.. _booleans:

Boolean Expressions
======================

Relational and Logical Operators
---------------------------------

    Relational Operators
    ``< > <= >=``

    Equality Operators
    ``== !=``

    Logical Operators
    ``! && ||``


The main rule for *order of precedence* to note is that AND (&&) has higher
precedence than OR (``||``). Other than this, they evaluate left to right as
expected.

::

   a && b || c && d                --->   (a && b) || (c && d)
   (i<0)&&(j<0)||(i>=20)&&(j>=20)  ---> ((i<0)&&(j<0)) || ((i>=20)&&(j>=20))


These logical expressions all evaluate to binary values of 0 or 1.

The unary operator NOT (!) reverses the result of the expression.

Short Circuit Evaluation
---------------------------------

If the final result of an expression is determined after a portion
of the expression is evaluated, the rest of the expression is
skipped. Knowing this behavior can allow slightly more compact
code.

    ::

        if( (x != 0.0) && ((y = z/x) < 1.0) )
          /* z/x never evaluated if( x == 0 ) */
        
        if( (x < 0) || ((y = sqrt(x)) <= z ))  
          /* sqrt(x) never evaluated if x < 0 */
        
        if( (result == error) && exit(1) );
          /* the same as `if( result == error ) exit(1); */


Note in the last example, we used an *empty statement*, the ';' by
it self as an expression. The empty statement can also be useful in
writing control constructs.