Error message 'Error using fft Invalid data type. First argument must be double, single, int8, uint8, int16, uint16, int32, uint32, or logical."

I have got the following code to find third harmonic from a rheology test. There is some more code after this but I am stuck at this point because the fft function wont ren. Please could someone help me figure out how I can modify the code to evaluate the fft?

waitfor(msgbox('Select strain file, .txt')); % asks for strain file
[filename,pathname,filterindex] = uigetfile('*.txt'); % gets selected file
S = (importdata(strcat(pathname,filename))); % joins filename and path and imports data
Output = S.data(:,1:3); % sets Output to the first three columns of strain file (Strain, frequency, temperature)
Output = [Output zeros(size(Output,1),17)]; % adds 17 columns to output
Strain=(Output(:,1)/100); % calculates non dimensional strain from percentage
Output = [Output Strain]; % adds non dimensional strain to end of file
waitfor(msgbox('Select output folder')); % asks user where to put output file
outputfolder = uigetdir; % gets selected folder
count=1 %counter for each file processed, sets initially to 1
for Stress=PVDFPT3Hstress %for each text file in the folder of stress files
    w= S.data(count,2); %reads frequency from column 2 in strain file
    T = PVDFPT3Hstress(:,3:3); %time data from file
    B = PVDFPT3Hstress(1:21,4:4); %stress data from file
    Z = PVDFPT3Hstress(:,5:5); % strain data from file
    DT=6.673 ;% mean difference between steps
    cycletime = 1/w; % time for 1 cycles
    pointspercycle = cycletime/DT; %finds points per cycle
    initialsize = size(Z,1); %gets starting number of points
    initialsize2 = size(Z,1); %gets starting number of points (minus 1st 5% cycles)
    Fs=1/DT;
    N = size(B,1);
    r=round(N/2);
    freq= (Fs*(0:1/r:1-1/r))/2;   % 0 to r-1 give 0, 0 +1/r, 0+2/r etc. 
    y=B./r;
    x = fft(y); 

[DGM]

It's complaining about the datatype. Without appropriate data files, it's anybody's guess what PVDFPT3Hstress is, where it comes from, or what class it is -- and consequently, what the class of y is. So what class is y?

[Hafsah]

pvdfstress contains the time, stress and strain values from the experiment.

y is each stress value divided by an integer (r), hence, y is an array.

Please lmk if you have any other follow-up questions

[Walter Roberson]

If it contains a time... it's probably in the form of a table() object.

Answer 1

[Walter Roberson]

for Stress=PVDFPT3Hstress %for each text file in the folder of stress files

It is often overlooked in the documentation, but when you use a for loop, the looping is over the columns of the object on the right-hand side of the =

        T = PVDFPT3Hstress(:,3:3); %time data from file

That hints that the variable on the right side of the for loop is a 2D array

        Z = PVDFPT3Hstress(:,5:5); % strain data from file

with at least 5 columns.

So the first time through the for loop, the data would be copied from PVDFPT3Hstress(:,1) into Stress. The second time through the for loop, the data would be copied from PVDFPT3Hstress(:,2) into Stress . And so on.

My guess about what is happening is that I suspect that PVDFPT3Hstress is a table object, and therefore that B = PVDFPT3Hstress(1:21,4:4) is a table object with 21 rows and 1 variable.

You are using R2023b, and new in R2023b is that if you use a table object with an arithmetic operation such as in your y=B./r; line, then MATLAB will attempt to apply the arithmetic operation to each variable in the table object, producing a table object as output. So I suspect your y is a table object with one variable.

Reminder: if you have table objects, then it is recommended to access them by variable name, as in

        B = PVDFTL3Bstress.stress(1:21);

and if you must use numeric subscripts, then use { } to extract the data, as in

        B = PVDFPT3Hstress{1:21,4};

[Hafsah]

yes Y is a table with 1 variable. How can i proceed with the fft function if y is a table?

[Walter Roberson]

B = PVDFPT3Hstress{1:21,4};

and I doubt that you should be using a for loop.

[Hafsah]

you're probably right. I copied this code from a source and edited it so I think with this version I dont need the loop. Still the same issue though, fft wont work.

[Walter Roberson]

    B = PVDFPT3Hstress{1:21,4};    %notice the { and } rather than ( and )
    N = size(B,1);
    r=round(N/2);
    freq= (Fs*(0:1/r:1-1/r))/2;   % 0 to r-1 give 0, 0 +1/r, 0+2/r etc. 
    y=B./r;
    x = fft(y); 

[Walter Roberson]

Please show

fprintf('Variable names\n');
PVDFPT3Hstress.Properties.VariableNames(:)

fprintf('Variable Classes\n');
classes = varfun(@(V)string(class(V)), PVDFPT3Hstress);
classes{:,}(:)