38. 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++;
}

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.

38.1. 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);

Try it!

Try to run the above program.

38.2. 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.