@c Copyright (C) 1996, 1997 John W. Eaton
@c This is part of the Octave manual.
@c For copying conditions, see the file gpl.tex.

@node Variables, Expressions, Data Structures, Top
@chapter Variables
@cindex variables, user-defined
@cindex user-defined variables

Variables let you give names to values and refer to them later.  You have
already seen variables in many of the examples.  The name of a variable
must be a sequence of letters, digits and underscores, but it may not begin
with a digit.  Octave does not enforce a limit on the length of variable
names, but it is seldom useful to have variables with names longer than
about 30 characters.  The following are all valid variable names

@cindex job hunting
@cindex getting a good job
@cindex flying high and fast
@example
@group
x
x15
__foo_bar_baz__
fucnrdthsucngtagdjb
@end group
@end example

@noindent
However, names like @code{__foo_bar_baz__} that begin and end with two
underscores are understood to be reserved for internal use by Octave.
You should not use them in code you write, except to access Octave's
documented internal variables and built-in symbolic constants.

Case is significant in variable names.  The symbols @code{a} and
@code{A} are distinct variables.

A variable name is a valid expression by itself.  It represents the
variable's current value.  Variables are given new values with
@dfn{assignment operators} and @dfn{increment operators}.
@xref{Assignment Ops, ,Assignment Expressions}.

A number of variables have special built-in meanings.  For example,
@code{PWD} holds the current working directory, and @code{pi} names the
ratio of the circumference of a circle to its diameter. @xref{Summary of
Built-in Variables}, for a list of all the predefined variables.  Some
of these built-in symbols are constants and may not be changed.  Others
can be used and assigned just like all other variables, but their values
are also used or changed automatically by Octave.

Variables in Octave do not have fixed types, so it is possible to first
store a numeric value in a variable and then to later use the same name
to hold a string value in the same program.  Variables may not be used
before they have been given a value.  Doing so results in an error.

@menu
* Global Variables::            
* Status of Variables::         
* Summary of Built-in Variables::  
* Defaults from the Environment::  
@end menu

@node Global Variables, Status of Variables, Variables, Variables
@section Global Variables
@cindex global variables
@cindex @code{global} statement
@cindex variables, global

A variable that has been declared @dfn{global} may be accessed from
within a function body without having to pass it as a formal parameter.

A variable may be declared global using a @code{global} declaration
statement.  The following statements are all global declarations.

@example
@group
global a
global b = 2
global c = 3, d, e = 5
@end group
@end example

It is necessary declare a variable as global within a function body in
order to access it.  For example,

@example
@group
global x
function f ()
  x = 1;
endfunction
f ()
@end group
@end example

@noindent
does @emph{not} set the value of the global variable @code{x} to 1.  In
order to change the value of the global variable @code{x}, you must also
declare it to be global within the function body, like this

@example
@group
function f ()
  global x;
  x = 1;
endfunction
@end group
@end example

Passing a global variable in a function parameter list will
make a local copy and not modify the global value.  For example, given
the function

@example
@group
function f (x)
  x = 0
endfunction
@end group
@end example

@noindent
and the definition of @code{x} as a global variable at the top level,

@example
global x = 13
@end example

@noindent
the expression

@example
f (x)
@end example

@noindent
will display the value of @code{x} from inside the function as 0,
but the value of @code{x} at the top level remains unchanged, because
the function works with a @emph{copy} of its argument.

@defvr {Built-in Variable} warn_comma_in_global_decl
If the value of @code{warn_comma_in_global_decl} is nonzero, a
warning is issued for statements like

@example
global a = 1, b
@end example

@noindent
which makes the variables @code{a} and @code{b} global and assigns the
value 1 to the variable @code{a}, because in this context, the comma is
not interpreted as a statement separator.

The default value of @code{warn_comma_in_global_decl} is nonzero.
@end defvr

@deftypefn {Built-in Function} {} is_global (@var{name})
Return 1 if @var{name} is globally visible.  Otherwise, return 0.  For
example,

@example
@group
global x
is_global ("x")
     @result{} 1
@end group
@end example
@end deftypefn

@node Status of Variables, Summary of Built-in Variables, Global Variables, Variables
@section Status of Variables

@deffn {Command} clear options pattern @dots{}
Delete the names matching the given patterns from the symbol table.  The
pattern may contain the following special characters:
@table @code
@item ?
Match any single character.

@item *
Match zero or more characters.

@item [ @var{list} ]
Match the list of characters specified by @var{list}.  If the first
character is @code{!} or @code{^}, match all characters except those
specified by @var{list}.  For example, the pattern @samp{[a-zA-Z]} will
match all lower and upper case alphabetic characters. 
@end table

For example, the command

@example
clear foo b*r
@end example

@noindent
clears the name @code{foo} and all names that begin with the letter
@code{b} and end with the letter @code{r}.

If @code{clear} is called without any arguments, all user-defined
variables (local and global) are cleared from the symbol table.  If
@code{clear} is called with at least one argument, only the visible
names matching the arguments are cleared.  For example, suppose you have
defined a function @code{foo}, and then hidden it by performing the
assignment @code{foo = 2}.  Executing the command @kbd{clear foo} once
will clear the variable definition and restore the definition of
@code{foo} as a function.  Executing @kbd{clear foo} a second time will
clear the function definition.

This command may not be used within a function body.
@end deffn

@deffn {Command} who options pattern @dots{}
@deffnx {Command} whos options pattern @dots{}
List currently defined symbols matching the given patterns.  The
following are valid options.  They may be shortened to one character but
may not be combined.

@table @code
@item -all
List all currently defined symbols.

@item -builtins
List built-in variables and functions.  This includes all currently
compiled function files, but does not include all function files that
are in the @code{LOADPATH}.

@item -functions
List user-defined functions.

@item -long
Print a long listing including the type and dimensions of any symbols.
The symbols in the first column of output indicate whether it is
possible to redefine the symbol, and whether it is possible for it to be
cleared.

@item -variables
List user-defined variables.
@end table

Valid patterns are the same as described for the @code{clear} command
above.  If no patterns are supplied, all symbols from the given category
are listed.  By default, only user defined functions and variables
visible in the local scope are displayed.

The command @kbd{whos} is equivalent to @kbd{who -long}.
@end deffn

@deftypefn {Built-in Function} {} exist (@var{name})
Return 1 if the name exists as a variable, 2 if the name (after
appending @samp{.m}) is a function file in the path, 3 if the name is a
@samp{.oct} file in the path, or 5 if the name is a built-in function.
Otherwise, return 0.
@end deftypefn

@deftypefn {Built-in Function} {} document (@var{symbol}, @var{text})
Set the documentation string for @var{symbol} to @var{text}.
@end deftypefn

@deffn {Command} type options name @dots{}
Display the definition of each @var{name} that refers to a function.

Normally also displays if each @var{name} is user-defined or builtin;
the @code{-q} option suppresses this behaviour.

Currently, Octave can only display functions that can be compiled
cleanly, because it uses its internal representation of the function to
recreate the program text.

Comments are not displayed because Octave's parser currently discards
them as it converts the text of a function file to its internal
representation.  This problem may be fixed in a future release.
@end deffn

@deffn {Command} which name @dots{}
Display the type of each @var{name}.  If @var{name} is defined from a
function file, the full name of the file is also displayed.
@end deffn

@node Summary of Built-in Variables, Defaults from the Environment, Status of Variables, Variables
@section Summary of Built-in Variables

Here is a summary of all of Octave's built-in variables along with
cross references to additional information and their default values.  In
the following table @var{octave-home} stands for the root directory
where all of Octave is installed (the default is @file{@value{OCTAVEHOME}},
@var{version} stands for the Octave version number (for example,
@value{VERSION}) and @var{arch} stands for the type of system for which
Octave was compiled (for example, @code{@value{TARGETHOSTTYPE}}).

@vtable @code
@item EDITOR
@xref{Commands For History}.

Default value: @code{"emacs"}.

@item EXEC_PATH
@xref{Controlling Subprocesses}.

Default value: @code{":$PATH"}.

@item INFO_FILE
@xref{Getting Help}.

Default value: @code{"@var{octave-home}/info/octave.info"}.

@item INFO_PROGRAM
@xref{Getting Help}.

Default value: @code{"@var{octave-home}/libexec/octave/@var{version}/exec/@var{arch}/info"}.

@item LOADPATH
@xref{Function Files}.

Default value: @code{".:@var{octave-home}/lib/@var{version}"}.

@item OCTAVE_HOME

Default value: @code{"@value{OCTAVEHOME}"}.

@item PAGER
@xref{Input and Output}.

Default value: @code{"less", or "more"}.

@item PS1
@xref{Customizing the Prompt}.

Default value: @code{"\s:\#> "}.

@item PS2
@xref{Customizing the Prompt}.

Default value: @code{"> "}.

@item PS4
@xref{Customizing the Prompt}.

Default value: @code{"+ "}.

@item automatic_replot
@xref{Two-Dimensional Plotting}.

Default value: 0.

@item beep_on_error
@xref{Error Handling}.

Default value: 0.

@item completion_append_char
@xref{Commands For Completion}.

Default value: @code{" "}.

@item default_eval_print_flag
@xref{Evaluation}.

Default value: 1.

@item default_return_value
@xref{Multiple Return Values}.

Default value: @code{[]}.

@item default_save_format
@xref{Simple File I/O}.

Default value: @code{"ascii"}.

@item do_fortran_indexing
@xref{Index Expressions}.

Default value: 0.

@item define_all_return_values
@xref{Multiple Return Values}.

Default value: 0.

@item empty_list_elements_ok
@xref{Empty Matrices}.

Default value: @code{"warn"}.

@item gnuplot_binary
@xref{Three-Dimensional Plotting}.

Default value: @code{"gnuplot"}.

@item history_file
@xref{Commands For History}.

Default value: @code{"~/.octave_hist"}.

@item history_size
@xref{Commands For History}.

Default value: 1024.

@item ignore_function_time_stamp
@xref{Function Files}.

Default value: @code{"system"}.

@item implicit_str_to_num_ok
@xref{String Conversions}.

Default value: 0.

@item ok_to_lose_imaginary_part
@xref{Special Utility Matrices}.

Default value: @code{"warn"}.

@item output_max_field_width
@xref{Matrices}.

Default value: 10.

@item output_precision
@xref{Matrices}.

Default value: 5.

@item page_screen_output
@xref{Input and Output}.

Default value: 1.

@item prefer_column_vectors
@xref{Index Expressions}.

Default value: 0.

@item print_answer_id_name
@xref{Terminal Output}.

Default value: 1.

@item print_empty_dimensions
@xref{Empty Matrices}.

Default value: 1.

@item resize_on_range_error
@xref{Index Expressions}.

Default value: 1.

@item return_last_computed_value
@xref{Returning From a Function}.

Default value: 0.

@item save_precision
@xref{Simple File I/O}.

Default value: 17.

@item saving_history
@xref{Commands For History}.

Default value: 1.

@item silent_functions
@xref{Defining Functions}.

Default value: 0.

@item split_long_rows
@xref{Matrices}.

Default value: 1.

@item struct_levels_to_print
@xref{Data Structures}.

Default value: 2.

@item suppress_verbose_help_message
@xref{Getting Help}.

Default value: 1.

@item treat_neg_dim_as_zero
@xref{Special Utility Matrices}.

Default value: 0.

@item warn_assign_as_truth_value
@xref{The if Statement}.

Default value: 1.

@item warn_comma_in_global_decl
@xref{Global Variables}.

Default value: 1.

@item warn_divide_by_zero
@xref{Arithmetic Ops}.

Default value: 1.

@item warn_function_name_clash
@xref{Function Files}.

Default value: 1.

@item whitespace_in_literal_matrix
@xref{Matrices}.

Default value: @code{""}.
@end vtable


@node Defaults from the Environment,  , Summary of Built-in Variables, Variables
@section Defaults from the Environment

Octave uses the values of the following environment variables to set the
default values for the corresponding built-in variables.  In addition,
the values from the environment may be overridden by command-line
arguments.  @xref{Command Line Options}.

@vtable @code
@item EDITOR
@xref{Commands For History}.

Built-in variable: @code{EDITOR}.

@item OCTAVE_EXEC_PATH        
@xref{Controlling Subprocesses}.

Built-in variable: @code{EXEC_PATH}.
Command-line argument: @code{--exec-path}.

@item OCTAVE_PATH
@xref{Function Files}.

Built-in variable: @code{LOADPATH}.
Command-line argument: @code{--path}.

@item OCTAVE_INFO_FILE
@xref{Getting Help}.

Built-in variable: @code{INFO_FILE}.
Command-line argument: @code{--info-file}.

@item OCTAVE_INFO_PROGRAM
@xref{Getting Help}.

Built-in variable: @code{INFO_PROGRAM}.
Command-line argument: @code{--info-program}.

@item OCTAVE_HISTSIZE
@xref{Commands For History}.

Built-in variable: @code{history_size}.

@item OCTAVE_HISTFILE
@xref{Commands For History}.

Built-in variable: @code{history_file}.
@end vtable
