A function that outputs the mean and standard deviation

Question

Christopher Clarke 2020-5-31

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https://ww2.mathworks.cn/matlabcentral/answers/538999-a-function-that-outputs-the-mean-and-standard-deviation

编辑： And_Or 2020-5-31

function mean_stdev
    A=rand(1,100);
    
    % This function calculates the mean and standard deviation without
    %   using in-built functions
som=0;
for i=1:length(A)
  som=som+A(i);
end
M=som/length(A); %the mean
moy=0;
for i=1:length(A)
    moy = (A(i)-M)^2
    moy = sqrt((moy)/length(A));
end
VSD=moy/length(A); %Varaince
end

i get 100 results when i run this code. My aim is to call the function then enter a series of numbers then output the standard daviation and the mean. Where am i going wrong?

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Answer 1

And_Or 2020-5-31

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Hello Christopher

I think that what you want is to define a function like this:

function [M, VSD] = mean_stdev(A)
    
    % This function calculates the mean and standard deviation without
    %   using in-built functions
som=0;
for i=1:length(A)
  som=som+A(i);
end
M=som/length(A); %the mean
moy=0;
for i=1:length(A)
    moy = (A(i)-M)^2
    moy = sqrt((moy)/length(A));
end
VSD=moy/length(A); %Varaince
end

This function has an input (A) and two outputs (M, VSD). Once you have defined that function, you can call it from outside:

%Define your input values
A=rand(1,100);
%Call the function
[myMean myStdDev] = mean_stdev(A)

The function should return the mean value and the standard deviation in the outputs of the function, which I have called myMean and myStdDev.

Matlab includes some built-in functions that can be used to calculate the mean and the standard deviation of an array.

myMean = mean(A);       %Mean function
myStdDev = std(A);      %Standard deviation

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Christopher Clarke 2020-5-31

function [M, VSD] = mean_stdev(A)
    A=input('N numbers:')
    
    % This function calculates the mean and standard deviation without
    %   using in-built functions
som=0;
for i=1:length(A)
  som=som+A(i);
end
M=som/length(A); %the mean
moy=0;
for i=1:length(A)
    moy = (A(i)-M)^2;
    moy = (moy)/length(A);
end
VSD=sqrt(moy/length(A)); %Varaince
end

Hi,

Thank you for the advise.

I have changed the code so the user can enter their own series of numbers but when i run the function it only one outputs the mean result. I cannot figure out way that is. My results are below.

>> mean_stdev
N numbers:
[3,6,1,7,8,9]
A =
     3     6     1     7     8     9
ans =
    5.6667

And_Or 2020-5-31

编辑：And_Or 2020-5-31

Two things:

1-In the function I suggested, you could pass the data as an input for the function. The "input" function is not required if you pass the array as an put for the function.

[myMean, myStdDev] = mean_stdev([3,6,1,7,8,9])

2-If you want both mean and standard deviation outputs, you should call the function with two variables between brackets, as I have done. You would have the mean value stored in "myMean" and the standard deviation in "myStdDev".

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Answer 2

the cyclist 2020-5-31

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https://ww2.mathworks.cn/matlabcentral/answers/538999-a-function-that-outputs-the-mean-and-standard-deviation#answer_443587

编辑：the cyclist 2020-5-31

The reason you are displaying 100 results is that you do not have a semicolon after this line:

moy = (A(i)-M)^2

so the results are sent to the display for every iteration of the for loop.

You are calculating just the results you intend, I believe: M and VSD.

I won't correct the standard deviation result, as I assume this is a school assignment. But a hint is that you are not carrying out a sum in that calculation as you should be, and I don't think you want the square root as part of the for loop.