in reply to Re: (jcwren) Re: Still confused
in thread Still confused

I think your problem is in the algorithm, not just the Perl code.

You're calculating deviations from the average via ($score - $avg) as you go along. But this won't work because the "average" isn't the true average until you're done counting the data! I think that what you want to do is:
1. Read in all of the data, calculate $number and $sum as you go
2. Calculate $avg from the final $number and $sum
3. Loop through the data again and calculate $totd as you go

Or, you can use the module Statistics::Descriptive to do it for you. Unless this is a homework assignment...

buckaduck

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(tye)Re: Still confused
by tye (Sage) on Apr 11, 2001 at 22:47 UTC

    Actually, you should compute $number, $sum, and $sqsum (which is the sum of the squares of all of the numbers) as you go and then you can compute standard deviation without having to loop a second time.

    I'll omit the details of the calculation since you can probably just look them up in Statistics::Descriptive, though I find deriving the formula is a good way to see if you remember any of your math classes. (:

    This standard trick allows you to add numbers to the set (or even remove some) and recalculate the average and stddev without having to even keep the list of numbers around, much less loop through them.

            - tye (but my friends call me "Tye")
[reply]
      Oh holy smokes, that works nicely. I looked it up in Statistics::Descriptive and could have kicked myself. My way was pretty trivial and I think easy for anyone to understand. But it's not all that hard to do it your way, and it has significant advantages. Well worth a look.

      Thanks, tye.

      buckaduck

[reply]

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