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You can find the magnitude (the length) using the Pythagorean theorem. In a car game, you might need to find this value show show the speed of an object on the HUD display.
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The PVector class in processing has a method that returns this value: mag();
Normalising
A normalised vector or a unit vector has had its magnitude set to one, with the direction left unchanged. A unit vector shows us a direction alone, without a magnitude. Sometimes we are only interested in direction, and removing magnitude can make many calculations much simpler.
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