For a ``while``
===============

When we want to repeat some instructions until a certain condition is
satisfied, Javascript gives us a simpler way to write this using a new
keyword: ``while``. For example, suppose we want to have Reeborg keep
moving until it reaches a wall. Previously, we might have done something
like the following::

    function move_until_wall () {
        if ( front_is_clear() ){
            move();
        }
    }

    repeat(move_until_wall, 42);

and hoped that 42 would have been a number of repetitions sufficient to
reach a wall. Using ``while``, we can write the following::

    while ( front_is_clear() ){
        move();
    }

That's it! No more guessing and asking something to be performed a large
number of time just to ensure that it will be enough.

How to think about ``while``
----------------------------

Suppose we have the following::

    while ( condition() ){
        do_1();
        do_2();
        do_3();
    }

You can think of this as being equivalent to::

    if ( condition() ){
        do_1();
        do_2();
        do_3();
    }
    if ( condition() ){
        do_1();
        do_2();
        do_3();
    }
    if ( condition() ){
        do_1();
        do_2();
        do_3();
    }
    if ( condition() ){
        do_1();
        do_2();
        do_3();
    }
    ....

which is to say that the block of code is repeated as long as the
condition remains ``true``. So, what happens if the condition is always
``true``? The block of code is repeated for ever and the program never
ends.

This is bad.

Instead of using this description of repeated blocks of code,
programmers describe this as a **loop**: that is, you start with the
first instruction (``do_1();``) inside the code block, continue with all
the others until you reach the last instruction (``do_3();``), then
***loop* back**, or go back, to the test just before the beginning of
the block and see if the condition is satisfied; if not, you repeat once
again the cycle. If the condition never becomes ``false``, you keep
repeating and end up with an **infinite loop**.

Conclusion: you want to make sure that the condition will become
``false`` at some point.

Back to hurdles!
----------------

.. topic:: Your turn!

    Go back to **Hurdles 1**, **Hurdles 2** and **Hurdles 3**, and write a single
    program for all three world, using ``while`` and without using ``repeat()``.

.. hint::

  Your program might look like the following::

    function jump_over_hurdle() {
        // suitable definition
    }

    function run_or_jump () {
        // suitable definition
    }

    while ( !at_goal() ){
        run_or_jump();
    }

That's it! No more arbitrary repetitions! From now on, you should only
use ``repeat()`` when you know **exactly** how many times a given
function must be repeated.