Can I pass table as an argument to a function.?

Hi,

Currently I am working on migration of MATLAB scripts to C++ code.

I encountered a problem with the datastucture table. Basically I am reading a CSV file using readtable and stored it in a table DataTable. I want to send this table to a function which uses the DataTable for some calculations. But when I tried to access the members of the DataTable this is where the problem starts, The coder app is not allowing me to access the parameters in the DataTable and is showing the error "When indexing a table using variable names, the names of the table variables must be constant.". Example code is below

mycsv.csv is a simple csv file with feilds as X and Y

X,Y

1011,203

1026,210

DataTable = readtable("mycsv.csv");
MyCalulations(DataTable);

function MyCalulations(DataTable)
Xmean=mean(DataTable.X);
Ymean=mean(DataTable.Y)
end

Answer 1

[Ameer Hamza]

See this answer here: https://www.mathworks.com/matlabcentral/answers/572815-matlab-coder-table-variables-must-be-constant. It will be easier if you index the table using numeric values. For example

function MyCalulations(DataTable)
Xmean=mean(DataTable{:,1});
Ymean=mean(DataTable{:,2})
end

The C++ code is generated correctly.

These are some useful links:

https://www.mathworks.com/help/coder/ug/code-generation-for-tables.html

https://www.mathworks.com/help/simulink/ug/table-limitations-for-code-generation.html

Answer 2

[Image Analyst]

Can you attach "mycsv.csv" so we can try it? It might call the fields "Var1" and "Var2" instead of X and Y since you didn't specify field names for the table.

[Image Analyst]

Not sure why you didn't attach the CSV file in your edit, despite me directly asking for it. That is where the problem lies. Try again top attach, because this works fine:

% DataTable = readtable("mycsv.csv");
X = [1011;1026];
Y = [203;210];
DataTable = table(X, Y)
MyCalulations(DataTable);

function MyCalulations(DataTable)
Xmean=mean(DataTable.X)
Ymean=mean(DataTable.Y)
end

Again, I think it's because DataTable does not have the fields you expect. Leave the semicolon off the readtable() line and see what it puts as the column headers when it reports the table to the command window.

[C Mendonsa]

Thank you for the information.

This code is running succesfully. But when I tried to convert MyCalulations function to c++ code using the MATLAB coder it throws the error " When indexing a table using variable names, the names of the table variables must be constant" . Can we convert the code of MyCalculations function to c++?

[Image Analyst]

I don't have the Coder Toolbox like you do, so I can't help. Sorry. Contact tech support.

By the way, this works if you set ReadVariableNames to true:

DataTable = readtable("mycsv.csv", 'ReadVariableNames', true)
MyCalculations(DataTable)

function [Xmean, Ymean] = MyCalculations(DataTable)
Xmean = mean(DataTable.X)
Ymean = mean(DataTable.Y)
end

[Ameer Hamza]

For me, even this works fine

DataTable = readtable("mycsv.csv")

MATLAB automatically read the variable name. I am using R2020a. Maybe this behaviour was changed in some recent release.