(octave.info)The if Statement

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10.1 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
nonzero, 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 it is non-empty and _all_ of the elements are
nonzero.  The conceptually equivalent code when CONDITION is a matrix is
shown below.

     if (MATRIX) ≡ if (all (MATRIX(:)))

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.