Text file reading problem

Hi all,

I would like to code a MATLAB function that could perform the following operations:

• Read a given text file (see below)
• Extract the numeric values corresponding, for example, to the variables: Alpha, pb/2v, Beta, qc/2V, Mach, rb/2v, [...], CLa, CLb, CYa, CYb, [...], Xnp.
• Store (and return) these numeric values into a column vector or row vector. That is : [6.220 0.000 0.000 0.800 0.000 ... 3.770 0.000655 ... 10.885]

The problem is that the text file seems to be a bit complicated because it doesn't have a definite format.

Here is a sample text file:

---------------------------------------------------------------
Vortex Lattice Output -- Total Forces

Configuration: Airplane                                                   
     # Surfaces =  12
     # Strips   =  81
     # Vortices = 778

    Sref =  122.40       Cref =  4.1935       Bref =  37.573   
    Xref =  14.390       Yref = 0.31229E-04   Zref =-0.40414   

Standard axis orientation,  X fwd, Z down        

Run case: Cruise                                 

    Alpha =   6.22083     pb/2V =   0.00000     p'b/2V =   0.00000
    Beta  =   0.00000     qc/2V =   0.00000
    Mach  =     0.800     rb/2V =   0.00000     r'b/2V =   0.00000

    CXtot =   0.02889     Cltot =  -0.00001     Cl'tot =   0.00000
    CYtot =  -0.00006     Cmtot =   0.00000
    CZtot =  -0.48764     Cntot =   0.00003     Cn'tot =   0.00003

    CLtot =   0.48790
    CDtot =   0.02412
    CDvis =   0.00000     CDind =   0.02412
    CLff  =   0.48991     CDff  =   0.02138    | Trefftz
    CYff  =  -0.00006         e =    0.3098    | Plane 

     Elevator        =   8.77320
     Rudder          =   0.00000

---------------------------------------------------------------

Stability-axis derivatives...

                               alpha                beta
                    ----------------    ----------------
  z' force CL |    CLa =   3.770952    CLb =   0.000655
  y  force CY |    CYa =  -0.000593    CYb =  -0.895943
  x' mom.  Cl'|    Cla =  -0.000003    Clb =  -0.068255
  y  mom.  Cm |    Cma =   3.151285    Cmb =  -0.003040
  z' mom.  Cn'|    Cna =   0.000338    Cnb =   0.101545

                       roll rate  p'      pitch rate  q'        yaw rate  r'
                    ----------------    ----------------    ----------------
  z' force CL |    CLp =   0.000039    CLq =  -2.410821    CLr =  -0.000764
  y  force CY |    CYp =  -0.199752    CYq =  -0.007101    CYr =   0.709363
  x' mom.  Cl'|    Clp =  -0.375950    Clq =  -0.000470    Clr =   0.109807
  y  mom.  Cm |    Cmp =  -0.000244    Cmq = -58.421581    Cmr =   0.003568
  z' mom.  Cn'|    Cnp =  -0.067254    Cnq =   0.003484    Cnr =  -0.321462

                    Elevator     d1     Rudder       d2
                    ----------------    ----------------
  z' force CL |   CLd1 =   0.008195   CLd2 =   0.000008
  y  force CY |   CYd1 =   0.000001   CYd2 =  -0.005356
  x' mom.  Cl'|   Cld1 =   0.000000   Cld2 =  -0.000690
  y  mom.  Cm |   Cmd1 =  -0.038800   Cmd2 =  -0.000039
  z' mom.  Cn'|   Cnd1 =  -0.000001   Cnd2 =   0.002880
  Trefftz drag| CDffd1 =   0.001417 CDffd2 =   0.000001
  span eff.   |    ed1 =  -0.010030    ed2 =   0.000001

Neutral point  Xnp =  10.885436

Clb Cnr / Clr Cnb  =   1.967782    (  > 1 if spirally stable )

I tried to use the TEXTSCAN function but, because the format is not the same everywhere (in each line), the code is not very “clean” and efficient. I also tried to use the SSCANF function, combined with a “while loop”, in the following form (for example):

fid = fopen('Textfile.txt','r');
while ~feof(fid)
     A{1} = sscanf(fid,'z'' force CL |    CLa =   %f    CLb =   %f');
     A{2} = sscanf(fid,' y  force CY |    CYa =   %f    CYb =   %f');
     etc...
end
fclose(fid)

In which A is a cell array (or matrix). This doesn’t work because, each time the “while loop” restarts, the contents of the cell that was previously stored is cleared. Is it possible to read a text file line by line, and store the numerical data from each line into a cell array? Could you help me please ? Is there a faster and more efficient way for solving this "simple" problem?

Thanks in advance for your help!

Answer 1

[Matt Kindig]

Hi Oscar,

I've had a similar issue (reading data from a text file with an irregular format, and instead a series of "keys" that you need to find in the file). I solved it by using regular expressions (regex())--are you familiar with those. In general, I did it like this:

str = fileread(FILENAME_HERE);   %read file into string
pieces = regexp(str, '\-{2,}', 'split');  %split into sections indicated by repeated hyphens
%then process each piece.

The big issue that could arise is if your file is especially long--MATLAB may not have enough memory to read it in full all at once.

Does this help? How much regexp() experience do you have?

[Oscar]

Hi Matt,

Thanks for answering my question! I'm not familiar with the REGEXP function but it seems to be an interesting way to solve the problem. I'm going to read the Matlab help carefully and I will try to implement this function. I will keep you informed.

Thanks again Matt