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.
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
satement.
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. See section Functions and Script Files.
while
StatementIn 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 an if
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 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.
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
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 either
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.
In the for
statement, body stands for any statement or list
of statements.
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.
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
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
unwind_protect
StatementOctave 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 = "true"; 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.
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.
In some cases, Octave will allow you to continue lines without having to specify continuation characters. For example, it is possible to write statements like
if (big_long_variable_name == other_long_variable_name || not_so_short_variable_name > 4 && y > x) some (code, here); endif
without having to clutter up the if statement with continuation characters.