OS/2 Shareware BBS: 10 Tools/10-Tools.zip/octa21fb.zip/octave/doc/octave.i04

GNU Info File | 2000-01-15 | 48KB | 1,029 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: Evaluation,  Next: Statements,  Prev: Expressions,  Up: Top
Evaluation
**********
   Normally, you evaluate expressions simply by typing them at the
Octave prompt, or by asking Octave to interpret commands that you have
saved in a file.
   Sometimes, you may find it necessary to evaluate an expression that
has been computed and stored in a string, or use a string as the name
of a function to call.  The `eval' and `feval' functions allow you to
do just that, and are necessary in order to evaluate commands that are
not known until run time, or to write functions that will need to call
user-supplied functions.
 - Built-in Function:  eval (TRY, CATCH)
     Parse the string TRY and evaluate it as if it were an Octave
     program, returning the last value computed.  If that fails,
     evaluate the string CATCH.  The string TRY is evaluated in the
     current context, so any results remain available after `eval'
     returns.  For example,
          eval ("a = 13")
               -| a = 13
               => 13
     In this case, the value of the evaluated expression is printed and
     it is also returned returned from `eval'.  Just as with any other
     expression, you can turn printing off by ending the expression in a
     semicolon.  For example,
          eval ("a = 13;")
               => 13
     In this example, the variable `a' has been given the value 13, but
     the value of the expression is not printed.  You can also turn off
     automatic printing for all expressions executed by `eval' using the
     variable `default_eval_print_flag'.
 - Built-in Variable: default_eval_print_flag
     If the value of this variable is nonzero, Octave prints the
     results of commands executed by `eval' that do not end with
     semicolons.  If it is zero, automatic printing is suppressed.  The
     default value is 1.
 - Built-in Function:  feval (NAME, ...)
     Evaluate the function named NAME.  Any arguments after the first
     are passed on to the named function.  For example,
          feval ("acos", -1)
               => 3.1416
     calls the function `acos' with the argument `-1'.
     The function `feval' is necessary in order to be able to write
     functions that call user-supplied functions, because Octave does
     not have a way to declare a pointer to a function (like C) or to
     declare a special kind of variable that can be used to hold the
     name of a function (like `EXTERNAL' in Fortran).  Instead, you
     must refer to functions by name, and use `feval' to call them.
   Here is a simple-minded function using `feval' that finds the root
of a user-supplied function of one variable using Newton's method.
     function result = newtroot (fname, x)

     # usage: newtroot (fname, x)
     #
     #   fname : a string naming a function f(x).
     #   x     : initial guess

       delta = tol = sqrt (eps);
       maxit = 200;
       fx = feval (fname, x);
       for i = 1:maxit
         if (abs (fx) < tol)
           result = x;
           return;
         else
           fx_new = feval (fname, x + delta);
           deriv = (fx_new - fx) / delta;
           x = x - fx / deriv;
           fx = fx_new;
         endif
       endfor

       result = x;

     endfunction
   Note that this is only meant to be an example of calling
user-supplied functions and should not be taken too seriously.  In
addition to using a more robust algorithm, any serious code would check
the number and type of all the arguments, ensure that the supplied
function really was a function, etc.  See *Note Predicates for Numeric
Objects::, for example, for a list of predicates for numeric objects,
and *Note Status of Variables::, for a description of the `exist'
function.
   % DO NOT EDIT!  Generated automatically by munge-texi.
File: octave,  Node: Statements,  Next: Functions and Scripts,  Prev: Evaluation,  Up: Top
Statements
**********
   Statements may be a simple constant expression or a complicated list
of nested loops and conditional statements.
   "Control statements" such as `if', `while', and so on control the
flow of execution in Octave programs.  All the control statements start
with special keywords such as `if' and `while', to distinguish them
from simple expressions.  Many control statements contain other
statements; for example, the `if' statement contains another statement
which may or may not be executed.
   Each control statement has a corresponding "end" statement that
marks the end of the end of the control statement.  For example, the
keyword `endif' marks the end of an `if' statement, and `endwhile'
marks the end of a `while' statement.  You can use the keyword `end'
anywhere a more specific end keyword is expected, but using the more
specific keywords is preferred because if you use them, Octave is able
to provide better diagnostics for mismatched or missing end tokens.
   The list of statements contained between keywords like `if' or
`while' and the corresponding end statement is called the "body" of a
control statement.
* Menu:
* The if Statement::
* The switch Statement::
* The while Statement::
* The for Statement::
* The break Statement::
* The continue Statement::
* The unwind_protect Statement::
* The try Statement::
* Continuation Lines::
File: octave,  Node: The if Statement,  Next: The switch Statement,  Prev: Statements,  Up: Statements
The `if' Statement
==================
   The `if' statement is Octave's decision-making statement.  There are
three basic forms of an `if' statement.  In its simplest form, it looks
like this:
     if (CONDITION)
       THEN-BODY
     endif
CONDITION is an expression that controls what the rest of the statement
will do.  The THEN-BODY is executed only if CONDITION is true.
   The condition in an `if' statement is considered true if its value
is non-zero, and false if its value is zero.  If the value of the
conditional expression in an `if' statement is a vector or a matrix, it
is considered true only if *all* of the elements are non-zero.
   The second form of an if statement looks like this:
     if (CONDITION)
       THEN-BODY
     else
       ELSE-BODY
     endif
If CONDITION is true, THEN-BODY is executed; otherwise, ELSE-BODY is
executed.
   Here is an example:
     if (rem (x, 2) == 0)
       printf ("x is even\n");
     else
       printf ("x is odd\n");
     endif
   In this example, if the expression `rem (x, 2) == 0' is true (that
is, the value of `x' is divisible by 2), then the first `printf'
statement is evaluated, otherwise the second `printf' statement is
evaluated.
   The third and most general form of the `if' statement allows
multiple decisions to be combined in a single statement.  It looks like
this:
     if (CONDITION)
       THEN-BODY
     elseif (CONDITION)
       ELSEIF-BODY
     else
       ELSE-BODY
     endif
Any number of `elseif' clauses may appear.  Each condition is tested in
turn, and if one is found to be true, its corresponding BODY is
executed.  If none of the conditions are true and the `else' clause is
present, its body is executed.  Only one `else' clause may appear, and
it must be the last part of the statement.
   In the following example, if the first condition is true (that is,
the value of `x' is divisible by 2), then the first `printf' statement
is executed.  If it is false, then the second condition is tested, and
if it is true (that is, the value of `x' is divisible by 3), then the
second `printf' statement is executed.  Otherwise, the third `printf'
statement is performed.
     if (rem (x, 2) == 0)
       printf ("x is even\n");
     elseif (rem (x, 3) == 0)
       printf ("x is odd and divisible by 3\n");
     else
       printf ("x is odd\n");
     endif
   Note that the `elseif' keyword must not be spelled `else if', as is
allowed in Fortran.  If it is, the space between the `else' and `if'
will tell Octave to treat this as a new `if' statement within another
`if' statement's `else' clause.  For example, if you write
     if (C1)
       BODY-1
     else if (C2)
       BODY-2
     endif
Octave will expect additional input to complete the first `if'
statement.  If you are using Octave interactively, it will continue to
prompt you for additional input.  If Octave is reading this input from a
file, it may complain about missing or mismatched `end' statements, or,
if you have not used the more specific `end' statements (`endif',
`endfor', etc.), it may simply produce incorrect results, without
producing any warning messages.
   It is much easier to see the error if we rewrite the statements above
like this,
     if (C1)
       BODY-1
     else
       if (C2)
         BODY-2
       endif
using the indentation to show how Octave groups the statements.  *Note
Functions and Scripts::.
 - Built-in Variable: warn_assign_as_truth_value
     If the value of `warn_assign_as_truth_value' is nonzero, 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 0, 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 1.
File: octave,  Node: The switch Statement,  Next: The while Statement,  Prev: The if Statement,  Up: Statements
The `switch' Statement
======================
   The `switch' statement was introduced in Octave 2.0.5.  It should be
considered experimental, and details of the implementation may change
slightly in future versions of Octave.  If you have comments or would
like to share your experiences in trying to use this new command in real
programs, please send them to ([e-mail]).
(But if you think you've found a bug, please report it to
([e-mail]).
   The general form of the `switch' statement is
     switch EXPRESSION
       case LABEL
         COMMAND_LIST
       case LABEL
         COMMAND_LIST
       ...

       otherwise
         COMMAND_LIST
     endswitch
   * The identifiers `switch', `case', `otherwise', and `endswitch' are
     now keywords.
   * The LABEL may be any expression.
   * Duplicate LABEL values are not detected.  The COMMAND_LIST
     corresponding to the first match will be executed.
   * You must have at least one `case LABEL COMMAND_LIST' clause.
   * The `otherwise COMMAND_LIST' clause is optional.
   * As with all other specific `end' keywords, `endswitch' may be
     replaced by `end', but you can get better diagnostics if you use
     the specific forms.
   * Cases are exclusive, so they don't `fall through' as do the cases
     in the switch statement of the C language.
   * The COMMAND_LIST elements are not optional.  Making the list
     optional would have meant requiring a separator between the label
     and the command list.  Otherwise, things like
          switch (foo)
            case (1) -2
            ...
     would produce surprising results, as would
          switch (foo)
            case (1)
            case (2)
              doit ();
            ...
     particularly for C programmers.
   * The implementation is simple-minded and currently offers no real
     performance improvement over an equivalent `if' block, even if all
     the labels are integer constants.  Perhaps a future variation on
     this could detect all constant integer labels and improve
     performance by using a jump table.
 - Built-in Variable: warn_variable_switch_label
     If the value of this variable is nonzero, Octave will print a
     warning if a switch label is not a constant or constant expression
File: octave,  Node: The while Statement,  Next: The for Statement,  Prev: The switch Statement,  Up: Statements
The `while' Statement
=====================
   In programming, a "loop" means a part of a program that is (or at
least can be) executed two or more times in succession.
   The `while' statement is the simplest looping statement in Octave.
It repeatedly executes a statement as long as a condition is true.  As
with the condition in an `if' statement, the condition in a `while'
statement is considered true if its value is non-zero, and false if its
value is zero.  If the value of the conditional expression in a `while'
statement is a vector or a matrix, it is considered true only if *all*
of the elements are non-zero.
   Octave's `while' statement looks like this:
     while (CONDITION)
       BODY
     endwhile
Here BODY is a statement or list of statements that we call the "body"
of the loop, and CONDITION is an expression that controls how long the
loop keeps running.
   The first thing the `while' statement does is test CONDITION.  If
CONDITION is true, it executes the statement BODY.  After BODY has been
executed, CONDITION is tested again, and if it is still true, BODY is
executed again.  This process repeats until CONDITION is no longer
true.  If CONDITION is initially false, the body of the loop is never
executed.
   This example creates a variable `fib' that contains the first ten
elements of the Fibonacci sequence.
     fib = ones (1, 10);
     i = 3;
     while (i <= 10)
       fib (i) = fib (i-1) + fib (i-2);
       i++;
     endwhile
Here the body of the loop contains two statements.
   The loop works like this: first, the value of `i' is set to 3.
Then, the `while' tests whether `i' is less than or equal to 10.  This
is the case when `i' equals 3, so the value of the `i'-th element of
`fib' is set to the sum of the previous two values in the sequence.
Then the `i++' increments the value of `i' and the loop repeats.  The
loop terminates when `i' reaches 11.
   A newline is not required between the condition and the body; but
using one makes the program clearer unless the body is very simple.
   *Note The if Statement:: for a description of the variable
`warn_assign_as_truth_value'.
File: octave,  Node: The for Statement,  Next: The break Statement,  Prev: The while Statement,  Up: Statements
The `for' Statement
===================
   The `for' statement makes it more convenient to count iterations of a
loop.  The general form of the `for' statement looks like this:
     for VAR = EXPRESSION
       BODY
     endfor
where BODY stands for any statement or list of statements, EXPRESSION
is any valid expression, and VAR may take several forms.  Usually it is
a simple variable name or an indexed variable.  If the value of
EXPRESSION is a structure, VAR may also be a list.  *Note Looping Over
Structure Elements::, below.
   The assignment expression in the `for' statement works a bit
differently than Octave's normal assignment statement.  Instead of
assigning the complete result of the expression, it assigns each column
of the expression to VAR in turn.  If EXPRESSION is a range, a row
vector, or a scalar, the value of VAR will be a scalar each time the
loop body is executed.  If VAR is a column vector or a matrix, VAR will
be a column vector each time the loop body is executed.
   The following example shows another way to create a vector containing
the first ten elements of the Fibonacci sequence, this time using the
`for' statement:
     fib = ones (1, 10);
     for i = 3:10
       fib (i) = fib (i-1) + fib (i-2);
     endfor
This code works by first evaluating the expression `3:10', to produce a
range of values from 3 to 10 inclusive.  Then the variable `i' is
assigned the first element of the range and the body of the loop is
executed once.  When the end of the loop body is reached, the next
value in the range is assigned to the variable `i', and the loop body
is executed again.  This process continues until there are no more
elements to assign.
   Although it is possible to rewrite all `for' loops as `while' loops,
the Octave language has both statements because often a `for' loop is
both less work to type and more natural to think of.  Counting the
number of iterations is very common in loops and it can be easier to
think of this counting as part of looping rather than as something to
do inside the loop.
* Menu:
* Looping Over Structure Elements::
File: octave,  Node: Looping Over Structure Elements,  Prev: The for Statement,  Up: The for Statement
Looping Over Structure Elements
-------------------------------
   A special form of the `for' statement allows you to loop over all
the elements of a structure:
     for [ VAL, KEY ] = EXPRESSION
       BODY
     endfor
In this form of the `for' statement, the value of EXPRESSION must be a
structure.  If it is, KEY and VAL are set to the name of the element
and the corresponding value in turn, until there are no more elements.
For example,
     x.a = 1
     x.b = [1, 2; 3, 4]
     x.c = "string"
     for [val, key] = x
       key
       val
     endfor

          -| key = a
          -| val = 1
          -| key = b
          -| val =
          -|
          -|   1  2
          -|   3  4
          -|
          -| key = c
          -| val = string
   The elements are not accessed in any particular order.  If you need
to cycle through the list in a particular way, you will have to use the
function `struct_elements' and sort the list yourself.
   The KEY variable may also be omitted.  If it is, the brackets are
also optional.  This is useful for cycling through the values of all the
structure elements when the names of the elements do not need to be
known.
File: octave,  Node: The break Statement,  Next: The continue Statement,  Prev: The for Statement,  Up: Statements
The `break' Statement
=====================
   The `break' statement jumps out of the innermost `for' or `while'
loop that encloses it.  The `break' statement may only be used within
the body of a loop.  The following example finds the smallest divisor
of a given integer, and also identifies prime numbers:
     num = 103;
     div = 2;
     while (div*div <= num)
       if (rem (num, div) == 0)
         break;
       endif
       div++;
     endwhile
     if (rem (num, div) == 0)
       printf ("Smallest divisor of %d is %d\n", num, div)
     else
       printf ("%d is prime\n", num);
     endif
   When the remainder is zero in the first `while' statement, Octave
immediately "breaks out" of the loop.  This means that Octave proceeds
immediately to the statement following the loop and continues
processing.  (This is very different from the `exit' statement which
stops the entire Octave program.)
   Here is another program equivalent to the previous one.  It
illustrates how the CONDITION of a `while' statement could just as well
be replaced with a `break' inside an `if':
     num = 103;
     div = 2;
     while (1)
       if (rem (num, div) == 0)
         printf ("Smallest divisor of %d is %d\n", num, div);
         break;
       endif
       div++;
       if (div*div > num)
         printf ("%d is prime\n", num);
         break;
       endif
     endwhile
File: octave,  Node: The continue Statement,  Next: The unwind_protect Statement,  Prev: The break Statement,  Up: Statements
The `continue' Statement
========================
   The `continue' statement, like `break', is used only inside `for' or
`while' loops.  It skips over the rest of the loop body, causing the
next cycle around the loop to begin immediately.  Contrast this with
`break', which jumps out of the loop altogether.  Here is an example:
     # print elements of a vector of random
     # integers that are even.

     # first, create a row vector of 10 random
     # integers with values between 0 and 100:

     vec = round (rand (1, 10) * 100);

     # print what we're interested in:

     for x = vec
       if (rem (x, 2) != 0)
         continue;
       endif
       printf ("%d\n", x);
     endfor
   If one of the elements of VEC is an odd number, this example skips
the print statement for that element, and continues back to the first
statement in the loop.
   This is not a practical example of the `continue' statement, but it
should give you a clear understanding of how it works.  Normally, one
would probably write the loop like this:
     for x = vec
       if (rem (x, 2) == 0)
         printf ("%d\n", x);
       endif
     endfor
File: octave,  Node: The unwind_protect Statement,  Next: The try Statement,  Prev: The continue Statement,  Up: Statements
The `unwind_protect' Statement
==============================
   Octave supports a limited form of exception handling modelled after
the unwind-protect form of Lisp.
   The general form of an `unwind_protect' block looks like this:
     unwind_protect
       BODY
     unwind_protect_cleanup
       CLEANUP
     end_unwind_protect
Where BODY and CLEANUP are both optional and may contain any Octave
expressions or commands.  The statements in CLEANUP are guaranteed to
be executed regardless of how control exits BODY.
   This is useful to protect temporary changes to global variables from
possible errors.  For example, the following code will always restore
the original value of the built-in variable `do_fortran_indexing' even
if an error occurs while performing the indexing operation.
     save_do_fortran_indexing = do_fortran_indexing;
     unwind_protect
       do_fortran_indexing = 1;
       elt = a (idx)
     unwind_protect_cleanup
       do_fortran_indexing = save_do_fortran_indexing;
     end_unwind_protect
   Without `unwind_protect', the value of DO_FORTRAN_INDEXING would not
be restored if an error occurs while performing the indexing operation
because evaluation would stop at the point of the error and the
statement to restore the value would not be executed.
File: octave,  Node: The try Statement,  Next: Continuation Lines,  Prev: The unwind_protect Statement,  Up: Statements
The `try' Statement
===================
   In addition to unwind_protect, Octave supports another limited form
of exception handling.
   The general form of a `try' block looks like this:
     try
       BODY
     catch
       CLEANUP
     end_try_catch
   Where BODY and CLEANUP are both optional and may contain any Octave
expressions or commands.  The statements in CLEANUP are only executed
if an error occurs in BODY.
   No warnings or error messages are printed while BODY is executing.
If an error does occur during the execution of BODY, CLEANUP can access
the text of the message that would have been printed in the builtin
constant `__error_text__'.  This is the same as `eval (TRY, CATCH)'
(which may now also use `__error_text__') but it is more efficient
since the commands do not need to be parsed each time the TRY and CATCH
statements are evaluated.  *Note Error Handling::, for more information
about the `__error_text__' variable.
   Octave's TRY block is a very limited variation on the Lisp
condition-case form (limited because it cannot handle different classes
of errors separately).  Perhaps at some point Octave can have some sort
of classification of errors and try-catch can be improved to be as
powerful as condition-case in Lisp.
File: octave,  Node: Continuation Lines,  Prev: The try Statement,  Up: Statements
Continuation Lines
==================
   In the Octave language, most statements end with a newline character
and you must tell Octave to ignore the newline character in order to
continue a statement from one line to the next.  Lines that end with the
characters `...' or `\' are joined with the following line before they
are divided into tokens by Octave's parser.  For example, the lines
     x = long_variable_name ...
         + longer_variable_name \
         - 42
form a single statement.  The backslash character on the second line
above is interpreted a continuation character, *not* as a division
operator.
   For continuation lines that do not occur inside string constants,
whitespace and comments may appear between the continuation marker and
the newline character.  For example, the statement
     x = long_variable_name ...     # comment one
         + longer_variable_name \   # comment two
         - 42                       # last comment
is equivalent to the one shown above.  Inside string constants, the
continuation marker must appear at the end of the line just before the
newline character.
   Input that occurs inside parentheses can be continued to the next
line without having to use a continuation marker.  For example, it is
possible to write statements like
     if (fine_dining_destination == on_a_boat
         || fine_dining_destination == on_a_train)
       suess (i, will, not, eat, them, sam, i, am, i,
              will, not, eat, green, eggs, and, ham);
     endif
without having to add to the clutter with continuation markers.
   % DO NOT EDIT!  Generated automatically by munge-texi.
File: octave,  Node: Functions and Scripts,  Next: Error Handling,  Prev: Statements,  Up: Top
Functions and Script Files
**************************
   Complicated Octave programs can often be simplified by defining
functions.  Functions can be defined directly on the command line during
interactive Octave sessions, or in external files, and can be called
just like built-in functions.
* Menu:
* Defining Functions::
* Multiple Return Values::
* Variable-length Argument Lists::
* Variable-length Return Lists::
* Returning From a Function::
* Function Files::
* Script Files::
* Dynamically Linked Functions::
* Organization of Functions::
File: octave,  Node: Defining Functions,  Next: Multiple Return Values,  Prev: Functions and Scripts,  Up: Functions and Scripts
Defining Functions
==================
   In its simplest form, the definition of a function named NAME looks
like this:
     function NAME
       BODY
     endfunction
A valid function name is like a valid variable name: a sequence of
letters, digits and underscores, not starting with a digit.  Functions
share the same pool of names as variables.
   The function BODY consists of Octave statements.  It is the most
important part of the definition, because it says what the function
should actually *do*.
   For example, here is a function that, when executed, will ring the
bell on your terminal (assuming that it is possible to do so):
     function wakeup
       printf ("\a");
     endfunction
   The `printf' statement (*note Input and Output::.) simply tells
Octave to print the string `"\a"'.  The special character `\a' stands
for the alert character (ASCII 7).  *Note Strings::.
   Once this function is defined, you can ask Octave to evaluate it by
typing the name of the function.
   Normally, you will want to pass some information to the functions you
define.  The syntax for passing parameters to a function in Octave is
     function NAME (ARG-LIST)
       BODY
     endfunction
where ARG-LIST is a comma-separated list of the function's arguments.
When the function is called, the argument names are used to hold the
argument values given in the call.  The list of arguments may be empty,
in which case this form is equivalent to the one shown above.
   To print a message along with ringing the bell, you might modify the
`beep' to look like this:
     function wakeup (message)
       printf ("\a%s\n", message);
     endfunction
   Calling this function using a statement like this
     wakeup ("Rise and shine!");
will cause Octave to ring your terminal's bell and print the message
`Rise and shine!', followed by a newline character (the `\n' in the
first argument to the `printf' statement).
   In most cases, you will also want to get some information back from
the functions you define.  Here is the syntax for writing a function
that returns a single value:
     function RET-VAR = NAME (ARG-LIST)
       BODY
     endfunction
The symbol RET-VAR is the name of the variable that will hold the value
to be returned by the function.  This variable must be defined before
the end of the function body in order for the function to return a
value.
   Variables used in the body of a function are local to the function.
Variables named in ARG-LIST and RET-VAR are also local to the function.
*Note Global Variables::, for information about how to access global
variables inside a function.
   For example, here is a function that computes the average of the
elements of a vector:
     function retval = avg (v)
       retval = sum (v) / length (v);
     endfunction
   If we had written `avg' like this instead,
     function retval = avg (v)
       if (is_vector (v))
         retval = sum (v) / length (v);
       endif
     endfunction
and then called the function with a matrix instead of a vector as the
argument, Octave would have printed an error message like this:
     error: `retval' undefined near line 1 column 10
     error: evaluating index expression near line 7, column 1
because the body of the `if' statement was never executed, and `retval'
was never defined.  To prevent obscure errors like this, it is a good
idea to always make sure that the return variables will always have
values, and to produce meaningful error messages when problems are
encountered.  For example, `avg' could have been written like this:
     function retval = avg (v)
       retval = 0;
       if (is_vector (v))
         retval = sum (v) / length (v);
       else
         error ("avg: expecting vector argument");
       endif
     endfunction
   There is still one additional problem with this function.  What if
it is called without an argument?  Without additional error checking,
Octave will probably print an error message that won't really help you
track down the source of the error.  To allow you to catch errors like
this, Octave provides each function with an automatic variable called
`nargin'.  Each time a function is called, `nargin' is automatically
initialized to the number of arguments that have actually been passed
to the function.  For example, we might rewrite the `avg' function like
this:
     function retval = avg (v)
       retval = 0;
       if (nargin != 1)
         usage ("avg (vector)");
       endif
       if (is_vector (v))
         retval = sum (v) / length (v);
       else
         error ("avg: expecting vector argument");
       endif
     endfunction
   Although Octave does not automatically report an error if you call a
function with more arguments than expected, doing so probably indicates
that something is wrong.  Octave also does not automatically report an
error if a function is called with too few arguments, but any attempt to
use a variable that has not been given a value will result in an error.
To avoid such problems and to provide useful messages, we check for both
possibilities and issue our own error message.
 - Automatic Variable: nargin
     When a function is called, this local variable is automatically
     initialized to the number of arguments passed to the function.  At
     the top level, `nargin' holds the number of command line arguments
     that were passed to Octave.
 - Built-in Variable: silent_functions
     If the value of `silent_functions' is nonzero, 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 0.
     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'.
 - Built-in Variable: warn_missing_semicolon
     If the value of this variable is nonzero, Octave will warn when
     statements in function definitions don't end in semicolons.  The
     default value is 0.
File: octave,  Node: Multiple Return Values,  Next: Variable-length Argument Lists,  Prev: Defining Functions,  Up: Functions and Scripts
Multiple Return Values
======================
   Unlike many other computer languages, Octave allows you to define
functions that return more than one value.  The syntax for defining
functions that return multiple values is
     function [RET-LIST] = NAME (ARG-LIST)
       BODY
     endfunction
where NAME, ARG-LIST, and BODY have the same meaning as before, and
RET-LIST is a comma-separated list of variable names that will hold the
values returned from the function.  The list of return values must have
at least one element.  If RET-LIST has only one element, this form of
the `function' statement is equivalent to the form described in the
previous section.
   Here is an example of a function that returns two values, the maximum
element of a vector and the index of its first occurrence in the vector.
     function [max, idx] = vmax (v)
       idx = 1;
       max = v (idx);
       for i = 2:length (v)
         if (v (i) > max)
           max = v (i);
           idx = i;
         endif
       endfor
     endfunction
   In this particular case, the two values could have been returned as
elements of a single array, but that is not always possible or
convenient.  The values to be returned may not have compatible
dimensions, and it is often desirable to give the individual return
values distinct names.
   In addition to setting `nargin' each time a function is called,
Octave also automatically initializes `nargout' to the number of values
that are expected to be returned.  This allows you to write functions
that behave differently depending on the number of values that the user
of the function has requested.  The implicit assignment to the built-in
variable `ans' does not figure in the count of output arguments, so the
value of `nargout' may be zero.
   The `svd' and `lu' functions are examples of built-in functions that
behave differently depending on the value of `nargout'.
   It is possible to write functions that only set some return values.
For example, calling the function
     function [x, y, z] = f ()
       x = 1;
       z = 2;
     endfunction
     [a, b, c] = f ()
produces:
     a = 1

     b = [](0x0)

     c = 2
provided that the built-in variable `define_all_return_values' is
nonzero and the value of `default_return_value' is `[]'.  *Note Summary
of Built-in Variables::.
 - Automatic Variable: nargout
     When a function is called, this local variable is automatically
     initialized to the number of arguments expected to be returned.
     For example,
          f ()
     will result in `nargout' being set to 0 inside the function `f' and
          [s, t] = f ()
     will result in `nargout' being set to 2 inside the function `f'.
     At the top level, `nargout' is undefined.
 - Built-in Variable: default_return_value
     The value given to otherwise uninitialized return values if
     `define_all_return_values' is nonzero.  The default value is `[]'.
 - Built-in Variable: define_all_return_values
     If the value of `define_all_return_values' is nonzero, 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 0.
 - Function File:  nargchk (NARGIN_MIN, NARGIN_MAX, N)
     If N is in the range NARGIN_MIN through NARGIN_MAX  inclusive,
     return the empty matrix.  Otherwise, return a message  indicating
     whether N is too large or too small.
     This is useful for checking to see that the number of arguments
     supplied  to a function is within an acceptable range.
File: octave,  Node: Variable-length Argument Lists,  Next: Variable-length Return Lists,  Prev: Multiple Return Values,  Up: Functions and Scripts
Variable-length Argument Lists
==============================
   Octave has a real mechanism for handling functions that take an
unspecified number of arguments, so it is not necessary to place an
upper bound on the number of optional arguments that a function can
accept.
   Here is an example of a function that uses the new syntax to print a
header followed by an unspecified number of values:
     function foo (heading, ...)
       disp (heading);
       va_start ();
       ## Pre-decrement to skip `heading' arg.
       while (--nargin)
         disp (va_arg ());
       endwhile
     endfunction
   The ellipsis that marks the variable argument list may only appear
once and must be the last element in the list of arguments.
   va_arg (): return next argument in a function that takes a variable
number of parameters
   va_start (): reset the pointer to the list of optional arguments to
the beginning
   Sometimes it is useful to be able to pass all unnamed arguments to
another function.  The keyword ALL_VA_ARGS makes this very easy to do.
For example,
     function f (...)
       while (nargin--)
         disp (va_arg ())
       endwhile
     endfunction

     function g (...)
       f ("begin", all_va_args, "end")
     endfunction

     g (1, 2, 3)

          -| begin
          -| 1
          -| 2
          -| 3
          -| end
 - Keyword: all_va_args
     This keyword stands for the entire list of optional argument, so
     it is possible to use it more than once within the same function
     without having to call `va_start'.  It can only be used within
     functions that take a variable number of arguments.  It is an
     error to use it in other contexts.
File: octave,  Node: Variable-length Return Lists,  Next: Returning From a Function,  Prev: Variable-length Argument Lists,  Up: Functions and Scripts
Variable-length Return Lists
============================
   Octave also has a real mechanism for handling functions that return
an unspecified number of values, so it is no longer necessary to place
an upper bound on the number of outputs that a function can produce.
   Here is an example of a function that uses a variable-length return
list to produce N values:
     function [...] = f (n, x)
       for i = 1:n
         vr_val (i * x);
       endfor
     endfunction

     [dos, quatro] = f (2, 2)
          => dos = 2
          => quatro = 4
   As with variable argument lists, the ellipsis that marks the variable
return list may only appear once and must be the last element in the
list of returned values.
   vr_val (X): append X to the list of optional return values for a
function that allows a variable number of return values
File: octave,  Node: Returning From a Function,  Next: Function Files,  Prev: Variable-length Return Lists,  Up: Functions and Scripts
Returning From a Function
=========================
   The body of a user-defined function can contain a `return' statement.
This statement returns control to the rest of the Octave program.  It
looks like this:
     return
   Unlike the `return' statement in C, Octave's `return' statement
cannot be used to return a value from a function.  Instead, you must
assign values to the list of return variables that are part of the
`function' statement.  The `return' statement simply makes it easier to
exit a function from a deeply nested loop or conditional statement.
   Here is an example of a function that checks to see if any elements
of a vector are nonzero.
     function retval = any_nonzero (v)
       retval = 0;
       for i = 1:length (v)
         if (v (i) != 0)
           retval = 1;
           return;
         endif
       endfor
       printf ("no nonzero elements found\n");
     endfunction
   Note that this function could not have been written using the
`break' statement to exit the loop once a nonzero value is found
without adding extra logic to avoid printing the message if the vector
does contain a nonzero element.
 - Keyword: return
     When Octave encounters the keyword `return' inside a function or
     script, it returns control to be caller immediately.  At the top
     level, the return statement is ignored.  A `return' statement is
     assumed at the end of every function definition.
 - Built-in Variable: 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 0.
     For example, the function
          function f ()
            2 + 2;
          endfunction
     will either return nothing, if the value of
     `return_last_computed_value' is 0, or 4, if the value of
     `return_last_computed_value' is nonzero.
File: octave,  Node: Function Files,  Next: Script Files,  Prev: Returning From a Function,  Up: Functions and Scripts
Function Files
==============
   Except for simple one-shot programs, it is not practical to have to
define all the functions you need each time you need them.  Instead, you
will normally want to save them in a file so that you can easily edit
them, and save them for use at a later time.
   Octave does not require you to load function definitions from files
before using them.  You simply need to put the function definitions in a
place where Octave can find them.
   When Octave encounters an identifier that is undefined, it first
looks for variables or functions that are already compiled and currently
listed in its symbol table.  If it fails to find a definition there, it
searches the list of directories specified by the built-in variable
`LOADPATH' for files ending in `.m' that have the same base name as the
undefined identifier.(1)  Once Octave finds a file with a name that
matches, the contents of the file are read.  If it defines a *single*
function, it is compiled and executed.  *Note Script Files::, for more
information about how you can define more than one function in a single
file.
   When Octave defines a function from a function file, it saves the
full name of the file it read and the time stamp on the file.  After
that, it checks the time stamp on the file every time it needs the
function.  If the time stamp indicates that the file has changed since
the last time it was read, Octave reads it again.
   Checking the time stamp allows you to edit the definition of a
function while Octave is running, and automatically use the new function
definition without having to restart your Octave session.  Checking the
time stamp every time a function is used is rather inefficient, but it
has to be done to ensure that the correct function definition is used.
   To avoid degrading performance unnecessarily by checking the time
stamps on functions that are not likely to change, Octave assumes that
function files in the directory tree
`OCTAVE-HOME/share/octave/VERSION/m' will not change, so it doesn't
have to check their time stamps every time the functions defined in
those files are used.  This is normally a very good assumption and
provides a significant improvement in performance for the function
files that are distributed with Octave.
   If you know that your own function files will not change while you
are running Octave, you can improve performance by setting the variable
`ignore_function_time_stamp' to `"all"', so that Octave will ignore the
time stamps for all function files.  Setting it to `"system"' gives the
default behavior.  If you set it to anything else, Octave will check
the time stamps on all function files.
 - Built-in Variable: DEFAULT_LOADPATH
     A colon separated list of directories in which to search for
     function files by default.  The value of this variable is also
     automatically substituted for leading, trailing, or doubled colons
     that appear in the built-in variable `LOADPATH'.
 - Built-in Variable: LOADPATH
     A colon separated list of directories in which to search for
     function files.  *Note Functions and Scripts::.  The value of
     `LOADPATH' overrides the environment variable `OCTAVE_PATH'.
     *Note Installation::.
     `LOADPATH' is now handled in the same way as TeX handles
     `TEXINPUTS'.  Leading, trailing, or doubled colons that appear in
     `LOADPATH' are replaced by the value of `DEFAULT_LOADPATH'.  The
     default value of `LOADPATH' is `":"', which tells Octave to search
     in the directories specified by `DEFAULT_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.
     *Note Organization of Functions:: for a description of the
     function file directories that are distributed with Octave.
 - Built-in Variable: ignore_function_time_stamp
     This 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
     `OCTAVE-HOME/lib/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"'.
 - Built-in Variable: warn_function_name_clash
     If the value of `warn_function_name_clash' is nonzero, 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 names
     disagree, the name declared inside the file is ignored.)  If the
     value is 0, the warning is omitted.  The default value is 1.
   ---------- Footnotes ----------
   (1)  The `.m' suffix was chosen for compatibility with MATLAB.