Syntax help with script

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Greg
Greg 2013-6-20
I am wanting to put this equation into matlab, but I am having problems.
I am wanting to find the standard deviation without the built in matlab function. I have a mean_temp and a mean_pres already set up and displaying properly. Can I get some help how to incorporate those values (i have i=1:30) for each temp and pressure.
The standard deviation equation is:
(sum i=1 to n (xi^2-nxbar^2))/(n-1)
My xi should be my values each 1 through 30. xbar is my mean.
Thank you in advance.

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采纳的回答

John
John 2013-6-20
assuming x is an nx1 column vector...
sum(x.^2-mean(x)^2)/size(x,1)
also I think your equation is wrong: http://en.wikipedia.org/wiki/Standard_deviation you'll want to square after subtracting, not before.
sum((x-mean(x)).^2)/size(x,1)

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Greg
Greg 2013-6-20
You are correct, I had entered the equation wrong. Thanks. For your response, when I am substituting my variable for yours, can I enter display value as x?
Here is some of the code that I am trying to work with. Does the code you provided above operate for each of my i values or just for the mean that is calculated?
------------------------------------
sum_temp=0; sum_pres=0;
% Mean value of Temperature
for i=1:length(temperature) sum_temp=sum_temp+temperature(i); end mean_temp=sum_temp/length(temperature)
% Mean value of Pressure
for i=1:length(pressure) sum_pres=sum_pres+pressure(i); end mean_pres=sum_pres/length(pressure)
% Standard deviation Value of Temperature
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John
John 2013-6-20
oh sorry, I was assuming i was the length of your vector x. If you want the standard deviation of "temperature", then you would just do:
sum((temperature-mean(temperature)).^2)/size(temperature,1)
assuming temperature has i rows and 1 column. No for-loop is necessary.

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