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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.

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 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.

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.

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 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.

See section The if Statement for a description of the variable warn_assign_as_truth_value.

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. See section 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 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.

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.

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,

octave:1> x.a = 1; x.b = [1, 2; 3, 4]; x.c = "string";
octave:2> for [val, key] = x; key, val, endfor

will print

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.

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

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

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 = "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.

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.

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.

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.

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.


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