Make for loop and extract data from different tables within a structure.

Could you help me cxtract data from different tables in loops?

Samples are separated into separate csv files with 4 columns of data. Each of the columns in a file has the same number of rows but the number of rows in different files differs.

clc;clear;

%% Call Samples
n=18;                  %Number of specimens/ samples

for i=1:n
    n_strPadded = sprintf('%02d',i);      %if files contain leading zeros (mine does)
    fname = ['Sample' n_strPadded]          %File name as string
    temp_var = strcat('Sample',num2str(i));     %Trying to convert file name from a string to to a variable
    Samples.(temp_var)=readtable(fname); %convert csv files into tables and import them.
end
% now each csv file (each sample data is located within a table within a
% structure. Sample1, Sample2,.... in structure Samples

%I want to extract data from the tables by indexing it like
%Samples(i).Sample(i) , where samplei is a table (from within the
%structure). Then the following steps could be completed once in a loop.
%I have tried to use some commands to call out a table without using the
%tables unique name (below). The only way I have been able
%to do this is Samples.Samplex.var (x= sample # and var is the variable or
%column name) or Samples.Samplex(m,n).
%{
for i=1:n
    T1=Samples(i)
end

C=struct2cell(Samples)
x=C(1)
x=Samples(2).Sample(2,2)
%}
%% Stress Sample 2
Cross_sectionalArea=readtable('Cross_sectionalArea.csv');
StressSample2=Samples.Sample2.Force*(10^3) / Cross_sectionalArea.m(2);

%% Strain Sample 2

StrainSample2=Samples.Sample2.Strain1;

%% Plot Stress Strain Sample 2
plot(StrainSample2,StressSample2)

Answer 1

[Stephen23]

You are confusing the field names with an index, but really you should just be using an index. Rather than awkward messing about with dynamic fieldnames in a scalar structure, you would be much better off using basic efficient indexing with a cell array:

n = 18;
c = cell(1,n);
for k = 1:n
    fname = sprintf('Sample%02d',k); 
    c{k} = readtable(fname); % or READMATRIX
end

https://www.mathworks.com/help/matlab/matlab_prog/access-data-in-a-cell-array.html

Storing the imported data in a simple cell array is exactly what the MATLAB documentation recommends:

https://www.mathworks.com/help/matlab/import_export/process-a-sequence-of-files.html

Answer 2

[Ayush Gupta]

The excel files here can be read directly with the help of readmatrix function. Suppose there is an excel file of name abc.xlsx, the file can be read, and first column can be stored as follows

data = readmatrix('abc.xlsx'); 
dolumn1 = data(:,1) 

To refer to examples on how to use readmatrix and examples on how to use it, refer here.

[Jordan Blake]

Thank you Ayush! The readmatrix is better for my purpose.

I do still have one issue though. At the end of the first For Loop I attemp to save each matrix as its own unique variable. So I will have 18 matrices after the loop. If I simply do what is below, I would overwrite each matrice and only have the final. If I use my code above, I end up with a structure and will again have issues calling on the elements later.

%% Call Samples
n=18;                  %Number of specimens/ samples
for i=1:n
    n_strPadded = sprintf('%02d',i);      %if files contain leading zeros (mine does)
    fname = ['Sample' n_strPadded]          %File name as string
   %temp_var = strcat('Sample',num2str(i));     %Trying to convert file name from a string to to a variable
    Sample_n=readmatrix(fname); %convert csv files into tables and import them.
end