Control statements such as
while, and so on
control the flow of execution in Octave programs. All the control
statements start with special keywords such as
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
endwhile marks the end of a
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
while and the corresponding end statement is called the
body of a control statement.
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
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
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
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
In the following example, if the first condition is true (that is, the
x is divisible by 2), then the first
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,
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
as is allowed in Fortran. If it is, the space between the
if will tell Octave to treat this as a new
statement within another
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
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
or, if you have not used the more specific
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.
In programming, a loop means a part of a program that is (or at least can be) executed two or more times in succession.
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
if statement is a vector or a matrix, it is considered true
only if all of the elements are non-zero.
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.
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
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 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
The following example shows another way to create a vector containing
the first ten elements of the Fibonacci sequence, this time using the
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.
for statement, body stands for any statement or list
Although it is possible to rewrite all
for loops as
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 jumps out of the innermost
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
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
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, like
break, is used only inside
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
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
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
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
\ are joined with the following line
before they are divided into tokens by Octave's parser. For example,
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.