Calling table from matlab workspace as input for function

Hi,

Disclaimer I'm relatively new to Matlab/coding in general so apologies if it's messy/unclear!

I am trying to create a function that takes a table in the workspace as an input for a function but receive an error. I have tried using 'FILENAME', "FILENAME", as well as trying to take the original excel file i.e. 'filename.xlsx' and a few other varieties but can't seem to get it right. I'm guessing it's because I need to define the file path but was wondering if there was a way to avoid that as I'm trying to help a few people in my research group by creating a generic bit of code that allows someone with practically no coding knowledge just to supply the four inputs as simply as possible (either directly from an excel file or from the table variable in matlab).

[m1,m2]=IRmax(FILENAME,1750,400,2)

function [mx1,mx2]=IRmax(FILENAME, UpperWn, LowerWn, NumMaxima)

    %import data
    data=readtable(FILENAME);
    
    %labels variables
    data.Properties.Variablenames{1}='wavenumber';
    data.Properties.Variablenames{2}='absorbance';
    
    %trim 
    trim=data(data.wavenumber>LowerWn & data.wavenumber<UpperWn,:);
    
    %get local maxima
    maxTS=table();
    maxIR = (islocalmax(trim,"MaxNumExtrema",NumMaxima,"DataVariables","absorbance"));
    
    %finds wavenumbers corresponding to maxima     
    maxTS.FILENAME=trim.wavenumber(maxIR(:,2)==1);
    mx1=maxTS.FILENAME(1,1);
    mx2=maxTS.FILENAME(2,1);
end

I receive these errors when running the code. I'm guessing that I'm going to encouter erros further down the line but I've been unable to get past this point

Error using readtable (line 318)
"filename" must be a string scalar or character vector.

Error in IRmax (line 4)
data=readtable(FILENAME);

Thanks in advance

Answer 1

[Dave B]

when you call a function, you're passing in some values to be used in the function. So when you have:

[m1,m2]=IRmax(FILENAME,1750,400,2)
function [mx1,mx2]=IRmax(FILENAME, UpperWn, LowerWn, NumMaxima)

whatever is in the variable FILENAME in the outer workspace will get passed to FILENAME in IRmax's workspace. readtable expects that thing to be a string (or character vector) but it isn't. So what is in the variable FILENAME? Is that your table which you already read earlier? If so, you don't need to do readtable again, just pass that in as the function parameter data. If you're expecting it to be the name of the file, where did you define it above?

Here are some illustrative examples:

[m1,m2]=IRmax('mydata.xlsx',1750,400,2)
function [mx1,mx2]=IRmax(fn, UpperWn, LowerWn, NumMaxima)
    %import data
    data=readtable(fn);
    %...
end

myfilename = 'mydata.xlsx';
[m1,m2]=IRmax(myfilename,1750,400,2)
function [mx1,mx2]=IRmax(fn, UpperWn, LowerWn, NumMaxima)
    %import data
    data=readtable(fn);
    %...
end

mydata = readtable('mydata.xlsx');
[m1,m2]=IRmax(mydata,1750,400,2)
function [mx1,mx2]=IRmax(data, UpperWn, LowerWn, NumMaxima)
    % data already imported, can just start working with it!
end
    %...
end

[R L]

Many thanks for your answer Dave, I just tried this by commenting out the "data=readtable" part and it worked (after I had also capitalised the N in VariableNames!)

Answer 2

[Benjamin Pommer]

Hi,

I hava a related question. I have a function which shall have as an argument either variables from a table from the workspace or directly the .mat or .csv file and extract the necessary data from it. The function arguments shall be used for further calculation of what so ever.

The first time, I tried to get access to the variables velocity through the csv file, but it didnt work out.

function [A,X,res,x_vec,x_pred] = DMD_Kopie(filename,n,N_pred,states)

data = readtable(filename);

v = data.velocity(1:end)';

a = data.acc(1:end)';

t = data.t(1:end)';

E = data.energy(1:end)';

%further code

end

The first time, I tried to get access to the variables velocity through the corresponding .mat file. In that case, I got a struct array but again. It didnt work out.

function [A,X,res,x_vec,x_pred] = DMD_Kopie(filename,n,N_pred,states)

data = load(filename);

v = data.velocity(1:end)';

a = data.acc(1:end)';

t = data.t(1:end)';

E = data.energy(1:end)';

%further code

end

The last time, I loaded the variable into the workspace and tried to get access to them but again nothing worked out.

function [A,X,res,x_vec,x_pred] = DMD_Kopie(n,N_pred,states)

v = tablexyz.velocity(1:end)';

a = tablexyz.acc(1:end)';

t = tablexyz.t(1:end)';

E = tablexyz.energy(1:end)';

%further code

end

Do you have any idea how to solve this?

[Dave B]

@Benjamin Pommer - I'd suggest you post this as a new question as that will help get lots of eyes on it. Also it's really helpful if you expand on 'didnt work out' because it's not clear what didn't work and how to help you. Folks will always want to know whether you got an error, and if so which one, and on which line of code. Or maybe you didn't get an error but the values weren't as expected.

Your first two code snippets look reasonable, aalthough the (1:end) part doesn't seem like it would do anything in this case.

I'd be curious whether there was something in the variables, is it that readtable didn't give you what you expected? Trying readtable outside of the function, or setting a breakpoint and stepping through the code are great ways to diagnose that sort of thing. How did you call the function, are you sure you correctly passed in 'filename'?

Your third code snippet wouldn't work because you didn't pass in the table, functions in general have a scope - they don't know about variables that weren't passed in as arguments.