Hi, I have a three questions about loops. The question I attempted was 'Create normal random data for 10 rows and 5 columns. Compute the means and standard deviations of each row by using a loop. Display the results for each row.'

Question

Sara Renmiu 2020-5-16

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/525883-hi-i-have-a-three-questions-about-loops-the-question-i-attempted-was-create-normal-random-data-fo

关闭： MATLAB Answer Bot 2021-8-20

在 MATLAB Online 中打开

The answer is

Create normal random data for 10 rows and 5 columns.

Compute the means and standard deviations of each row by using a loop.

Display the results for each row.

%}

data = randn([10 5])
for i = 1:size(data,1)
    m = mean(data(i,:),2);
    s = std (data(i,:),[],2);
    disp('Row data:')
    disp(m)
    disp(s)    
end

My questions are

1)What does size mean in this context (for i)?

2)What does mean(data(i, : ), 2); mean?

3)What does std (data(i, : ), [] 2); mean? And why are the [] square brackets used here?

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Mehmed Saad 2020-5-16

编辑：Mehmed Saad 2020-5-16

在 MATLAB Online 中打开

The best way of understanding that is

Open MATLAB
On the top right corner in MATLAB app, there's a search documentation field. Type size in it. it will show you array size, read that document
After that, read Array Indexing, mean and std by following step 2
There are examples given in each document, read those examples for better understanding
Also you can get help on any MATLAB command by typing help size (or whatever command you want to see)

help size
size   Size of array.  
    D = size(X), for M-by-N matrix X, returns the two-element row vector
    D = [M,N] containing the number of rows and columns in the matrix.
    For N-D arrays, size(X) returns a 1-by-N vector of dimension lengths.
    Trailing singleton dimensions are ignored.
 
 .
 .
 .
 
    See also length, ndims, numel.
    Reference page for size
    Other functions named size

此问题已关闭。

Answer 1

Walter Roberson 2020-5-16

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/525883-hi-i-have-a-three-questions-about-loops-the-question-i-attempted-was-create-normal-random-data-fo#answer_432790

size(data,1) is number of rows
mean(something, 2) is mean along the columns. The number of columns will be reduced to 1, with the other dimensions remaining the same as they were.
std() along the columns. The [] exist to occupy an argument so that MATLAB can tell the difference between giving information about which normalization scheme to use (normalize by N-1 or normalize by N), versus giving information about which dimension to act along.