How do I use fprintf with MATLAB Coder?

I'm trying to use fprintf with MATLAB Coder but struggling to understand the limitation of "The formatSpec parameter must be constant."

I'm simply trying to save an array as an CSV or text file, and using the following code/function:

function write2CSV(varname, fname)

% Writes a particular variable to CSV file.

if nargin<2, fname = 'test.csv';end

fileID = fopen(fname,'w');

%fprintf(fileID,'%7.4f\n',varname);

fprintf(fileID,"%c\n",varname);

fclose(fileID);

You can call this function inside a script with any simple array as input such as:

write2CSV([1;2;3;4;5]);

It works fine in basic MATLAB. But when I try to generate C-code using MATLAB Coder, the specific error I'm getting is ... "argument corresponding to conversion character 'c' in the formatspec parameter is not scalar. It has to be fixed size scalar". HOW DO I SPECIFY A FIXED-SIZE SCALAR IN THE FORMAT SPEC?

I have tried various combinations for the format spec, but don't understand how to make it a 'constant'.

Any ideas? thanks a bunch!

Answer 1

[Darshan Ramakant Bhat]

How are you generating code ? Are you using Coder App or Command Line ?

As the error says, you cannot pass an array as input when the format specifier is "%c". I modified the code like below and it worked for me :

function write2CSV(varname, fname)
% Writes a particular variable to CSV file. 
if nargin<2, fname = 'test.csv';end
fileID = fopen(fname,'w');
%fprintf(fileID,'%7.4f\n',varname);
n = length(varname);

for i = 1:n
fprintf(fileID,"%c\n",varname(i));
end
fclose(fileID);

Code generation using command line :

codegen write2CSV -args {'hello','myFile.txt'} -report -config:lib

Hope this will be helpful

[Arfoss Andy]

Thanks a bunch Darshan! This did work. I suspected I might have to do it 1 value at a time. But the whole formatspec has to be a constant threw me off. It'd be nice to document this limitation directly in the Help - to state it has to write 1 value at a time.

[Ryan Livingston]

You can also consider using the %s format specifier in place of %c. That will print out all elememts at once.

[Arfoss Andy]

I did try %s, but that gave the same error. Looks like either way, I have to use a For loop to print it one at a time.

Also, can anyone point out how to create a EXE that'll execute this code on my host PC as a standalone program? Looks like I might have to modify the generated Main.c - and it's not straightforward to create a standlone EXE.

All I'm trying to do here is to simply read in using fscanf, and write it back using frpintf a bunch of numbers from CSV. Once this works, I plan to introduce processing within this code to modify the numbers that are read in, before I save them again.

Here's the full MATLAB Code - that now work for generating code. I'll try to explore creating an EXE next.

%% MAIN FUNCTION TO DO THE READING AND WRITING

function coderTests(file2write)

fname = 'myInputFile.csv';

%Read a specific CSV file

a = getCSV2MAT(fname); %

% Some Processing goes here eventually on a

% Write it back to another name.

write2CSV(a,file2write);

%% FUNCTION TO READ CSV

function out = getCSV2MAT(fname)

% Uses fscanf to read variables from csv file into a matrix

fileID = fopen(fname,'r');

fgets(fileID); % Skip the first line which is the header

formatSpec = '%d\n';

out = fscanf(fileID,formatSpec);

fclose(fileID);

%% FUNCTION TO WRITE TO CSV

function write2CSV(varname, fname)

% Writes a particular variable to CSV file.

if nargin<2, fname = 'test.csv';end

fileID = fopen(fname,'w');

for i = 1: length(varname)

fprintf(fileID,"%d\n",int32(varname(i)));

end

fclose(fileID);

[Darshan Ramakant Bhat]

I tried below code and it worked for me without error

function write2CSV(varname, fname)
% Writes a particular variable to CSV file. 
if nargin<2, fname = 'test.csv';end
fileID = fopen(fname,'w');
%fprintf(fileID,'%7.4f\n',varname);
% n = length(varname);
% 
% for i = 1:n
% fprintf(fileID,"%c\n",varname(i));
% end
fprintf(fileID,"%s\n",varname);
fclose(fileID);

codegen -config cfg write2CSV -args {coder.typeof('a',[inf inf]),coder.typeof('a',[inf inf])} -report

For EXE :

cfg = coder.config('exe');
cfg.GenerateExampleMain = "GenerateCodeAndCompile";

Above will generate EXE file with the default example main. We also generate makefile inside the codegen folder. You can modify the generated main and try to rebuild EXE using the makefile.