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@node Statements
@chapter Statements
@cindex statements

Statements may be a simple constant expression or a complicated list of
nested loops and conditional statements.

@dfn{Control statements} such as @code{if}, @code{while}, and so on
control the flow of execution in Octave programs.  All the control
statements start with special keywords such as @code{if} and
@code{while}, to distinguish them from simple expressions.
Many control statements contain other statements; for example, the
@code{if} statement contains another statement which may or may not be
executed.

@cindex @code{end} statement
Each control statement has a corresponding @dfn{end} statement that
marks the end of the control statement.  For example, the
keyword @code{endif} marks the end of an @code{if} statement, and
@code{endwhile} marks the end of a @code{while} statement.  You can use
the keyword @code{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 @code{if} or
@code{while} and the corresponding end statement is called the
@dfn{body} of a control statement.

@menu
* The @code{if} Statement::            
* The @code{switch} Statement::        
* The @code{while} Statement::         
* The @code{do-until} Statement::      
* The @code{for} Statement::           
* The @code{break} Statement::         
* The @code{continue} Statement::      
* The @code{unwind_protect} Statement::  
* The @code{try} Statement::           
* Continuation Lines::          
@end menu

@node The @code{if} Statement
@section The @code{if} Statement
@cindex @code{if} statement
@cindex @code{else} statement
@cindex @code{elseif} statement
@cindex @code{endif} statement

The @code{if} statement is Octave's decision-making statement.  There
are three basic forms of an @code{if} statement.  In its simplest form,
it looks like this:

@example
@group
if (@var{condition})
  @var{then-body}
endif
@end group
@end example

@noindent
@var{condition} is an expression that controls what the rest of the
statement will do.  The @var{then-body} is executed only if
@var{condition} is true.

The condition in an @code{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 @code{if} statement is a vector or a
matrix, it is considered true only if it is non-empty and @emph{all}
of the elements are non-zero.

The second form of an if statement looks like this:

@example
@group
if (@var{condition})
  @var{then-body}
else
  @var{else-body}
endif
@end group
@end example

@noindent
If @var{condition} is true, @var{then-body} is executed; otherwise,
@var{else-body} is executed.

Here is an example:

@example
@group
if (rem (x, 2) == 0)
  printf ("x is even\n");
else
  printf ("x is odd\n");
endif
@end group
@end example

In this example, if the expression @code{rem (x, 2) == 0} is true (that
is, the value of @code{x} is divisible by 2), then the first
@code{printf} statement is evaluated, otherwise the second @code{printf}
statement is evaluated.

The third and most general form of the @code{if} statement allows
multiple decisions to be combined in a single statement.  It looks like
this:

@example
@group
if (@var{condition})
  @var{then-body}
elseif (@var{condition})
  @var{elseif-body}
else
  @var{else-body}
endif
@end group
@end example

@noindent
Any number of @code{elseif} clauses may appear.  Each condition is
tested in turn, and if one is found to be true, its corresponding
@var{body} is executed.  If none of the conditions are true and the
@code{else} clause is present, its body is executed.  Only one
@code{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 @code{x} is divisible by 2), then the first @code{printf}
statement is executed.  If it is false, then the second condition is
tested, and if it is true (that is, the value of @code{x} is divisible
by 3), then the second @code{printf} statement is executed.  Otherwise,
the third @code{printf} statement is performed.

@example
@group
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
@end group
@end example

Note that the @code{elseif} keyword must not be spelled @code{else if},
as is allowed in Fortran.  If it is, the space between the @code{else}
and @code{if} will tell Octave to treat this as a new @code{if}
statement within another @code{if} statement's @code{else} clause.  For
example, if you write

@example
@group
if (@var{c1})
  @var{body-1}
else if (@var{c2})
  @var{body-2}
endif
@end group
@end example

@noindent
Octave will expect additional input to complete the first @code{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 @code{end} statements,
or, if you have not used the more specific @code{end} statements
(@code{endif}, @code{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,

@example
@group
if (@var{c1})
  @var{body-1}
else
  if (@var{c2})
    @var{body-2}
  endif
@end group
@end example

@noindent
using the indentation to show how Octave groups the statements.
@xref{Functions and Scripts}.

@node The @code{switch} Statement
@section The @code{switch} Statement
@cindex @code{switch} statement
@cindex @code{case} statement
@cindex @code{otherwise} statement
@cindex @code{endswitch} statement

It is very common to take different actions depending on the value of
one variable.  This is possible using the @code{if} statement in the
following way

@example
@group
if (X == 1)
  do_something ();
elseif (X == 2)
  do_something_else ();
else
  do_something_completely_different ();
endif
@end group
@end example

@noindent
This kind of code can however be very cumbersome to both write and
maintain.  To overcome this problem Octave supports the @code{switch}
statement.  Using this statement, the above example becomes

@example
@group
switch (X)
  case 1
    do_something ();
  case 2
    do_something_else ();
  otherwise
    do_something_completely_different ();
endswitch
@end group
@end example

@noindent
This code makes the repetitive structure of the problem more explicit,
making the code easier to read, and hence maintain.  Also, if the
variable @code{X} should change its name, only one line would need
changing compared to one line per case when @code{if} statements are
used.

The general form of the @code{switch} statement is

@example
@group
switch @var{expression}
  case @var{label}
    @var{command_list}
  case @var{label}
    @var{command_list}
  @dots{}

  otherwise
    @var{command_list}
endswitch
@end group
@end example

@noindent
where @var{label} can be any expression.  However, duplicate
@var{label} values are not detected, and only the @var{command_list}
corresponding to the first match will be executed.  For the
@code{switch} statement to be meaningful at least one
@code{case @var{label} @var{command_list}} clause must be present,
while the @code{otherwise @var{command_list}} clause is optional.

If @var{label} is a cell array the corresponding @var{command_list}
is executed if @emph{any} of the elements of the cell array match
@var{expression}.  As an example, the following program will print
@samp{Variable is either 6 or 7}.

@example
@group
A = 7;
switch A
  case @{ 6, 7 @}
    printf ("variable is either 6 or 7\n");
  otherwise
    printf ("variable is neither 6 nor 7\n");
endswitch
@end group
@end example

As with all other specific @code{end} keywords, @code{endswitch} may be
replaced by @code{end}, but you can get better diagnostics if you use
the specific forms.

@c Strings can be matched

One advantage of using the @code{switch} statement compared to using
@code{if} statements is that the @var{label}s can be strings.  If an
@code{if} statement is used it is @emph{not} possible to write

@example
if (X == "a string") # This is NOT valid
@end example

@noindent
since a character-to-character comparison between @code{X} and the
string will be made instead of evaluating if the strings are equal.
This special-case is handled by the @code{switch} statement, and it
is possible to write programs that look like this

@example
@group
switch (X)
  case "a string"
    do_something
  @dots{}
endswitch
@end group
@end example

@menu
* Notes for the C Programmer::  
@end menu

@node Notes for the C Programmer
@subsection Notes for the C Programmer

The @code{switch} statement is also available in the widely used C
programming language.  There are, however, some differences
between the statement in Octave and C

@itemize @bullet
@item
Cases are exclusive, so they don't `fall through' as do the cases
in the @code{switch} statement of the C language.

@item
The @var{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

@example
@group
switch (foo)
  case (1) -2
  @dots{}
@end group
@end example

@noindent
would produce surprising results, as would

@example
@group
switch (foo)
  case (1)
  case (2)
    doit ();
  @dots{}
@end group
@end example

@noindent
particularly for C programmers.  If @code{doit()} should be executed if
@var{foo} is either @code{1} or @code{2}, the above code should be
written with a cell array like this

@example
@group
switch (foo)
  case @{ 1, 2 @}
    doit ();
  @dots{}
@end group
@end example

@end itemize

@node The @code{while} Statement
@section The @code{while} Statement
@cindex @code{while} statement
@cindex @code{endwhile} statement
@cindex loop
@cindex body of a loop

In programming, a @dfn{loop} means a part of a program that is (or at least can
be) executed two or more times in succession.

The @code{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 @code{if} statement, the condition in a
@code{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 @code{while} statement is a vector or a matrix, it is considered
true only if it is non-empty and @emph{all} of the elements are non-zero.

Octave's @code{while} statement looks like this:

@example
@group
while (@var{condition})
  @var{body}
endwhile
@end group
@end example

@noindent
Here @var{body} is a statement or list of statements that we call the
@dfn{body} of the loop, and @var{condition} is an expression that
controls how long the loop keeps running.

The first thing the @code{while} statement does is test @var{condition}.
If @var{condition} is true, it executes the statement @var{body}.  After
@var{body} has been executed, @var{condition} is tested again, and if it
is still true, @var{body} is executed again.  This process repeats until
@var{condition} is no longer true.  If @var{condition} is initially
false, the body of the loop is never executed.

This example creates a variable @code{fib} that contains the first ten
elements of the Fibonacci sequence.

@example
@group
fib = ones (1, 10);
i = 3;
while (i <= 10)
  fib (i) = fib (i-1) + fib (i-2);
  i++;
endwhile
@end group
@end example

@noindent
Here the body of the loop contains two statements.

The loop works like this: first, the value of @code{i} is set to 3.
Then, the @code{while} tests whether @code{i} is less than or equal to
10.  This is the case when @code{i} equals 3, so the value of the
@code{i}-th element of @code{fib} is set to the sum of the previous two
values in the sequence.  Then the @code{i++} increments the value of
@code{i} and the loop repeats.  The loop terminates when @code{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.

@node The @code{do-until} Statement
@section The @code{do-until} Statement
@cindex @code{do-until} statement

The @code{do-until} statement is similar to the @code{while} statement,
except that it repeatedly executes a statement until a condition becomes
true, and the test of the condition is at the end of the loop, so the
body of the loop is always executed at least once.  As with the
condition in an @code{if} statement, the condition in a @code{do-until}
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
@code{do-until} statement is a vector or a matrix, it is considered 
true only if it is non-empty and @emph{all} of the elements are non-zero.

Octave's @code{do-until} statement looks like this:

@example
@group
do
  @var{body}
until (@var{condition})
@end group
@end example

@noindent
Here @var{body} is a statement or list of statements that we call the
@dfn{body} of the loop, and @var{condition} is an expression that
controls how long the loop keeps running.

This example creates a variable @code{fib} that contains the first ten
elements of the Fibonacci sequence.

@example
@group
fib = ones (1, 10);
i = 2;
do
  i++;
  fib (i) = fib (i-1) + fib (i-2);
until (i == 10)
@end group
@end example

A newline is not required between the @code{do} keyword and the
body; but using one makes the program clearer unless the body is very
simple.

@node The @code{for} Statement
@section The @code{for} Statement
@cindex @code{for} statement
@cindex @code{endfor} statement

The @code{for} statement makes it more convenient to count iterations of a
loop.  The general form of the @code{for} statement looks like this:

@example
@group
for @var{var} = @var{expression}
  @var{body}
endfor
@end group
@end example

@noindent
where @var{body} stands for any statement or list of statements,
@var{expression} is any valid expression, and @var{var} may take several
forms.  Usually it is a simple variable name or an indexed variable.  If
the value of @var{expression} is a structure, @var{var} may also be a
vector with two elements.  @xref{Looping Over Structure Elements}, below.

The assignment expression in the @code{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{var} in turn.  If @var{expression} is a range,
a row vector, or a scalar, the value of @var{var} will be a scalar each
time the loop body is executed.  If @var{var} is a column vector or a
matrix, @var{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
@code{for} statement:

@example
@group
fib = ones (1, 10);
for i = 3:10
  fib (i) = fib (i-1) + fib (i-2);
endfor
@end group
@end example

@noindent
This code works by first evaluating the expression @code{3:10}, to
produce a range of values from 3 to 10 inclusive.  Then the variable
@code{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 @code{i}, and the
loop body is executed again.  This process continues until there are no
more elements to assign.

Within Octave is it also possible to iterate over matrices or cell arrays
using the @code{for} statement.  For example consider

@example
@group
disp("Loop over a matrix")
for i = [1,3;2,4]
  i
endfor
disp("Loop over a cell array")
for i = @{1,"two";"three",4@}
  i
endfor
@end group 
@end example

@noindent
In this case the variable @code{i} takes on the value of the columns of
the matrix or cell matrix.  So the first loop iterates twice, producing
two column vectors @code{[1;2]}, followed by @code{[3;4]}, and likewise
for the loop over the cell array.  This can be extended to loops over
multi-dimensional arrays.  For example:

@example
@group
a = [1,3;2,4]; b = cat(3, a, 2*a);
for i = c
  i
endfor
@end group 
@end example

@noindent
In the above case, the multi-dimensional matrix @var{c} is reshaped to a
two-dimensional matrix as @code{reshape (c, rows(c),
prod(size(c)(2:end)))} and then the same behavior as a loop over a two
dimensional matrix is produced.

Although it is possible to rewrite all @code{for} loops as @code{while}
loops, the Octave language has both statements because often a
@code{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::  
@end menu

@node Looping Over Structure Elements
@subsection Looping Over Structure Elements
@cindex structure elements, looping over
@cindex looping over structure elements

A special form of the @code{for} statement allows you to loop over all
the elements of a structure:

@example
@group
for [ @var{val}, @var{key} ] = @var{expression}
  @var{body}
endfor
@end group
@end example

@noindent
In this form of the @code{for} statement, the value of @var{expression}
must be a structure.  If it is, @var{key} and @var{val} are set to the
name of the element and the corresponding value in turn, until there are
no more elements.  For example:

@example
@group
x.a = 1
x.b = [1, 2; 3, 4]
x.c = "string"
for [val, key] = x
  key
  val
endfor

     @print{} key = a
     @print{} val = 1
     @print{} key = b
     @print{} val =
     @print{} 
     @print{}   1  2
     @print{}   3  4
     @print{} 
     @print{} key = c
     @print{} val = string
@end group
@end example

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 @code{fieldnames} and sort the list yourself.

The @var{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.

@node The @code{break} Statement
@section The @code{break} Statement
@cindex @code{break} statement

The @code{break} statement jumps out of the innermost @code{for} or
@code{while} loop that encloses it.  The @code{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:

@example
@group
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
@end group
@end example

When the remainder is zero in the first @code{while} statement, Octave
immediately @dfn{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 @code{exit} statement
which stops the entire Octave program.)

Here is another program equivalent to the previous one.  It illustrates
how the @var{condition} of a @code{while} statement could just as well
be replaced with a @code{break} inside an @code{if}:

@example
@group
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
@end group
@end example

@node The @code{continue} Statement
@section The @code{continue} Statement
@cindex @code{continue} statement

The @code{continue} statement, like @code{break}, is used only inside
@code{for} or @code{while} loops.  It skips over the rest of the loop
body, causing the next cycle around the loop to begin immediately.
Contrast this with @code{break}, which jumps out of the loop altogether.
Here is an example:

@example
@group
# 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
@end group
@end example

If one of the elements of @var{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 @code{continue} statement, but it
should give you a clear understanding of how it works.  Normally, one
would probably write the loop like this:

@example
@group
for x = vec
  if (rem (x, 2) == 0)
    printf ("%d\n", x);
  endif
endfor
@end group
@end example

@node The @code{unwind_protect} Statement
@section The @code{unwind_protect} Statement
@cindex @code{unwind_protect} statement
@cindex @code{unwind_protect_cleanup}
@cindex @code{end_unwind_protect}

Octave supports a limited form of exception handling modelled after the
unwind-protect form of Lisp.  

The general form of an @code{unwind_protect} block looks like this:

@example
@group
unwind_protect
  @var{body}
unwind_protect_cleanup
  @var{cleanup}
end_unwind_protect
@end group
@end example

@noindent
where @var{body} and @var{cleanup} are both optional and may contain any
Octave expressions or commands.  The statements in @var{cleanup} are 
guaranteed to be executed regardless of how control exits @var{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 global variable @code{frobnosticate}
even if an error occurs in the first part of the @code{unwind_protect}
block.

@example
@group
save_frobnosticate = frobnosticate;
unwind_protect
  frobnosticate = true;
  @dots{}
unwind_protect_cleanup
  frobnosticate = save_frobnosticate;
end_unwind_protect
@end group
@end example

@noindent
Without @code{unwind_protect}, the value of @var{frobnosticate}
would not be restored if an error occurs while evaluating the first part
of the @code{unwind_protect} block because evaluation would stop at the
point of the error and the statement to restore the value would not be
executed.

@node The @code{try} Statement
@section The @code{try} Statement
@cindex @code{try} statement
@cindex @code{catch}
@cindex @code{end_try_catch}

In addition to unwind_protect, Octave supports another limited form of
exception handling.

The general form of a @code{try} block looks like this:

@example
@group
try
  @var{body}
catch
  @var{cleanup}
end_try_catch
@end group
@end example

@noindent
where @var{body} and @var{cleanup} are both optional and may contain any
Octave expressions or commands.  The statements in @var{cleanup} are
only executed if an error occurs in @var{body}.

No warnings or error messages are printed while @var{body} is
executing.  If an error does occur during the execution of @var{body},
@var{cleanup} can use the function @code{lasterr} to access the text
of the message that would have been printed.  This is the same
as @code{eval (@var{try}, @var{catch})} but it is more efficient since
the commands do not need to be parsed each time the @var{try} and
@var{catch} statements are evaluated.  @xref{Errors and Warnings}, for more
information about the @code{lasterr} function.

@cindex continuation lines
@cindex @code{...} continuation marker
@cindex @code{\} continuation marker

@node Continuation Lines
@section 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 @code{...} or @code{\} are joined with the following line
before they are divided into tokens by Octave's parser.  For example,
the lines

@example
@group
x = long_variable_name ...
    + longer_variable_name \
    - 42
@end group
@end example

@noindent
form a single statement.  The backslash character on the second line
above is interpreted as a continuation character, @emph{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

@example
@group
x = long_variable_name ...     # comment one
    + longer_variable_name \   # comment two
    - 42                       # last comment
@end group
@end example

@noindent
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

@example
@group
if (fine_dining_destination == on_a_boat
    || fine_dining_destination == on_a_train)
  seuss (i, will, not, eat, them, sam, i, am, i,
         will, not, eat, green, eggs, and, ham);
endif
@end group
@end example

@noindent
without having to add to the clutter with continuation markers.
