.. _selection:

Selection Statements
=====================

.. index:: selection

.. _else:
.. _if:

if -- else
-----------

.. index:: if, else

We discussed this in Topic 1. Recall the basic form of 
:keyword:`if` statements::

        if( expr) statement;
        
        if( expr ) {
           statement1;
           statement2;
        }
        
        if( expr ) {
           code block;
        } else {
           code block;
        }


Also, recall the dangers of the "dangling" or ambiguous else.

    ::

        if( expr )
           if( expr )
              statement;
           else
              statement;

.. _goto:

The goto statement
-------------------

.. index:: goto

The :keyword:`goto` statement is **strongly discouraged**.

    ::

        label: begin

        ...

        if( attempts == 0 ) goto begin;


The only possible argument for a :keyword:`goto` is if the code is nested
several layers deep into loops and an immediate switch to elsewhere
is needed. Even here, other methods achieve the same result.

.. _break:
.. _continue:

The break and continue statements
----------------------------------

.. index:: break, continue

:keyword:`break` and :keyword:`continue` provide additional controls for
loops. [1]_

:keyword:`break` tells the program counter to exit the loop and continue
executing the code following the loop.

:keyword:`continue` is an instruction to stop with the current execution of
the loop body and skip to the end of the body of the loop and
continue as if the bottom of the loop had been reached via normal
program execution.

.. _case:
.. _switch:
.. _default:

The switch statement
------------------------

.. index:: switch, case, default

The :keyword:`switch` is a multi-way conditional statement generalizing the
:keyword:`if` -- :keyword:`else` statement.

    ::

        switch( val ) {
        case 1:
           counter++;
           break;
        case 2:
        case 3:
           counter--;
           break;
        default:
           counter = 0;
        }



- 
   The *controlling expression* of a switch statement, must be an
   integer.

- Notice that the end of each :keyword:`case` block is a :keyword:`break`
  statement.

- 
   If :keyword:`break` is missing from a :keyword:`case` block, execution
   *falls through* to the next block. In the example above, if
   `val == 2`, then the same code for when `val == 3` is executed.


.. _conditional:

Conditional Operator
----------------------

.. index:: conditional operator

- 
   The operator ``? :``  is a *ternary* operator. It takes three
   variables or expressions as its operands.

-  Syntax: ``expr1 ? expr2: expr3``

- 
   First, ``expr1`` is evaluated, if it is true (non-zero), then the
   whole expression reduces to ``expr2``, which is then evaluated.

- 
   If ``expr1`` is false (zero), then the whole expression reduces to
   ``expr3``, which is then evaluated.


The following two expressions are identical.

    ::

        if (y < z)
           x = y;
        else
           x = z;
        
        
        x = (y < z) ? y : z;


Here is another example.

    ::

        int a = 1, b = 2;
        double x = 7.07, y;
        
        printf( "%d\n", ((a == b) ? a -1 : b + 1));     ---  3
        printf( "%lf\n", ((a-b) < 0 ? x : a + b));      ---  7.07
        printf( "%lf\n", ((a-b) > 0 ? x : a + b));      ---  3.0


In the last example, because of the mixed data type in the
conditional operation, all data types are converted to double
type.

.. rubric:: Footnotes

.. [1]  :keyword:`break` is also used with the :keyword:`switch` statement.