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Most Octave variables are available for you to use for your own purposes; they never change except when your program assigns values to them, and never affect anything except when your program examines them.
A few variables have special built-in meanings. Some of them, like
pi and eps provide useful predefined constant values.
Others, like do_fortran_indexing and page_screen_output
are examined automatically by Octave, so that you can to tell Octave how
to do certain things. There are also two special variables, ans
and PWD, that are set automatically by Octave and carry
information from the internal workings of Octave to your program.
This chapter documents all the built-in variables of Octave. Most of them are also documented in the chapters that describe functions that use them, or are affected by their values.
| 1.1 Predefined Constants | ||
| 1.2 User Preferences | ||
| 1.3 Other Built-in Variables | ||
| 1.4 Summary of Preference Variables |
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I, i, J, j
A pure imaginary number, defined as
The I and J forms are true constants, and cannot be
modified. The i and j forms are like ordinary variables,
and may be used for other purposes. However, unlike other variables,
they once again assume their special predefined values if they are
cleared @xref{Miscellaneous Utilities}.
Inf, inf
Infinity. This is the result of an operation like 1/0, or an operation that results in a floating point overflow.
NaN, nan
Not a number. This is the result of an operation like ‘0/0’, or ‘Inf - Inf’, or any operation with a NaN.
SEEK_SET
SEEK_CUR
SEEK_END
These variables may be used as the optional third argument for the
function fseek.
eps
The machine precision. More precisely, eps is the smallest value
such that ‘1+eps’ is not equal to 1. This number is
system-dependent. On machines that support 64 bit IEEE floating point
arithmetic, eps is approximately
pi
The ratio of the circumference of a circle to its diameter.
Internally, pi is computed as ‘4.0 * atan (1.0)’.
realmax
The largest floating point number that is representable. The actual
value is system-dependent. On machines that support 64 bit IEEE
floating point arithmetic, realmax is approximately
realmin
The smallest floating point number that is representable. The actual
value is system-dependent. On machines that support 64 bit IEEE
floating point arithmetic, realmin is approximately
stdin
stdout
stderr
These variables are the file numbers corresponding to the standard input, standard output, and standard error streams. These streams are preconnected and available when Octave starts.
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This section describes the variables that you can use to customize Octave’s behavior.
Normally, preferences are set in the file ‘~/.octaverc’, so that you can customize your environment in the same way each time you use Octave without having to remember and retype all the necessary commands. @xref{Startup Files} for more information.
EDITOR
A string naming the editor to use with the edit_history command.
If the environment variable EDITOR is set when Octave starts, its
value is used as the default. Otherwise, EDITOR is set to
"vi".
IMAGEPATH
A colon separated list of directories in which to search for image files. @xref{Image Processing} for a description of Octave’s image processing capabilities.
INFO_FILE
A string naming the location of the Octave info file.
The default value is "{No value for `OCTAVEHOME'}/info/octave.info".
LOADPATH
A colon separated list of directories in which to search for function
files. @xref{Functions and Scripts}. The value of LOADPATH
overrides the environment variable OCTAVE_PATH. @xref{Installation}.
LOADPATH is now handled in the same way as TeX handles
TEXINPUTS. If the path starts with ‘:’, the standard path
is prepended to the value of LOADPATH. If it ends with ‘:’
the standard path is appended to the value of LOADPATH.
In addition, if any path element ends in ‘//’, that directory and
all subdirectories it contains are searched recursively for function
files. This can result in a slight delay as Octave caches the lists of
files found in the LOADPATH the first time Octave searches for a
function. After that, searching is usually much faster because Octave
normally only needs to search its internal cache for files.
To improve performance of recursive directory searching, it is best for each directory that is to be searched recursively to contain either additional subdirectories or function files, but not a mixture of both.
@xref{Organization of Functions} for a description of the function file directories that are distributed with Octave.
OCTAVE_VERSION
The version number of Octave, as a string.
PAGER
The default value is "less", or, if less is not available
on your system, "more". @xref{Installation}, and
@ref{Input and Output}.
PS1
The primary prompt string. When executing interactively, Octave
displays the primary prompt PS1 when it is ready to read a
command. Octave allows the prompt to be customized by inserting a
number of backslash-escaped special characters that are decoded as
follows:
The time.
The date.
Begins a new line by printing the equivalent of a carriage return followed by a line feed.
The name of the program (usually just octave).
The current working directory.
The basename of the current working directory.
The username of the current user.
The hostname.
The command number of this command, counting from when Octave starts.
The history number of this command. This differs from ‘\#’ by the number of commands in the history list when Octave starts.
If the effective UID is 0, a #, otherwise a $.
The character whose character code in octal is ‘nnn’.
A backslash.
The default value of PS1 is "\s:\#> ". To change it, use a
command like
octave:13> PS1 = "\\u@\\h> "
which will result in the prompt ‘boris@kremvax> ’ for the user ‘boris’ logged in on the host ‘kremvax’. Note that two backslashes are required to enter a backslash into a string. @xref{String Constants}.
PS2
The secondary prompt string, which is printed when Octave is
expecting additional input to complete a command. For example, when
defining a function over several lines, Octave will print the value of
PS1 at the beginning of each line after the first. Octave allows
PS2 to be customized in the same way as PS1. The default
value of PS2 is "> ".
PS4
If Octave is invoked with the --echo-input option, the value of
PS4 is printed before each line of input that is echoed. Octave
allows PS4 to be customized in the same way as PS1. The
default value of PS4 is "+ ". @xref{Invoking Octave}, for
a description of --echo-input.
automatic_replot
If this variable is "true", Octave will automatically send a
replot command to gnuplot each time the plot changes.
Since this is fairly inefficient, the default value is "false".
default_return_value
The value given to otherwise unitialized return values if
define_all_return_values is "true". The default value is
[].
default_save_format
Specify the default format used by the save command. Options are
"ascii", "binary", "mat-binary", or
"float-binary". The default value is ascii.
define_all_return_values
If the value of define_all_return_values is "true", Octave
will substitute the value specified by default_return_value for
any return values that remain undefined when a function returns. The
default value is "false".
do_fortran_indexing
If the value of do_fortran_indexing is "true", Octave allows
you to select elements of a two-dimensional matrix using a single index
by treating the matrix as a single vector created from the columns of
the matrix. The default value is "false".
empty_list_elements_ok
This variable controls whether Octave ignores empty matrices in a matrix list.
For example, if the value of empty_list_elements_ok is
"true", Octave will ignore the empty matrices in the expression
a = [1, [], 3, [], 5]
and the variable ‘a’ will be assigned the value ‘[ 1 3 5 ]’.
The default value is "warn".
gnuplot_binary
The name of the program invoked by the plot command. The default value
is "gnuplot". @xref{Installation}.
ignore_function_time_stamp
This variable variable can be used to prevent Octave from making the
system call stat() each time it looks up functions defined in
function files. If ignore_function_time_stamp to
"system", Octave will not automatically recompile function files
in subdirectories of {No value for `OCTAVEHOME'}/lib/{No value for `VERSION'} if
they have changed since they were last compiled, but will recompile
other function files in the LOADPATH if they change. If set to
"all", Octave will not recompile any function files unless their
definitions are removed with clear. For any other value of
ignore_function_time_stamp, Octave will always check to see if
functions defined in function files need to recompiled. The default
value of ignore_function_time_stamp is "system".
implicit_str_to_num_ok
If the value of implicit_str_to_num_ok is "true", implicit
conversions of strings to their numeric ASCII equivalents are allowed.
Otherwise, an error message is printed and control is returned to the
top level. The default value is "false".
ok_to_lose_imaginary_part
If the value of ok_to_lose_imaginary_part is "true",
implicit conversions of complex numbers to real numbers are allowed (for
example, by fsolve). If the value is "warn", the conversion is allowed,
but a warning is printed. Otherwise, an error message is printed and
control is returned to the top level. The default value is "warn".
output_max_field_width
This variable specifies the maximum width of a numeric output field. The default value is 10.
It is possible to achieve a wide range of output styles by using
different values of output_precision and
output_max_field_width. Reasonable combinations can be set using
the format function. @xref{Basic Input and Output}.
output_precision
This variable specifies the minimum number of significant figures to display for numeric output. The default value is 5.
It is possible to achieve a wide range of output styles by using
different values of output_precision and
output_max_field_width. Reasonable combinations can be set using
the format function. @xref{Basic Input and Output}.
page_screen_output
If the value of page_screen_output is "true", all output
intended for the screen that is longer than one page is sent through a
pager. This allows you to view one screenful at a time. Some pagers
(such as less—see @ref{Installation}) are also capable of moving
backward on the output. The default value is "true".
@xref{Input and Output}.
You can choose the program to use as the pager by setting the variable
PAGER.
prefer_column_vectors
If prefer_column_vectors is "true", operations like
for i = 1:10 a (i) = i; endfor
(for ‘a’ previously undefined) produce column vectors. Otherwise, row
vectors are preferred. The default value is "false".
If a variable is already defined to be a vector (a matrix with a single
row or column), the original orientation is respected, regardless of the
value of prefer_column_vectors.
prefer_zero_one_indexing
If the value of prefer_zero_one_indexing is "true", Octave
will perform zero-one style indexing when there is a conflict with the
normal indexing rules. @xref{Index Expressions}. For example, given a
matrix
a = [1, 2, 3, 4]
with prefer_zero_one_indexing is set to "true", the
expression
a ([1, 1, 1, 1])
results in the matrix ‘[ 1 2 3 4 ]’. If the value of
prefer_zero_one_indexing set to "false", the result would be
the matrix ‘[ 1 1 1 1 ]’.
In the first case, Octave is selecting each element corresponding to a ‘1’ in the index vector. In the second, Octave is selecting the first element multiple times.
The default value for prefer_zero_one_indexing is "false".
print_answer_id_name
If the value of print_answer_id_name is "true", variable
names are printed along with the result. Otherwise, only the result
values are printed. The default value is "true".
print_empty_dimensions
If the value of print_empty_dimensions is "true", the
dimensions of empty matrices are printed along with the empty matrix
symbol, ‘[]’. For example, the expression
zeros (3, 0)
will print
ans = [](3x0)
propagate_empty_matrices
If the value of propagate_empty_matrices is "true",
functions like inverse and svd will return an empty matrix
if they are given one as an argument. The default value is "true".
@xref{Empty Matrices}.
resize_on_range_error
If the value of resize_on_range_error is "true", expressions
like
for i = 1:10 a (i) = i; endfor
(for ‘a’ previously undefined) result in the variable ‘a’
being resized to be just large enough to hold the new value. Otherwise
uninitialized elements are set to zero. If the value of
resize_on_range_error is "false", an error message is
printed and control is returned to the top level. The default value is
"true".
return_last_computed_value
If the value of return_last_computed_value is true, and a
function is defined without explicitly specifying a return value, the
function will return the value of the last expression. Otherwise, no
value will be returned. The default value is "false".
For example, the function
function f () 2 + 2; endfunction
will either return nothing, if return_last_computed_value is
"false", or 4, if it is "true".
save_precision
This variable specifies the number of digits to keep when saving data
with the save command. The default value is 17.
silent_functions
If the value of silent_functions is "true", internal output
from a function is suppressed. Otherwise, the results of expressions
within a function body that are not terminated with a semicolon will
have their values printed. The default value is "false".
For example, if the function
function f () 2 + 2 endfunction
is executed, Octave will either print ‘ans = 4’ or nothing
depending on the value of silent_functions.
split_long_rows
For large matrices, Octave may not be able to display all the columns of a given row on one line of your screen. This can result in missing information or output that is nearly impossible to decipher, depending on whether your terminal truncates or wraps long lines.
If the value of split_long_rows is "true", Octave will
display the matrix in a series of smaller pieces, each of which can fit
within the limits of your terminal width. Each set of rows is labeled
so that you can easily see which columns are currently being displayed.
For example:
octave:13> rand (2, 9) ans = Columns 1 through 7: 0.92205 0.72628 0.99841 0.62590 0.82422 0.77486 0.30258 0.15999 0.79484 0.75443 0.86995 0.91430 0.23980 0.64591 Columns 8 and 9: 0.08894 0.13266 0.28008 0.65575
The default value of split_long_rows is "true".
suppress_verbose_help_message
If the value of suppress_verbose_help_message is "true",
Octave will not add additional help information to the end of the output
from the help command and usage messages for built-in commands.
treat_neg_dim_as_zero
If the value of treat_neg_dim_as_zero is "true", expressions
like
eye (-1)
produce an empty matrix (i.e., row and column dimensions are zero).
Otherwise, an error message is printed and control is returned to the
top level. The default value is "false".
warn_assign_as_truth_value
If the value of warn_assign_as_truth_value is "true", a
warning is issued for statements like
if (s = t) ...
since such statements are not common, and it is likely that the intent was to write
if (s == t) ...
instead.
There are times when it is useful to write code that contains
assignments within the condition of a while or if
statement. For example, statements like
while (c = getc()) ...
are common in C programming.
It is possible to avoid all warnings about such statements by setting
warn_assign_as_truth_value to "false", but that may also
let real errors like
if (x = 1) # intended to test (x == 1)! ...
slip by.
In such cases, it is possible suppress errors for specific statements by writing them with an extra set of parentheses. For example, writing the previous example as
while ((c = getc())) ...
will prevent the warning from being printed for this statement, while allowing Octave to warn about other assignments used in conditional contexts.
The default value of warn_assign_as_truth_value is "true".
warn_comma_in_global_decl
If the value of warn_comma_in_global_decl is "true", a
warning is issued for statements like
global a = 1, b
which makes the variables ‘a’ and ‘b’ global and assigns the value 1 to the variable ‘a’, because in this context, the comma is not interpreted as a statement separator.
The default value of warn_comma_in_global_decl is "true".
warn_divide_by_zero
If the value of warn_divide_by_zero is "true", a warning
is issued when Octave encounters a division by zero. If the value is
"false", the warning is omitted. The default value is
"true".
warn_function_name_clash
If the value of warn_function_name_clash is "true", a
warning is issued when Octave finds that the name of a function defined
in a function file differs from the name of the file. If the value is
"false", the warning is omitted. The default value is
"true".
whitespace_in_literal_matrix
This variable allows some control over how Octave decides to convert spaces to commas and semicolons in matrix expressions like ‘[m (1)]’ or
[ 1, 2, 3, 4 ]
If the value of whitespace_in_literal_matrix is "ignore",
Octave will never insert a comma or a semicolon in a literal matrix
list. For example, the expression ‘[1 2]’ will result in an error
instead of being treated the same as ‘[1, 2]’, and the expression
[ 1, 2, 3, 4 ]
will result in the vector [1 2 3 4] instead of a matrix.
If the value of whitespace_in_literal_matrix is "traditional",
Octave will convert spaces to a comma between identifiers and ‘(’. For
example, given the matrix
m = [3 2]
the expression
[m (1)]
will be parsed as
[m, (1)]
and will result in
[3 2 1]
and the expression
[ 1, 2, 3, 4 ]
will result in a matrix because the newline character is converted to a semicolon (row separator) even though there is a comma at the end of the first line (trailing commas or semicolons are ignored). This is apparently how MATLAB behaves.
Any other value for whitespace_in_literal_matrix results in behavior
that is the same as traditional, except that Octave does not
convert spaces to a comma between identifiers and ‘(’. For
example, the expression
[m (1)]
will produce ‘3’. This is the way Octave has always behaved.
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In addition to these variables, there are two other special built-in variables whose values are automatically updated.
ans
This variable holds the most recently computed result that was not explicitly assigned to a variable. For example, after the expression
3^2 + 4^2
is evaluated, the value of ans is ‘25’.
PWD
The current working directory. The value of PWD is updated each
time the current working directory is changed with the ‘cd’
command. @xref{System Utilities}.
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Here is a summary of all of Octave’s preference variables and their
default values. In the following table OCTAVE_HOME stands for
the root directory where Octave is installed, and VERSION stands
for the Octave version number.
EDITOR EDITOR environment variable, or "vi"
INFO_FILE "OCTAVE_HOME/info/octave.info"
LOADPATH OCTAVE_PATH environment variable, or
".:OCTAVE_HOME/lib/VERSION"
PAGER "less", or "more"
PS1 "\s:\#> "
PS2 "> "
PS4 "+ "
automatic_replot "false"
default_return_value []
do_fortran_indexing "false"
define_all_return_values "false"
empty_list_elements_ok "warn"
gnuplot_binary "gnuplot"
ignore_function_time_stamp "system"
implicit_str_to_num_ok "false"
ok_to_lose_imaginary_part "warn"
output_max_field_width 10
output_precision 5
page_screen_output "true"
prefer_column_vectors "false"
prefer_zero_one_indexing "false"
print_answer_id_name "true"
print_empty_dimensions "true"
resize_on_range_error "true"
return_last_computed_value "false"
save_precision 17
silent_functions "false"
split_long_rows "true"
suppress_verbose_help_message "true"
treat_neg_dim_as_zero "false"
warn_assign_as_truth_value "true"
warn_comma_in_global_decl "true"
warn_divide_by_zero "true"
warn_function_name_clash "true"
whitespace_in_literal_matrix ""
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