Attempted to access data(1); index out of bounds because numel(data)=0. Error in RunAirfoilCase (line 62) alpha = data(1) ;

i dont know how to fix it, i would appreciate your help.

function [alpha,cl,cd,cdp,cm,xtr1,xtr2] = RunAirfoilCase(  p , counter ,  camb , xcamb , thickness )
%
Reynolds = 1e6 ;
Mach     = 0.05 ;
%
FileName = sprintf( 'NACA%04d.dat' , counter ) ;
fid  = fopen( FileName , 'wt' ) ;
fprintf( fid , 'NACA %10.6f %10.6f %10.6f\n' , camb , xcamb , thickness);
for i = 1 : length(p)
  fprintf( fid , '  %10.6f   %10.6f\n' , p(i,1) , p(i,2) ) ;
end
fclose(fid);

Filename = 'xfoil.inp' ;
fid = fopen( Filename , 'wt' );
%fprintf( fid , 'Y\n' );
Line = sprintf( 'load NACA%04d.dat\n' , counter ) ;
fprintf( fid , Line ) ;
fprintf( fid ,'pane\n');
fprintf( fid ,'oper\n');
fprintf( fid ,'iter 200\n');
fprintf( fid ,'visc %20.6f\n', Reynolds);
fprintf( fid ,'mach %10.6f\n', Mach);
fprintf( fid ,'pacc\n');
%Line = sprintf( 'NACA%04d.pol\n' , ii ) ;
%fprintf( fid , Line );
fprintf( fid ,'\n');
fprintf( fid ,'\n');
fprintf( fid ,'alfa 0.0\n');
fprintf( fid ,'pacc\n');
fprintf( fid , 'pwrt\n' );
Line = sprintf( 'NACA%04d.pol\n' , counter ) ;
fprintf( fid , Line );
fprintf( fid ,'\n');
fprintf( fid ,'\n');
fprintf( fid ,'\n');
fprintf( fid ,'quit\n');
fclose(fid) ;
!xfoil.exe < xfoil.inp
%!./xfoil < xfoil.inp
%
%implementar la lectura de cl, cd y cm del archivo de resultados
%
PolarFile = sprintf( 'NACA%04d.pol' , counter ) ;
PolarFileID = fopen ( PolarFile , 'rt') ;
read = 1 ;
counter = 0 ;
alpha = 100.0 ;
cl    =  10.0 ;
cd    = 100.0 ;
cdp   = 100.0 ;
cm    = 100.0 ;
xtr1  =   1.0 ;
xtr2  =   1.0 ;
while ( read == 1)
  Line = fgetl ( PolarFileID ) ;
  counter = counter + 1 ;
  if Line == -1 
      read = 0 ;
  elseif counter == 13
      data = sscanf ( Line , '%f %f %f %f %f %f %f') ;
      alpha = data(1) ;
      cl    = data(2) ;
      cd    = data(3) ;
      cdp   = data(4) ;
      cm    = data(5) ;
      xtr1  = data(6) ;
      xtr2  = data(7) ;
  end
end
fclose ( PolarFileID ) ;

[dpb]

Several issues; can't tell just what is the root cause w/o data, but--

...
while ( read == 1)
  Line = fgetl ( PolarFileID ) ;
  counter = counter + 1 ;
  if Line == -1 
      read = 0 ;
  elseif counter == 13
      data = sscanf ( Line , '%f %f %f %f %f %f %f') ;
      alpha = data(1) ;
      cl    = data(2) ;
      ...

There's nothing to terminate the while loop except an error; the variable read is never set to anything other than initial 1 so it's an infinite loop. Use

while ~feof(PolarFileID)

or other similar logic instead.

The actual error comes because the sscanf line fails to convert anything -- why we have no way to tell; perhaps there's not the data expected at record 13 in the file or there's some other issue regarding the format of the data expected.

Set a breakpoint and step through and see what the issue is with the file and parsing it.

Why wouldn't one use textscan and a value for the 'headerlines' parameter as being more straightforward?

[Ameer Hamza]

fgetl() does return -1 when an end of file is reached, so the while loop will terminate eventually.

[dpb]

Bad eyes...missed it, that is correct.

Answer 1

[Ameer Hamza]

As error points out the variable data is empty. The problem appears to be happening in the line

data = sscanf ( Line , '%f %f %f %f %f %f %f') ;

You are making the assumption that if fgetl return -1 then stop the loop otherwise sscanf() it will always 7 floating point numbers. But sprintf can fail this assumption. For example, here I am using 3 floating point number in each case.

sscanf('a 12 32', '%f %f %f') % a non-numeric character is present
ans =
   []

sscanf('10 20', '%f%f%f')  % less then 3 numbers are present
ans =
  10
  20

Here are few solutions

1. You either need to make sure that file strictly follows the format, i.e. only contain 7 numbers in each line.
2. You need to change formatSpec to match the format of your file.
3. Add extra checks in you code to see if the read line follow the required format.