Syntax help with script

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.

Answer 1

[John]

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)

Answer 2

[Greg]

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

[John]

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.