*******************
Control Structures
*******************

Types
=======

.. role:: bmagenta
.. role:: bblue
.. role:: green
.. role:: red
.. highlight:: r

Böhm and Jacopini's work [#f1]_ in 1966, showed that the computer programs can be developed using only 
three *control structures*:

1. :bmagenta:`Sequence Structure`: the instructions are executed in the sequential order they have been written, 
unless the contrary is specified. In R, this behaviour is inherent to the interactive execution (through the 
R interpreter) and it is also the way in which instructions are executed in a script.

2. :bmagenta:`Selection Structure`: different instructions can be executed depending on a condition. In R this is implemented
through:

::

  > if(cond) expr
  > if(cond) cons.expr  else  alt.expr
  
3. :bmagenta:`Repetition Structure`: the execution of a group of instructions can be repeated inside a loop. This can be 
accomplished by:

::

  > for (name in expr_1) expr_2
  > while (condition) expr
  > repeat expr
  
Every algorithm can be resolved using the control structures described above. These structures can be nested so 
the use of braces "{...}" and proper indentation make the blocks of instructions clearer:

.. code-block:: ini

  for (x in seq(-3,3)) {
    if (x < 0) {
      print("Caso A:")
      y <- sqrt(-x)
      cat("y=",y,"\n")
    } else {
      print("Caso B:")
      y <- sqrt(x)
      cat("y=",y,"\n")
    }
  }

Control Flow: break, next, return
==================================

These three commands are used to alter the normal execution of the control structures. From R *help*:

**break**: breaks out of a ‘for’, ‘while’ or ‘repeat’ loop (applies only to the innermost loop).

**next**: halts the processing of the current iteration and advances the looping 
index (applies only to the innermost loop.)

**return**: returns a value in a function and exits it.



.. rubric:: Footnotes

.. [#f1] Böhm C., and Jacopini G,. Flow Diagrams, Turing Machines, and Languages with Only Two Formation Rules, Communications of the ACM, Vol 9., No. 5, 1966, 336–371.