37. Increment

Make sure you have the Editor, the World and the Diary panels open. Select world Around 1.

Suppose we wanted to count the number of steps taken by Reeborg to reach the wall on the right from its starting position. One way to do this is to use a variable which I will name number_of_steps and give it an initial value of 0. Then, each time that Reeborg takes a step, I will add 1 to the previous value of number_of_steps. This is done in Javascript using the following syntax:

number_of_steps = number_of_steps + 1;

Do not think of this as a mathematical equation! Clearly, if this were a mathematical equation, the variable on the left hand side of the “equal sign” would not have the same value as the terms on the right hand side. However, in Javascript, and many other programming languages, “=” is known as the assignment operator: Javascript determines the value on the right hand side of “=” and use the variable name on the left hand side as a way to remember that value. Thus, if we have:

var n;
n = 1;
n = n + 3;
write(n);  // outputs 4

it is equivalent to:

var n;
n = 1;
n = 1 + 3;  // notice how we use "1" instead of "n"
write(n);  // outputs 4

Try it!

Try to run the above code and see what is printed in Reeborg’s Diary.

Because this type of operation, known as incrementing a variable occurs so often, Javascript, and many other languages, allow the use of the following shortened notation, using the += operator:

var n;
n = 1;
n += 3;
write(n);  // outputs 4

If the value by which we want to increment the variable is 1, we can even use a shorter expression:

var n;
n = 1;
n++;
write(n);  // outputs 2

Try it!

Try running programs like the ones above to make sure you understand these ways to increment the numerical value of a variable.

37.1. Counting steps

We are now ready to write a program to have Reeborg count the number of steps using world Around 1.

Try it!

Copy the code below in the editor to run it. Ideally, you should then modify it to try the other ways we have seen and that can be used to increment a variable:

var number_of_steps = 0;

function move_and_count_steps() {
    move();
    number_of_steps++;
}

while (front_is_clear()){
    move_and_count_steps();
}

write(number_of_steps);  // should be 9

Your turn!

Write a program so that Reeborg goes all the way around world Around 1 and have him count the number of steps and the number of left turns, writing them in his diary at the end. You should start by putting a token down to mark what will be the end position.