======================
 The ternary operator
======================

In languages like C and Java, there is a pattern that
often occurs in programs:

.. code:: java

   // Example 1
   if (age >= 18)
      price = 30;
   else
      price = 25;

   // Example 2
   if (turningRight)
      angle = 90;
   else
      angle = -90;

   // Example 3
   if (value.equals("1"))
      b = true;
   else
      b = false;

The common pattern in these examples is “set a variable to some value
if a condition holds, otherwise set it to another value.” We can
abstract it as follows:

  ``if (`` <condition> ``)``

      <name> ``=`` <value1> ``;``

  ``else``

      <name> ``=`` <value2> ``;``

This pattern is so often used that the designers of the C language
have created a shorthand construct for this, that is also available in
C++ and Java.

The conditional expression
==========================

Syntax:

   <condition> ``?`` <expression1> ``:`` <expression2>

   (a conditional expression, followed by a question mark, followed by
   an expression, followed by a colon “``:``”, followed by another expression)

Semantics:

   The expression on the left is evaluated first. If it evaluates
   to “true”, the expression in the middle is evaluated and its result is returned.
   Otherwise, the expression on the right is evaluated and its result is returned.

For example:

.. code:: java

   price = (age >= 18) ? 30 : 25;

   angle = turningRight ? 90 : -90;

   b = value.equals("1") ? true : false;

This construct abstracts over the general pattern of conditional assignment to
a variable. It is even slightly more powerful, because we can nest it!

For example:

.. code:: java

   price = (age <= 6) ? 5 : ((age <= 18) ? 25 : 30);

   // is equivalent to:
   if (age <= 6)
      price = 5;
   else
      if (age <= 18)
         price = 25;
      else
         price = 30;

This construct is called the *conditional expression*, not to be
confused with the conditional statement that uses “``if``”.

We name:

- “conditional expression” an instance of the construct, when
  the three expressions are filled in;

- *“conditional operator”* the punctuation characters “``?:``”.

In other words the conditional expression combines three expressions
with the conditional operator.

Ternary operator
================

The conditional operator is also called *ternary operator* because it
has three positions. It is called “the” ternary operator in C and Java
because it is the only operator with 3 positions in the language.

Important concepts
==================

- definition of the *conditional expression*;
- difference between conditional expression (using “?:”)  and conditional statement (using “``if``”);
- difference between conditional expression and conditional operator
- the alternative name *“ternary operator”* for the conditional operator

Further reading
===============

- Introduction to Programming, section 2.5.5 (p. 51)
- Absolute Java, section 3.1 (pp. 112-113)

----

Copyright and licensing
=======================

Copyright © 2014, Raphael ‘kena’ Poss.
Permission is granted to distribute, reuse and modify this document
according to the terms of the Creative Commons Attribution-ShareAlike
4.0 International License.  To view a copy of this license, visit
`http://creativecommons.org/licenses/by-sa/4.0/
<http://creativecommons.org/licenses/by-sa/4.0/>`_.