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Multiplication and division, as we just saw, are means by which the length of the vector can be changed without affecting direction. Perhaps you’re wondering: “OK, so how do I know what the length of a vector is? I know the components (x and y), but how long (in pixels) is the actual arrow?” Understanding how to calculate the length (also known as magnitude) of a vector is incredibly useful and important.

Figure 1.10: The length or “magnitude” of a vector \vec{v}  is often written as: ||\vec{v}||

Notice in the above diagram how the vector, drawn as an arrow and two components (x and y), creates a right triangle. The sides are the components and the hypotenuse is the arrow itself. We’re very lucky to have this right triangle, because once upon a time, a Greek mathematician named Pythagoras developed a lovely formula to describe the relationship between the sides and hypotenuse of a right triangle.

The Pythagorean theorem is a squared plus b squared equals c squared. 

Armed with this formula, we can now compute the magnitude of \vec{v} as follows:

||\vec{v}|| = \sqrt{v_x * v_x + v_y * v_y}

The code for implementing in the PVector object would thus be:

PVector.prototype.mag = function() {
    return sqrt(this.x*this.x + this.y*this.y);
};

The following example visualizes the magnitude of a vector with a bar at the top:

Calculating the magnitude of a vector is only the beginning. The magnitude function opens the door to many possibilities, the first of which is normalization. Normalizing refers to the process of making something “standard” or, well, “normal.” In the case of vectors, let’s assume for the moment that a standard vector has a length of 1. To normalize a vector, therefore, is to take a vector of any length and, keeping it pointing in the same direction, change its length to 1, turning it into what is called a unit vector.

Since it describes a vector’s direction without regard to its length, it’s useful to have the unit vector readily accessible. We’ll see this come in handy once we start to work with forces in the next section.

For any given vector \vec{u}, its unit vector (written as \hat{u}) is calculated as follows:

\hat{u} = \dfrac{\vec{u}}{||\vec{u}||}

In other words, to normalize a vector, simply divide each component by its magnitude. This is pretty intuitive. Say a vector is of length 5. Well, 5 divided by 5 is 1. So, looking at our right triangle, we then need to scale the hypotenuse down by dividing by 5. In that process the sides shrink, divided by 5 as well.

In the PVector object, we therefore write our normalization function as follows:

PVector.prototype.normalize = function() {
  var m = this.mag();
  this.div(m);
};

Of course, there’s one small issue. What if the magnitude of the vector is 0? We can’t divide by 0! Some quick error checking will fix that right up:

PVector.prototype.normalize = function() {
  var m = this.mag();
  if (m > 0) {
    this.div(m);
  }
};

Here's a program where we always normalize the vector that represents the mouse position from the center (and then multiply it so we can see it, since 1 pixel is tiny!):


This "Natural Simulations" course is a derivative of "The Nature of Code" by Daniel Shiffman, used under a Creative Commons Attribution-NonCommercial 3.0 Unported License.