fprintf in nested for loops

Trying to fopen and fprintf the multiple files in nested for loops. Able to get the file opening to work:

for i=1:3 
  for j=1:4
    fname=['filei' num2str(i) 'j' num2str(j) '.txt'];
    fidsum=j+(i-1)*4;
    sprintf('fid%d',fidsum)=fopen(fname,'w');
  end 
end

But the fprintf part doesn't work. The following fails:

for k=1:largeNumber
  for i=1:3
    for j=1:4
      fidsum=j+(i-1)*4;
      if ((var1(k)==i) & (var2(k)==j))
        fprintf(sprintf('fid%d',fidsum),'%d\n',kDependentParameter)
      end
    end
  end
end

This also fails:

for k=1:largeNumber
  for i=1:3
    for j=1:4
      fidsum=j+(i-1)*4;
      if ((var1(k)==i) & (var2(k)==j))
        fid=sprintf('fid%d',fidsum);
        fprintf(fid,'%d\n',kDependentParameter)
      end
    end
  end
end

How to use the numerically increasing fid's for multiple file writings?

[Stephen23]

Ugh, no, this is the complete opposite of how to write good code. Do not try to create numbered variable names like that, it will be slow, buggy, complicated code that is hard to debug. The much better solution (as dpb showed you) is to preallocate an array and then simply use indexing.

Some beginners think that numbered variables are a good idea: it isn't. Read this to know why: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

[b]

Thanks. Did not realize that indexing can be used for file identifiers also.

[dpb]

>> help fopen
fopen  Open file.
  FID = fopen(FILENAME) opens the file FILENAME for read access.
  ... 
  FID is a scalar MATLAB integer valued double, called a file identifier. 
 ...

Answer 1

[dpb]

"Able to get the file opening to work"

Well, no, not really. What you've done is alias sprintf to a variable by assigning it in what looks to be array indexing syntax--

>> i=1;j=1;  % assign initial values
   fname=['filei' num2str(i) 'j' num2str(j) '.txt'];  % ok
   fidsum=j+(i-1)*4;                                  % fidsum --> 1 ok, but not sure what intend
   sprintf('fid%d',fidsum)=fopen(fname,'w');     % it all goes south here...
>>

>> whos sprintf
Name           Size            Bytes  Class     Attributes
sprintf      105x1               840  double              
>> which sprintf
sprintf is a variable.
>>

Do you really need all these files open at one time would be first question...but if do, keep the handles in an array...

I=3; J=4;           % loop limits
fid=zeros(I,J);     % preallocate to hold way too many fids at once in all likelihood
for i=1:I
  for j=1:J
    fname=sprintf('filei%02dj%02d.txt',i,j);  % build a file name
    fid(i,j)=fopen(fname,'w');                % and open a file, return fid in array  
  end
end

I don't know what part the fidsum variable was to play so left it out...

Now, when you're ready to write to one of these, then you'd have to know which i, j index pair you want and would write something like

fprintf(fid(i,j),fmt,variables)

but it's highly unlikely you really, really need that many files at once.

What is the actual end objective you're wanting to output? Almost certainly there's a far better way to construct the logic to do so.