Cream of the Crop 26/Cream of the Crop 26.iso/os2/octa209b.zip/octave-2.09/doc/octave.i03

GNU Info File | 1997-08-20 | 49KB | 1,125 lines

This is Info file octave, produced by Makeinfo-1.64 from the input file
octave.tex.
START-INFO-DIR-ENTRY
* Octave: (octave).     Interactive language for numerical computations.
END-INFO-DIR-ENTRY
   Copyright (C) 1996, 1997 John W. Eaton.
   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions.
File: octave,  Node: Data Structures,  Next: Variables,  Prev: Strings,  Up: Top
Data Structures
***************
   Octave includes support for organizing data in structures.  The
current implementation uses an associative array with indices limited to
strings, but the syntax is more like C-style structures.  Here are some
examples of using data structures in Octave.
   Elements of structures can be of any value type.  For example, the
three expressions
     x.a = 1
     x.b = [1, 2; 3, 4]
     x.c = "string"
create a structure with three elements.  To print the value of the
structure, you can type its name, just as for any other variable:
     octave:2> x
     x =
     {
       a = 1
       b =

         1  2
         3  4

       c = string
     }
Note that Octave may print the elements in any order.
   Structures may be copied.
     octave:1> y = x
     y =
     {
       a = 1
       b =

         1  2
         3  4

       c = string
     }
   Since structures are themselves values, structure elements may
reference other structures.  The following statements change the value
of the element `b' of the structure `x' to be a data structure
containing the single element `d', which has a value of 3.
     octave:1> x.b.d = 3
     x.b.d = 3
     octave:2> x.b
     ans =
     {
       d = 3
     }
     octave:3> x
     x =
     {
       a = 1
       b =
       {
         d = 3
       }

       c = string
     }
   Note that when Octave prints the value of a structure that contains
other structures, only a few levels are displayed.  For example,
     octave:1> a.b.c.d.e = 1;
     octave:2> a
     a =
     {
       b =
       {
         c = <structure>
       }
     }
This prevents long and confusing output from large deeply nested
structures.
 - Built-in Variable: struct_levels_to_print
     You can tell Octave how many structure levels to display by
     setting the built-in variable `struct_levels_to_print'.  The
     default value is 2.
   Functions can return structures.  For example, the following function
separates the real and complex parts of a matrix and stores them in two
elements of the same structure variable.
     octave:1> function y = f (x)
     > y.re = real (x);
     > y.im = imag (x);
     > endfunction
   When called with a complex-valued argument, `f' returns the data
structure containing the real and imaginary parts of the original
function argument.
     octave:2> f (rand (3) + rand (3) * I);
     ans =
     {
       im =

         0.26475  0.14828
         0.18436  0.83669

       re =

         0.040239  0.242160
         0.238081  0.402523
     }
   Function return lists can include structure elements, and they may be
indexed like any other variable.  For example,
     octave:1> [ x.u, x.s(2:3,2:3), x.v ] = svd ([1, 2; 3, 4])
     x.u =

       -0.40455  -0.91451
       -0.91451   0.40455

     x.s =

       0.00000  0.00000  0.00000
       0.00000  5.46499  0.00000
       0.00000  0.00000  0.36597

     x.v =

       -0.57605   0.81742
       -0.81742  -0.57605
   It is also possible to cycle through all the elements of a structure
in a loop, using a special form of the `for' statement (*note The for
Statement::.)
   The following functions are available to give you information about
structures.
 - Built-in Function:  is_struct (EXPR)
     Return 1 if the value of the expression EXPR is a structure.
 - Built-in Function:  struct_contains (EXPR, NAME)
     Return 1 if the expression EXPR is a structure and it includes an
     element named NAME.  The first argument must be a structure and
     the second must be a string.
 - Built-in Function:  struct_elements (STRUCT)
     Return a list of strings naming the elements of the structure
     STRUCT.  It is an error to call `struct_elements' with an argument
     that is not a structure.
File: octave,  Node: Variables,  Next: Expressions,  Prev: Data Structures,  Up: Top
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
     x
     x15
     __foo_bar_baz__
     fucnrdthsucngtagdjb
However, names like `__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 `a' and `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
"assignment operators" and "increment operators".  *Note Assignment
Expressions: Assignment Ops.
   A number of variables have special built-in meanings.  For example,
`PWD' holds the current working directory, and `pi' names the ratio of
the circumference of a circle to its diameter. *Note 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::
File: octave,  Node: Global Variables,  Next: Status of Variables,  Prev: Variables,  Up: Variables
Global Variables
================
   A variable that has been declared "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 `global' declaration
statement.  The following statements are all global declarations.
     global a
     global b = 2
     global c = 3, d, e = 5
   It is necessary declare a variable as global within a function body
in order to access it.  For example,
     global x
     function f ()
       x = 1;
     endfunction
     f ()
does *not* set the value of the global variable `x' to 1.  In order to
change the value of the global variable `x', you must also declare it
to be global within the function body, like this
     function f ()
       global x;
       x = 1;
     endfunction
   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
     function f (x)
       x = 0
     endfunction
and the definition of `x' as a global variable at the top level,
     global x = 13
the expression
     f (x)
will display the value of `x' from inside the function as 0, but the
value of `x' at the top level remains unchanged, because the function
works with a *copy* of its argument.
 - Built-in Variable: warn_comma_in_global_decl
     If the value of `warn_comma_in_global_decl' is nonzero, 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 nonzero.
 - Built-in Function:  is_global (NAME)
     Return 1 if NAME is globally visible.  Otherwise, return 0.  For
     example,
          global x
          is_global ("x")
               => 1
File: octave,  Node: Status of Variables,  Next: Summary of Built-in Variables,  Prev: Global Variables,  Up: Variables
Status of Variables
===================
 - Command: clear OPTIONS PATTERN ...
     Delete the names matching the given patterns from the symbol
     table.  The pattern may contain the following special characters:
    `?'
          Match any single character.
    `*'
          Match zero or more characters.
    `[ LIST ]'
          Match the list of characters specified by LIST.  If the first
          character is `!' or `^', match all characters except those
          specified by LIST.  For example, the pattern `[a-zA-Z]' will
          match all lower and upper case alphabetic characters.
     For example, the command
          clear foo b*r
     clears the name `foo' and all names that begin with the letter `b'
     and end with the letter `r'.
     If `clear' is called without any arguments, all user-defined
     variables (local and global) are cleared from the symbol table.  If
     `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 `foo', and then hidden it by
     performing the assignment `foo = 2'.  Executing the command `clear
     foo' once will clear the variable definition and restore the
     definition of `foo' as a function.  Executing `clear foo' a second
     time will clear the function definition.
     This command may not be used within a function body.
 - Command: who OPTIONS PATTERN ...
 - Command: whos OPTIONS PATTERN ...
     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.
    `-all'
          List all currently defined symbols.
    `-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 `LOADPATH'.
    `-functions'
          List user-defined functions.
    `-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.
    `-variables'
          List user-defined variables.
     Valid patterns are the same as described for the `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 `whos' is equivalent to `who -long'.
 - Built-in Function:  exist (NAME)
     Return 1 if the name exists as a variable, 2 if the name (after
     appending `.m') is a function file in the path, 3 if the name is a
     `.oct' file in the path, or 5 if the name is a built-in function.
     Otherwise, return 0.
 - Built-in Function:  document (SYMBOL, TEXT)
     Set the documentation string for SYMBOL to TEXT.
 - Command: type OPTIONS NAME ...
     Display the definition of each NAME that refers to a function.
     Normally also displays if each NAME is user-defined or builtin;
     the `-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.
 - Command: which NAME ...
     Display the type of each NAME.  If NAME is defined from a function
     file, the full name of the file is also displayed.
File: octave,  Node: Summary of Built-in Variables,  Next: Defaults from the Environment,  Prev: Status of Variables,  Up: Variables
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 OCTAVE-HOME stands for the root directory where all
of Octave is installed (the default is `', VERSION stands for the
Octave version number (for example, 2.0.5) and ARCH stands for the type
of system for which Octave was compiled (for example, `i486-OS/2').
`EDITOR'
     *Note Commands For History::.
     Default value: `"emacs"'.
`EXEC_PATH'
     *Note Controlling Subprocesses::.
     Default value: `":$PATH"'.
`INFO_FILE'
     *Note Getting Help::.
     Default value: `"OCTAVE-HOME/info/octave.info"'.
`INFO_PROGRAM'
     *Note Getting Help::.
     Default value:
     `"OCTAVE-HOME/libexec/octave/VERSION/exec/ARCH/info"'.
`LOADPATH'
     *Note Function Files::.
     Default value: `".:OCTAVE-HOME/lib/VERSION"'.
`OCTAVE_HOME'
     Default value: `""'.
`PAGER'
     *Note Input and Output::.
     Default value: `"less", or "more"'.
`PS1'
     *Note Customizing the Prompt::.
     Default value: `"\s:\#> "'.
`PS2'
     *Note Customizing the Prompt::.
     Default value: `"> "'.
`PS4'
     *Note Customizing the Prompt::.
     Default value: `"+ "'.
`automatic_replot'
     *Note Two-Dimensional Plotting::.
     Default value: 0.
`beep_on_error'
     *Note Error Handling::.
     Default value: 0.
`completion_append_char'
     *Note Commands For Completion::.
     Default value: `" "'.
`default_eval_print_flag'
     *Note Evaluation::.
     Default value: 1.
`default_return_value'
     *Note Multiple Return Values::.
     Default value: `[]'.
`default_save_format'
     *Note Simple File I/O::.
     Default value: `"ascii"'.
`do_fortran_indexing'
     *Note Index Expressions::.
     Default value: 0.
`define_all_return_values'
     *Note Multiple Return Values::.
     Default value: 0.
`empty_list_elements_ok'
     *Note Empty Matrices::.
     Default value: `"warn"'.
`gnuplot_binary'
     *Note Three-Dimensional Plotting::.
     Default value: `"gnuplot"'.
`history_file'
     *Note Commands For History::.
     Default value: `"~/.octave_hist"'.
`history_size'
     *Note Commands For History::.
     Default value: 1024.
`ignore_function_time_stamp'
     *Note Function Files::.
     Default value: `"system"'.
`implicit_str_to_num_ok'
     *Note String Conversions::.
     Default value: 0.
`ok_to_lose_imaginary_part'
     *Note Special Utility Matrices::.
     Default value: `"warn"'.
`output_max_field_width'
     *Note Matrices::.
     Default value: 10.
`output_precision'
     *Note Matrices::.
     Default value: 5.
`page_screen_output'
     *Note Input and Output::.
     Default value: 1.
`prefer_column_vectors'
     *Note Index Expressions::.
     Default value: 0.
`print_answer_id_name'
     *Note Terminal Output::.
     Default value: 1.
`print_empty_dimensions'
     *Note Empty Matrices::.
     Default value: 1.
`resize_on_range_error'
     *Note Index Expressions::.
     Default value: 1.
`return_last_computed_value'
     *Note Returning From a Function::.
     Default value: 0.
`save_precision'
     *Note Simple File I/O::.
     Default value: 17.
`saving_history'
     *Note Commands For History::.
     Default value: 1.
`silent_functions'
     *Note Defining Functions::.
     Default value: 0.
`split_long_rows'
     *Note Matrices::.
     Default value: 1.
`struct_levels_to_print'
     *Note Data Structures::.
     Default value: 2.
`suppress_verbose_help_message'
     *Note Getting Help::.
     Default value: 1.
`treat_neg_dim_as_zero'
     *Note Special Utility Matrices::.
     Default value: 0.
`warn_assign_as_truth_value'
     *Note The if Statement::.
     Default value: 1.
`warn_comma_in_global_decl'
     *Note Global Variables::.
     Default value: 1.
`warn_divide_by_zero'
     *Note Arithmetic Ops::.
     Default value: 1.
`warn_function_name_clash'
     *Note Function Files::.
     Default value: 1.
`whitespace_in_literal_matrix'
     *Note Matrices::.
     Default value: `""'.
File: octave,  Node: Defaults from the Environment,  Prev: Summary of Built-in Variables,  Up: Variables
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.  *Note Command Line Options::.
`EDITOR'
     *Note Commands For History::.
     Built-in variable: `EDITOR'.
`OCTAVE_EXEC_PATH'
     *Note Controlling Subprocesses::.
     Built-in variable: `EXEC_PATH'.  Command-line argument:
     `--exec-path'.
`OCTAVE_PATH'
     *Note Function Files::.
     Built-in variable: `LOADPATH'.  Command-line argument: `--path'.
`OCTAVE_INFO_FILE'
     *Note Getting Help::.
     Built-in variable: `INFO_FILE'.  Command-line argument:
     `--info-file'.
`OCTAVE_INFO_PROGRAM'
     *Note Getting Help::.
     Built-in variable: `INFO_PROGRAM'.  Command-line argument:
     `--info-program'.
`OCTAVE_HISTSIZE'
     *Note Commands For History::.
     Built-in variable: `history_size'.
`OCTAVE_HISTFILE'
     *Note Commands For History::.
     Built-in variable: `history_file'.
File: octave,  Node: Expressions,  Next: Evaluation,  Prev: Variables,  Up: Top
Expressions
***********
   Expressions are the basic building block of statements in Octave.  An
expression evaluates to a value, which you can print, test, store in a
variable, pass to a function, or assign a new value to a variable with
an assignment operator.
   An expression can serve as a statement on its own.  Most other kinds
of statements contain one or more expressions which specify data to be
operated on.  As in other languages, expressions in Octave include
variables, array references, constants, and function calls, as well as
combinations of these with various operators.
* Menu:
* Index Expressions::
* Calling Functions::
* Arithmetic Ops::
* Comparison Ops::
* Boolean Expressions::
* Assignment Ops::
* Increment Ops::
* Operator Precedence::
File: octave,  Node: Index Expressions,  Next: Calling Functions,  Prev: Expressions,  Up: Expressions
Index Expressions
=================
   An "index expression" allows you to reference or extract selected
elements of a matrix or vector.
   Indices may be scalars, vectors, ranges, or the special operator
`:', which may be used to select entire rows or columns.
   Vectors are indexed using a single expression.  Matrices require two
indices unless the value of the built-in variable `do_fortran_indexing'
is nonzero, in which case matrices may also be indexed by a single
expression.
 - Built-in Variable: do_fortran_indexing
     If the value of `do_fortran_indexing' is nonzero, 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 0.
   Given the matrix
     a = [1, 2; 3, 4]
all of the following expressions are equivalent
     a (1, [1, 2])
     a (1, 1:2)
     a (1, :)
and select the first row of the matrix.
   A special form of indexing may be used to select elements of a
matrix or vector.  If the indices are vectors made up of only ones and
zeros, the result is a new matrix whose elements correspond to the
elements of the index vector that are equal to one.  For example,
     a = [1, 2; 3, 4];
     a ([1, 0], :)
selects the first row of the matrix `a'.
   This operation can be useful for selecting elements of a matrix
based on some condition, since the comparison operators return matrices
of ones and zeros.
   This special zero-one form of indexing leads to a conflict with the
standard indexing operation.  For example, should the following
statements
     a = [1, 2; 3, 4];
     a ([1, 1], :)
return the original matrix, or the matrix formed by selecting the first
row twice?  Although this conflict is not likely to arise very often in
practice, you may select the behavior you prefer by setting the built-in
variable `prefer_zero_one_indexing'.
 - Built-in Variable: prefer_zero_one_indexing
     If the value of `prefer_zero_one_indexing' is nonzero, Octave will
     perform zero-one style indexing when there is a conflict with the
     normal indexing rules.  *Note Index Expressions::.  For example,
     given a matrix
          a = [1, 2, 3, 4]
     with `prefer_zero_one_indexing' is set to nonzero, 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 0, 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 0.
   Finally, indexing a scalar with a vector of ones can be used to
create a vector the same size as the the index vector, with each
element equal to the value of the original scalar.  For example, the
following statements
     a = 13;
     a ([1, 1, 1, 1])
produce a vector whose four elements are all equal to 13.
   Similarly, indexing a scalar with two vectors of ones can be used to
create a matrix.  For example the following statements
     a = 13;
     a ([1, 1], [1, 1, 1])
create a 2 by 3 matrix with all elements equal to 13.
   This is an obscure notation and should be avoided.  It is better to
use the function `ones' to generate a matrix of the appropriate size
whose elements are all one, and then to scale it to produce the desired
result.  *Note Special Utility Matrices::.
 - Built-in Variable: prefer_column_vectors
     If `prefer_column_vectors' is nonzero, 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 0.
     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'.
 - Built-in Variable: resize_on_range_error
     If the value of `resize_on_range_error' is nonzero, expressions
     like
          for i = 1:10
            a (i) = sqrt (i);
          endfor
     (for `a' previously undefined) result in the variable `a' being
     resized to be just large enough to hold the new value.  New
     elements that have not been given a value are set to zero.  If the
     value of `resize_on_range_error' is 0, an error message is printed
     and control is returned to the top level.  The default value is 1.
   Note that it is quite inefficient to create a vector using a loop
like the one shown in the example above.  In this particular case, it
would have been much more efficient to use the expression
     a = sqrt (1:10);
thus avoiding the loop entirely.  In cases where a loop is still
required, or a number of values must be combined to form a larger
matrix, it is generally much faster to set the size of the matrix first,
and then insert elements using indexing commands.  For example, given a
matrix `a',
     [nr, nc] = size (a);
     x = zeros (nr, n * nc);
     for i = 1:n
       x(:,(i-1)*n+1:i*n) = a;
     endfor
is considerably faster than
     x = a;
     for i = 1:n-1
       x = [x, a];
     endfor
particularly for large matrices because Octave does not have to
repeatedly resize the result.
File: octave,  Node: Calling Functions,  Next: Arithmetic Ops,  Prev: Index Expressions,  Up: Expressions
Calling Functions
=================
   A "function" is a name for a particular calculation.  Because it has
a name, you can ask for it by name at any point in the program.  For
example, the function `sqrt' computes the square root of a number.
   A fixed set of functions are "built-in", which means they are
available in every Octave program.  The `sqrt' function is one of
these.  In addition, you can define your own functions.  *Note
Functions and Scripts::, for information about how to do this.
   The way to use a function is with a "function call" expression,
which consists of the function name followed by a list of "arguments"
in parentheses. The arguments are expressions which give the raw
materials for the calculation that the function will do.  When there is
more than one argument, they are separated by commas.  If there are no
arguments, you can omit the parentheses, but it is a good idea to
include them anyway, to clearly indicate that a function call was
intended.  Here are some examples:
     sqrt (x^2 + y^2)      # One argument
     ones (n, m)           # Two arguments
     rand ()               # No arguments
   Each function expects a particular number of arguments.  For
example, the `sqrt' function must be called with a single argument, the
number to take the square root of:
     sqrt (ARGUMENT)
   Some of the built-in functions take a variable number of arguments,
depending on the particular usage, and their behavior is different
depending on the number of arguments supplied.
   Like every other expression, the function call has a value, which is
computed by the function based on the arguments you give it.  In this
example, the value of `sqrt (ARGUMENT)' is the square root of the
argument.  A function can also have side effects, such as assigning the
values of certain variables or doing input or output operations.
   Unlike most languages, functions in Octave may return multiple
values.  For example, the following statement
     [u, s, v] = svd (a)
computes the singular value decomposition of the matrix `a' and assigns
the three result matrices to `u', `s', and `v'.
   The left side of a multiple assignment expression is itself a list of
expressions, and is allowed to be a list of variable names or index
expressions.  See also *Note Index Expressions::, and *Note Assignment
Ops::.
* Menu:
* Call by Value::
* Recursion::
File: octave,  Node: Call by Value,  Next: Recursion,  Prev: Calling Functions,  Up: Calling Functions
Call by Value
-------------
   In Octave, unlike Fortran, function arguments are passed by value,
which means that each argument in a function call is evaluated and
assigned to a temporary location in memory before being passed to the
function.  There is currently no way to specify that a function
parameter should be passed by reference instead of by value.  This
means that it is impossible to directly alter the value of function
parameter in the calling function.  It can only change the local copy
within the function body.  For example, the function
     function f (x, n)
       while (n-- > 0)
         disp (x);
       endwhile
     endfunction
displays the value of the first argument N times.  In this function,
the variable N is used as a temporary variable without having to worry
that its value might also change in the calling function.  Call by
value is also useful because it is always possible to pass constants
for any function parameter without first having to determine that the
function will not attempt to modify the parameter.
   The caller may use a variable as the expression for the argument, but
the called function does not know this: it only knows what value the
argument had.  For example, given a function called as
     foo = "bar";
     fcn (foo)
you should not think of the argument as being "the variable `foo'."
Instead, think of the argument as the string value, `"bar"'.
   Even though Octave uses pass-by-value semantics for function
arguments, values are not copied unnecessarily.  For example,
     x = rand (1000);
     f (x);
does not actually force two 1000 by 1000 element matrices to exist
*unless* the function `f' modifies the value of its argument.  Then
Octave must create a copy to avoid changing the value outside the scope
of the function `f', or attempting (and probably failing!) to modify
the value of a constant or the value of a temporary result.
File: octave,  Node: Recursion,  Prev: Call by Value,  Up: Calling Functions
Recursion
---------
   With some restrictions(1), recursive function calls are allowed.  A
"recursive function" is one which calls itself, either directly or
indirectly.  For example, here is an inefficient(2) way to compute the
factorial of a given integer:
     function retval = fact (n)
       if (n > 0)
         retval = n * fact (n-1);
       else
         retval = 1;
       endif
     endfunction
   This function is recursive because it calls itself directly.  It
eventually terminates because each time it calls itself, it uses an
argument that is one less than was used for the previous call.  Once the
argument is no longer greater than zero, it does not call itself, and
the recursion ends.
   There is currently no limit on the recursion depth, so infinite
recursion is possible.  If this happens, Octave will consume more and
more memory attempting to store intermediate values for each function
call context until there are no more resources available.  This is
obviously undesirable, and will probably be fixed in some future version
of Octave by allowing users to specify a maximum allowable recursion
depth.
   ---------- Footnotes ----------
   (1)  Some of Octave's function are implemented in terms of functions
that cannot be called recursively.  For example, the ODE solver `lsode'
is ultimately implemented in a Fortran subroutine that cannot be called
recursively, so `lsode' should not be called either directly or
indirectly from within the user-supplied function that `lsode'
requires.  Doing so will result in undefined behavior.
   (2)  It would be much better to use `prod (1:n)', or `gamma (n+1)'
instead, after first checking to ensure that the value `n' is actually a
positive integer.
File: octave,  Node: Arithmetic Ops,  Next: Comparison Ops,  Prev: Calling Functions,  Up: Expressions
Arithmetic Operators
====================
   The following arithmetic operators are available, and work on scalars
and matrices.
`X + Y'
     Addition.  If both operands are matrices, the number of rows and
     columns must both agree.  If one operand is a scalar, its value is
     added to all the elements of the other operand.
`X .+ Y'
     Element by element addition.  This operator is equivalent to `+'.
`X - Y'
     Subtraction.  If both operands are matrices, the number of rows and
     columns of both must agree.
`X .- Y'
     Element by element subtraction.  This operator is equivalent to
     `-'.
`X * Y'
     Matrix multiplication.  The number of columns of X must agree with
     the number of rows of Y.
`X .* Y'
     Element by element multiplication.  If both operands are matrices,
     the number of rows and columns must both agree.
`X / Y'
     Right division.  This is conceptually equivalent to the expression
          (inverse (y') * x')'
     but it is computed without forming the inverse of Y'.
     If the system is not square, or if the coefficient matrix is
     singular, a minimum norm solution is computed.
`X ./ Y'
     Element by element right division.
`X \ Y'
     Left division.  This is conceptually equivalent to the expression
          inverse (x) * y
     but it is computed without forming the inverse of X.
     If the system is not square, or if the coefficient matrix is
     singular, a minimum norm solution is computed.
`X .\ Y'
     Element by element left division.  Each element of Y is divided by
     each corresponding element of X.
`X ^ Y'
`X ** Y'
     Power operator.  If X and Y are both scalars, this operator
     returns X raised to the power Y.  If X is a scalar and Y is a
     square matrix, the result is computed using an eigenvalue
     expansion.  If X is a square matrix. the result is computed by
     repeated multiplication if Y is an integer, and by an eigenvalue
     expansion if Y is not an integer.  An error results if both X and
     Y are matrices.
     The implementation of this operator needs to be improved.
`X .^ Y'
`X .** Y'
     Element by element power operator.  If both operands are matrices,
     the number of rows and columns must both agree.
     Negation.
     Unary plus.  This operator has no effect on the operand.
     Complex conjugate transpose.  For real arguments, this operator is
     the same as the transpose operator.  For complex arguments, this
     operator is equivalent to the expression
          conj (x.')
`X.''
     Transpose.
   Note that because Octave's element by element operators begin with a
`.', there is a possible ambiguity for statements like
     1./m
because the period could be interpreted either as part of the constant
or as part of the operator.  To resolve this conflict, Octave treats the
expression as if you had typed
     (1) ./ m
and not
     (1.) / m
Although this is inconsistent with the normal behavior of Octave's
lexer, which usually prefers to break the input into tokens by
preferring the longest possible match at any given point, it is more
useful in this case.
 - Built-in Variable: warn_divide_by_zero
     If the value of `warn_divide_by_zero' is nonzero, a warning is
     issued when Octave encounters a division by zero.  If the value is
     0, the warning is omitted.  The default value is 1.
File: octave,  Node: Comparison Ops,  Next: Boolean Expressions,  Prev: Arithmetic Ops,  Up: Expressions
Comparison Operators
====================
   "Comparison operators" compare numeric values for relationships such
as equality.  They are written using *relational operators*.
   All of Octave's comparison operators return a value of 1 if the
comparison is true, or 0 if it is false.  For matrix values, they all
work on an element-by-element basis.  For example,
     [1, 2; 3, 4] == [1, 3; 2, 4]
          =>  1  0
              0  1
   If one operand is a scalar and the other is a matrix, the scalar is
compared to each element of the matrix in turn, and the result is the
same size as the matrix.
`X < Y'
     True if X is less than Y.
`X <= Y'
     True if X is less than or equal to Y.
`X == Y'
     True if X is equal to Y.
`X >= Y'
     True if X is greater than or equal to Y.
`X > Y'
     True if X is greater than Y.
`X != Y'
`X ~= Y'
`X <> Y'
     True if X is not equal to Y.
   String comparisons may also be performed with the `strcmp' function,
not with the comparison operators listed above.  *Note Strings::.
File: octave,  Node: Boolean Expressions,  Next: Assignment Ops,  Prev: Comparison Ops,  Up: Expressions
Boolean Expressions
===================
* Menu:
* Element-by-element Boolean Operators::
* Short-circuit Boolean Operators::
File: octave,  Node: Element-by-element Boolean Operators,  Next: Short-circuit Boolean Operators,  Prev: Boolean Expressions,  Up: Boolean Expressions
Element-by-element Boolean Operators
------------------------------------
   An "element-by-element boolean expression" is a combination of
comparison expressions using the boolean operators "or" (`|'), "and"
(`&'), and "not" (`!'), along with parentheses to control nesting.  The
truth of the boolean expression is computed by combining the truth
values of the corresponding elements of the component expressions.  A
value is considered to be false if it is zero, and true otherwise.
   Element-by-element boolean expressions can be used wherever
comparison expressions can be used.  They can be used in `if' and
`while' statements.  However, if a matrix value used as the condition
in an `if' or `while' statement is only true if *all* of its elements
are nonzero.
   Like comparison operations, each element of an element-by-element
boolean expression also has a numeric value (1 if true, 0 if false) that
comes into play if the result of the boolean expression is stored in a
variable, or used in arithmetic.
   Here are descriptions of the three element-by-element boolean
operators.
`BOOLEAN1 & BOOLEAN2'
     Elements of the result are true if both corresponding elements of
     BOOLEAN1 and BOOLEAN2 are true.
`BOOLEAN1 | BOOLEAN2'
     Elements of the result are true if either of the corresponding
     elements of BOOLEAN1 or BOOLEAN2 is true.
`! BOOLEAN'
`~ BOOLEAN'
     Each element of the result is true if the corresponding element of
     BOOLEAN is false.
   For matrix operands, these operators work on an element-by-element
basis.  For example, the expression
     [1, 0; 0, 1] & [1, 0; 2, 3]
returns a two by two identity matrix.
   For the binary operators, the dimensions of the operands must
conform if both are matrices.  If one of the operands is a scalar and
the other a matrix, the operator is applied to the scalar and each
element of the matrix.
   For the binary element-by-element boolean operators, both
subexpressions BOOLEAN1 and BOOLEAN2 are evaluated before computing the
result.  This can make a difference when the expressions have side
effects.  For example, in the expression
     a & b++
the value of the variable B is incremented even if the variable A is
zero.
   This behavior is necessary for the boolean operators to work as
described for matrix-valued operands.
File: octave,  Node: Short-circuit Boolean Operators,  Prev: Element-by-element Boolean Operators,  Up: Boolean Expressions
Short-circuit Boolean Operators
-------------------------------
   Combined with the implicit conversion to scalar values in `if' and
`while' conditions, Octave's element-by-element boolean operators are
often sufficient for performing most logical operations.  However, it
is sometimes desirable to stop evaluating a boolean expression as soon
as the overall truth value can be determined.  Octave's "short-circuit"
boolean operators work this way.
`BOOLEAN1 && BOOLEAN2'
     The expression BOOLEAN1 is evaluated and converted to a scalar
     using the equivalent of the operation `all (all (BOOLEAN1))'.  If
     it is false, the result of the overall expression is 0.  If it is
     true, the expression BOOLEAN2 is evaluated and converted to a
     scalar using the equivalent of the operation `all (all
     (BOOLEAN1))'.  If it is true, the result of the overall expression
     is 1.  Otherwise, the result of the overall expression is 0.
`BOOLEAN1 || BOOLEAN2'
     The expression BOOLEAN1 is evaluated and converted to a scalar
     using the equivalent of the operation `all (all (BOOLEAN1))'.  If
     it is true, the result of the overall expression is 1.  If it is
     false, the expression BOOLEAN2 is evaluated and converted to a
     scalar using the equivalent of the operation `all (all
     (BOOLEAN1))'.  If it is true, the result of the overall expression
     is 1.  Otherwise, the result of the overall expression is 0.
   The fact that both operands may not be evaluated before determining
the overall truth value of the expression can be important.  For
example, in the expression
     a && b++
the value of the variable B is only incremented if the variable A is
nonzero.
   This can be used to write somewhat more concise code.  For example,
it is possible write
     function f (a, b, c)
       if (nargin > 2 && isstr (c))
         ...
instead of having to use two `if' statements to avoid attempting to
evaluate an argument that doesn't exist.  For example, without the
short-circuit feature, it would be necessary to write
     function f (a, b, c)
       if (nargin > 2)
         if (isstr (c))
           ...
   Writing
     function f (a, b, c)
       if (nargin > 2 & isstr (c))
         ...
would result in an error if `f' were called with one or two arguments
because Octave would be forced to try to evaluate both of the operands
for the operator `&'.
File: octave,  Node: Assignment Ops,  Next: Increment Ops,  Prev: Boolean Expressions,  Up: Expressions
Assignment Expressions
======================
   An "assignment" is an expression that stores a new value into a
variable.  For example, the following expression assigns the value 1 to
the variable `z':
     z = 1
   After this expression is executed, the variable `z' has the value 1.
Whatever old value `z' had before the assignment is forgotten.  The `='
sign is called an "assignment operator".
   Assignments can store string values also.  For example, the following
expression would store the value `"this food is good"' in the variable
`message':
     thing = "food"
     predicate = "good"
     message = [ "this " , thing , " is " , predicate ]
(This also illustrates concatenation of strings.)
   Most operators (addition, concatenation, and so on) have no effect
except to compute a value.  If you ignore the value, you might as well
not use the operator.  An assignment operator is different.  It does
produce a value, but even if you ignore the value, the assignment still
makes itself felt through the alteration of the variable.  We call this
a "side effect".
   The left-hand operand of an assignment need not be a variable (*note
Variables::.).  It can also be an element of a matrix (*note Index
Expressions::.) or a list of return values (*note Calling
Functions::.).  These are all called "lvalues", which means they can
appear on the left-hand side of an assignment operator.  The right-hand
operand may be any expression.  It produces the new value which the
assignment stores in the specified variable, matrix element, or list of
return values.
   It is important to note that variables do *not* have permanent types.
The type of a variable is simply the type of whatever value it happens
to hold at the moment.  In the following program fragment, the variable
`foo' has a numeric value at first, and a string value later on:
     octave:13> foo = 1
     foo = 1
     octave:13> foo = "bar"
     foo = bar
When the second assignment gives `foo' a string value, the fact that it
previously had a numeric value is forgotten.
   Assignment of a scalar to an indexed matrix sets all of the elements
that are referenced by the indices to the scalar value.  For example, if
`a' is a matrix with at least two columns,
     a(:, 2) = 5
sets all the elements in the second column of `a' to 5.
   Assigning an empty matrix `[]' works in most cases to allow you to
delete rows or columns of matrices and vectors.  *Note Empty Matrices::.
For example, given a 4 by 5 matrix A, the assignment
     A (3, :) = []
deletes the third row of A, and the assignment
     A (:, 1:2:5) = []
deletes the first, third, and fifth columns.
   An assignment is an expression, so it has a value.  Thus, `z = 1' as
an expression has the value 1.  One consequence of this is that you can
write multiple assignments together:
     x = y = z = 0
stores the value 0 in all three variables.  It does this because the
value of `z = 0', which is 0, is stored into `y', and then the value of
`y = z = 0', which is 0, is stored into `x'.
   This is also true of assignments to lists of values, so the
following is a valid expression
     [a, b, c] = [u, s, v] = svd (a)
that is exactly equivalent to
     [u, s, v] = svd (a)
     a = u
     b = s
     c = v
   In expressions like this, the number of values in each part of the
expression need not match.  For example, the expression
     [a, b, c, d] = [u, s, v] = svd (a)
is equivalent to the expression above, except that the value of the
variable `d' is left unchanged, and the expression
     [a, b] = [u, s, v] = svd (a)
is equivalent to
     [u, s, v] = svd (a)
     a = u
     b = s
   You can use an assignment anywhere an expression is called for.  For
example, it is valid to write `x != (y = 1)' to set `y' to 1 and then
test whether `x' equals 1.  But this style tends to make programs hard
to read.  Except in a one-shot program, you should rewrite it to get
rid of such nesting of assignments.  This is never very hard.
File: octave,  Node: Increment Ops,  Next: Operator Precedence,  Prev: Assignment Ops,  Up: Expressions
Increment Operators
===================
   *Increment operators* increase or decrease the value of a variable
by 1.  The operator to increment a variable is written as `++'.  It may
be used to increment a variable either before or after taking its value.
   For example, to pre-increment the variable X, you would write `++X'.
This would add one to X and then return the new value of X as the
result of the expression.  It is exactly the same as the expression `X
= X + 1'.
   To post-increment a variable X, you would write `X++'.  This adds
one to the variable X, but returns the value that X had prior to
incrementing it.  For example, if X is equal to 2, the result of the
expression `X++' is 2, and the new value of X is 3.
   For matrix and vector arguments, the increment and decrement
operators work on each element of the operand.
   Here is a list of all the increment and decrement expressions.
`++X'
     This expression increments the variable X.  The value of the
     expression is the *new* value of X.  It is equivalent to the
     expression `X = X + 1'.
`--X'
     This expression decrements the variable X.  The value of the
     expression is the *new* value of X.  It is equivalent to the
     expression `X = X - 1'.
`X++'
     This expression causes the variable X to be incremented.  The
     value of the expression is the *old* value of X.
`X--'
     This expression causes the variable X to be decremented.  The
     value of the expression is the *old* value of X.
   It is not currently possible to increment index expressions.  For
example, you might expect that the expression `V(4)++' would increment
the fourth element of the vector V, but instead it results in a parse
error.  This problem may be fixed in a future release of Octave.
File: octave,  Node: Operator Precedence,  Prev: Increment Ops,  Up: Expressions
Operator Precedence
===================
   "Operator precedence" determines how operators are grouped, when
different operators appear close by in one expression.  For example,
`*' has higher precedence than `+'.  Thus, the expression `a + b * c'
means to multiply `b' and `c', and then add `a' to the product (i.e.,
`a + (b * c)').
   You can overrule the precedence of the operators by using
parentheses.  You can think of the precedence rules as saying where the
parentheses are assumed if you do not write parentheses yourself.  In
fact, it is wise to use parentheses whenever you have an unusual
combination of operators, because other people who read the program may
not remember what the precedence is in this case.  You might forget as
well, and then you too could make a mistake.  Explicit parentheses will
help prevent any such mistake.
   When operators of equal precedence are used together, the leftmost
operator groups first, except for the assignment and exponentiation
operators, which group in the opposite order.  Thus, the expression `a
- b + c' groups as `(a - b) + c', but the expression `a = b = c' groups
as `a = (b = c)'.
   The precedence of prefix unary operators is important when another
operator follows the operand.  For example, `-x^2' means `-(x^2)',
because `-' has lower precedence than `^'.
   Here is a table of the operators in Octave, in order of increasing
precedence.
`statement separators'
     `;', `,'.
`assignment'
     `='.  This operator groups right to left.
`logical "or" and "and"'
     `||', `&&'.
`element-wise "or" and "and"'
     `|', `&'.
`relational'
     `<', `<=', `==', `>=', `>', `!=', `~=', `<>'.
`colon'
     `:'.
`add, subtract'
     `+', `-'.
`multiply, divide'
     `*', `/', `\', `.\', `.*', `./'.
`transpose'
     `'', `.''
`unary plus, minus, increment, decrement, and ``not'''
     `+', `-', `++', `--', `!', `~'.
`exponentiation'
     `^', `**', `.^', `.**'.