Keeping an average in a sketch.

Question

First off, thank you to everyone at the element14 community for professionally answering my novice questions. Its really great that everyone is willing to be so helpful!

This time around, I need a little help on my current project. I am making a arduino clone of a basic bicycle computer. I've got the basics of it set up, and I can accurately output my current speed, and distance traveled. My next goal is to calculate my average speed. The idea of storing all of my speed readings, and trying to average them all as I go weems ludicris. I would quickly fill the arduino up with an array of speeds. I was hoping there is a better way. Has anyone ever needed to do something similar? What would be the best equation to use?

My inital thought was something along the lines of: new average = (previous average + new value) / 2

However, it doesnt seem to be as accurate as properly averaging the numbers.

ntewinkel · Accepted Answer

Average = sum_of_all_values / number_of_values

So you could store one field named something like "sum" and keep adding to it.

And keep a field named something like "count" and keep incrementing it.

Then your average will always be sum/count.

For sum, you should use a "long" datatype or something that can hold really big numbers.

Hope that helps

Cheers,

-Nico

ntewinkel · Answer

ps, I think the formula you had in mind, to directly add to the average every time, would look like this:

newAverage = previousAverage * (count-1 / count) + newSpeed * (1 / count)

That would involve 2 multiplications, 2 divisions, 1 subtract, and 1 add.

The other method requires 2 additions (1 to add to the sum, 1 to increment the counter), and one division.

Cheers,

-Nico

Keeping an average in a sketch.

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