Standard Deviation of an array of numbers...

This is a discussion on Standard Deviation of an array of numbers... within the C++ Programming forums, part of the General Programming Boards category; I am writing a program that tells the user to enter 20 numbers, then it uses those and finds the ...

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    Standard Deviation of an array of numbers...

    I am writing a program that tells the user to enter 20 numbers, then it uses those and finds the average and standard deviation.

    I put the 20 numbers in an array.

    Does anyone know the c++ formula for standard deviation of an array? It's blowing my mind. I've been trying for hours.

  2. #2
    carry on JaWiB's Avatar
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    I don't know what "standar deviation" is...someone else here might, but in the meantime it would help if you could explain it a bit.

    Edit: after a google search I found one explanation:

    standard deviation is the square root of the variance of a mean of a set of numbers

    so for the set 1,2,3 the variance is:

    ( (1-avg)+(2-avg)+(3-avg) ) / n

    in this case avg is 2 and n is 3

    it also went on to say that you should divide by n-1 to give an unbiased estimate...although it didn't really explain why

    If this is correct it shouldn't be too hard to do...
    Last edited by JaWiB; 11-19-2003 at 09:26 PM.
    "Think not but that I know these things; or think
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    Of knowing what I ought."
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    It's a real weird formula.

    It's the square root of a summation of each number minus the average, squared and divided by the amount of numbers minus 1.

    It's hard to explain, even harder to code.

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    carry on JaWiB's Avatar
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    Sounds like i had it somewhat correct, but I had an error, I meant to square those terms:

    (1-avg)*(1-avg), etc

    So you should be able to do something like:
    Code:
    for (int i=0;i<n;i++)
       total+=(nums[i]-avg)*(nums[i]-avg);
    total/=n-1;
    std::cout<<sqrt(total);
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."
    -XSquared

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    Thanks!

    I got it to work, I think. Even if not so, you helped a lot. Awesome!

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