While counting
==============

Select world **Around 1**. You have seen before that Reeborg takes 9 steps
before reaching the first square where he is blocked by having a wall in
front of him. Let's use the increment operator to have Reeborg keep
track of the number of steps and stop just before hitting a wall::

    var number_of_steps = 0;
    while (number_of_steps < 9 ){  // "<" means "less than"
        move();
        number_of_steps++;
    }

.. topic:: Try it!


    Try to run the above. Then, modify the program to write the value of
    ``number_of_steps`` in Reeborg's Diary each time it is incremented.
    

Defining ``repeat()``?
----------------------

In the above program, Reeborg was counting and doing a number of steps
(``move``) up to a predetermined value. We have seen before how to
accomplish this using a single instruction::

    repeat(move, 9);

Let's define a function ``my_repeat`` that will *hide* the code with the
while loop above::

    function my_repeat() {
        var number_of_steps = 0;
        while (number_of_steps < 9 ){
            move();
            number_of_steps++;
        }
    }

    my_repeat();   // use it!

That's not very useful compared with ``repeat()`` since both the number
of steps to take and the specific instruction to repeat are
*hard-coded*. We can do better by *passing* **arguments** to
``my_repeat`` as follows::

    function my_repeat(some_function, max_value) {
        var number_of_steps = 0;
        while (number_of_steps < max_value ){
            some_function();
            number_of_steps++;
        }
    }

    my_repeat(move, 9);   // use it!
    my_repeat(turn_left, 4);

.. topic:: Try it!

   Try to run the above program.

Scope
-----

Make note of the colour of the variable ``number_of_steps`` in the
Javascript editor.  Once you have done that, enter the following code
instead::

    var number_of_steps = 0;
    function my_repeat(some_function, max_value) {
        while (number_of_steps < max_value ){
            some_function();
            number_of_steps++;
        }
    }

You should note that the colour of the
variable ``number_of_steps`` is different in both cases. If you look
more closely, you will find that in one case, this variable is defined
*inside* the function body whereas in the other it is defined *outside*.
When it is defined *inside*, we say that it is a **local** variable: its
value is only known locally inside the function and not in the larger
program. Otherwise, it is known as a **global** variable. Whether a
variable is *local* to a function or *global* is known as the **scope**
of the variable. Generally, it is recommended to use local variables
whenever possible so that if you reuse the same variable name (locally)
elsewhere in a program, it will be treated as a completely different
variable and won't affect the value of a similarly named variable
elsewhere. If you use global variables, a change in one part of the
program will change the value of that variable everywhere else - often
leading to some hard to trace bugs.