Info Catalog octave: Statements octave: The switch Statement

octave: The if Statement

 
 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.  
 Functions and Scripts.
 

Info Catalog octave: Statements octave: The switch Statement