point Type
Character Type
Sequence Type
Cons Cell Type
Array Type
String Type
Vector Type
Symbol Type
Function Type
Macro Type
Primitive Function Type
Byte-Code Type
Autoload Type
Dotted Pair Notation
Association List Type
Buffer Type
Window Type
Window Configuration Type
Marker Type
Process Type
Stream Type
Keymap Type
Overlay Type
Integer Basics
Float Basics
Predicates on Numbers
Comparison of Numbers
Arithmetic Operations
Bitwise Operations
Numeric Conversions
Math Functions
Random Numbers
String Basics
Predicates for Strings
Creating Strings
Text Comparison
String Conversion
Formatting Strings
Case Conversion
Cons Cells
Lists as Boxes
List-related Predicates
List Elements
Building Lists
Modifying Lists
Sets And Lists
Association Lists
Setcar
Setcdr
Rearrangement
Sequence Functions
Arrays
Array Functions
Vectors
Symbol Components
Definitions
Creating Symbols
Property Lists
Intro Eval
Eval
Forms
Quoting
Self-Evaluating Forms
Symbol Forms
Classifying Lists
Function Forms
Macro Forms
Special Forms
Autoloading
Sequencing
Conditionals
Combining Conditions
Iteration
Nonlocal Exits
Catch and Throw
Examples of Catch
Errors
Cleanups
Signaling Errors
Processing of Errors
Handling Errors
Error Symbols
Global Variables
Constant Variables
Local Variables
Void Variables
Defining Variables
Accessing Variables
Setting Variables
Variable Scoping
Buffer-Local Variables
Scope
Extent
Impl of Scope
Using Scoping
Intro to Buffer-Local
Creating Buffer-Local
Default Value
What Is a Function
Lambda Expressions
Function Names
Defining Functions
Calling Functions
Mapping Functions
Anonymous Functions
Function Cells
Related Topics
Lambda Components
Simple Lambda
Argument List
Function Documentation
Simple Macro
Expansion
Compiling Macros
Defining Macros
Backquote
Problems with Macros
How Programs Do Loading
Autoload
Named Features
Repeated Loading
Compilation Functions
Disassembly
Simple Advice
Defining Advice
Computed Advice
Activation of Advice
Enabling Advice
Preactivation
Argument Access in Advice
Subr Arguments
Combined Definition
Debugger
Syntax Errors
Compilation Errors
Edebug
Error Debugging
Function Debugging
Explicit Debug
Using Debugger
Debugger Commands
Invoking the Debugger
Internals of Debugger
Excess Open
Excess Close
Streams Intro
Input Streams
Input Functions
Output Streams
Output Functions
Intro to Minibuffers
Text from Minibuffer
Object from Minibuffer
Completion
Yes-or-No Queries
Minibuffer Misc
Basic Completion
Minibuffer Completion
Completion Commands
High-Level Completion
Reading File Names
Programmed Completion
Command Overview
Defining Commands
Interactive Call
Command Loop Info
Input Events
Reading Input
Waiting
Quitting
Prefix Command Arguments
Recursive Editing
Disabling Commands
Command History
Keyboard Macros
Using Interactive
Interactive Codes
Interactive Examples
Keymap Terminology
Format of Keymaps
Creating Keymaps
Inheritance and Keymaps
Prefix Keys
Menu Keymaps
Active Keymaps
Key Lookup
Functions for Key Lookup
Changing Key Bindings
Key Binding Commands
Scanning Keymaps
Major Modes
Minor Modes
Mode Line Format
Hooks
Major Mode Conventions
Example Major Modes
Auto Major Mode
Mode Help
Minor Mode Conventions
Keymaps and Minor Modes
Mode Line Data
Mode Line Variables
%-Constructs
Documentation Basics
Accessing Documentation
Keys in Documentation
Describing Characters
Help Functions
Visiting Files
Saving Buffers
Reading from Files
Writing to Files
File Locks
Information about Files
Contents of Directories
Changing Files
File Names
Visiting Functions
Subroutines of Visiting
Testing Accessibility
Kinds of Files
File Attributes
File Name Components
Directory Names
Relative File Names
File Name Expansion
Unique File Names
File Name Completion
Backup Files
Auto-Saving
Reverting
Making Backups
Rename or Copy
Numbered Backups
Backup Names
Buffer Basics
Buffer Names
Buffer File Name
Buffer Modification
Modification Time
Read Only Buffers
The Buffer List
Creating Buffers
Killing Buffers
Current Buffer
Basic Windows
Splitting Windows
Deleting Windows
Selecting Windows
Cyclic Window Ordering
Buffers and Windows
Displaying Buffers
Window Point
Window Start
Vertical Scrolling
Horizontal Scrolling
Size of Window
Resizing Windows
Window Configurations
Creating Frames
Multiple Displays
Frame Parameters
Frame Titles
Deleting Frames
Finding All Frames
Frames and Windows
Minibuffers and Frames
Input Focus
Visibility of Frames
Raising and Lowering
Frame Configurations
Mouse Tracking
Mouse Position
Pop-Up Menus
Dialog Boxes
Pointer Shapes
Window System Selections
Color Names
Resources
Display Feature Testing
Point
Motion
Excursions
Narrowing
Character Motion
Word Motion
Buffer End Motion
Text Lines
Screen Lines
List Motion
Skipping Characters
Overview of Markers
Predicates on Markers
Creating Markers
Information from Markers
Moving Markers
The Mark
The Region
Near Point
Buffer Contents
Insertion
Commands for Insertion
Deletion
User-Level Deletion
The Kill Ring
Undo
Auto Filling
Filling
Margins
Sorting
Indentation
Columns
Case Changes
Text Properties
Substitution
Transposition
Registers
Change Hooks
Kill Ring Concepts
Kill Functions
Yank Commands
Low-Level Kill Ring
Internals of Kill Ring
Primitive Indent
Mode-Specific Indent
Region Indent
Relative Indent
Indent Tabs
Motion by Indent
Examining Properties
Changing Properties
Property Search
Special Properties
Format Properties
Sticky Properties
Saving Properties
Lazy Properties
Clickable Text
Fields
Not Intervals
Text Representations
Converting Representations
Selecting a Representation
Character Codes
Character Sets
Chars and Bytes
Splitting Characters
Scanning Charsets
Translation of Characters
Coding Systems
Input Methods
Locales
String Search
Regular Expressions
Regexp Search
Match Data
Saving Match Data
Standard Regexps
Searching and Case
Syntax of Regexps
Regexp Example
Syntax Descriptors
Syntax Table Functions
Parsing Expressions
Standard Syntax Tables
Syntax Table Internals
Syntax Class Table
Syntax Flags
Abbrev Mode
Abbrev Tables
Defining Abbrevs
Abbrev Files
Abbrev Expansion
Standard Abbrev Tables
Subprocess Creation
Synchronous Processes
Asynchronous Processes
Deleting Processes
Process Information
Input to Processes
Signals to Processes
Output from Processes
Sentinels
Network
Process Buffers
Filter Functions
Accepting Output
Starting Up
Getting Out
System Environment
Terminal Input
Terminal Output
Flow Control
Batch Mode
Startup Summary
Init File
Terminal-Specific
Command-Line Arguments
Killing Emacs
Suspending Emacs
Refresh Screen
Truncation
The Echo Area
Selective Display
Overlay Arrow
Temporary Displays
Waiting
Blinking
Usual Display
Beeping
Window Systems
Building Emacs
Pure Storage
Garbage Collection
Object Internals
Writing Emacs Primitives
Buffer Internals
Window Internals
Process Internals
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