matlab, how to use the output that is printed by a function?

Question

I would like to store some values that are printed during the iterative procedure of a function, but I have no idea how. here is the code I am using:

a = 0
b = 2
tol = 1.e-6
trace = 1
F = @(x)1./(x.^3-2*x-5);
Q = quad(F,a,b,tol,trace);

the quad funciton gives the integral of F from a to b using the adaptive quadrature method. trace = 1 prints the values to the console [fcnt a ba Q] during the recursion, but does not store them to the variable.

I would like to store the values a and ba that are printed during the procedure, for later use. for instance, this code gives

>> quad(F,0,2,1.e-6,1);
   9     0.0000000000     5.43160000e-01    -0.0989460227
  11     0.5431600000     9.13680000e-01    -0.1584111746
  13     0.5431600000     4.56840000e-01    -0.0755952309
  15     1.0000000000     4.56840000e-01    -0.0828028464
  17     1.0000000000     2.28420000e-01    -0.0391911692
  19     1.2284200000     2.28420000e-01    -0.0436112507
  21     1.4568400000     5.43160000e-01    -0.2054245169
  23     1.4568400000     2.71580000e-01    -0.0667670196
  25     1.4568400000     1.35790000e-01    -0.0302481864
  27     1.5926300000     1.35790000e-01    -0.0365183194
  29     1.7284200000     2.71580000e-01    -0.1366285551
  31     1.7284200000     1.35790000e-01    -0.0492888403
  33     1.8642100000     1.35790000e-01    -0.0871164919
  35     1.8642100000     6.78950000e-02    -0.0350033472
  37     1.9321050000     6.78950000e-02    -0.0520998049
  39     1.9321050000     3.39475000e-02    -0.0228214919
  41     1.9660525000     3.39475000e-02    -0.0292778809

I need the values printed in the second and third columns above.

Thank you.

Answer 1

I'm not sure I've understood your question; if you want to store the trace values

9 0.0000000000 5.43160000e-01 -0.0989460227 11
0.5431600000 9.13680000e-01 -0.1584111746

etc ...

into an array, consider that the trace values are printed by the quad funcition by using the fprintf .

You can edit the quand function - edit quad - and see:

if trace
    fprintf('%8.0f %16.10f %18.8e %16.10f\n',fcnt,a,h,Q);
end

I see at least two possibilities:

1) Use the diary function

You can modify your code by calling diary right before calling quad ; this function create a log of the ouput displayed in the CommandWindow into a text file.

Then, you can load the content of that file to impport its content (the trace data) in the Workspace.

Do not forget to add ";" at the end of the call to quad otherwise also the output of the function will be stored into the diary file and this will prevent the possibility of loading it

a = 0
b = 2
tol = 1.e-6
trace = 1
F = @(x)1./(x.^3-2*x-5);
% Define the name of the diary file
diary_filename='trace_data.txt';
% Enable saving the data into the "trace_data.txt" output file
diary(diary_filename)
Q1 = my_quad(F,a,b,tol,trace);
% Turn off the diary log
diary
% Load the trace data into the "trace_data" array in the workspace
trace_data=load(diary_filename)

You might have a more "general" approach and dynamically generate the trace data output file, by using tempname .

( tempname generate the filename in the temporary folder, so,if you want to store it into you current directory you have to split it, extract the actual filename by using fileparts )

a = 0
b = 2
tol = 1.e-6
trace = 1
F = @(x)1./(x.^3-2*x-5);
% Dynamically generation of the output file name
tmpName = tempname
% Extract the actual filename
[pathstr,name,ext]=fileparts(tmpName)
% Build the filename and add the extension
diary_file=[name '.txt']
% Enable saving the data into the "trace_data.txt" output file
diary(diary_file)
Q1 = my_quad(F,a,b,tol,trace);
% Turn off the diary log
diary
%  Load the trace data into the "trace_data" array in the workspace
trace_data=load(diary_file)

2) Modify the quad function

Since the source code of the quad function is available, you can either directly modify the function (not recommended) or copy it in a folder on your path and modify it.

There are many way to modify the function.

One of them could be to:

• add an input parameter in which you can specify the name of the output file
• add in the function the code to open the file ( fopen )
• add the file handle in the fprintf call
• close the output file at the end ( fclose )

another possibility could be to add an output parameter in the definitin of the function in which to store the trace data; in thsi case you also have to add the code to store the trace data into an array at each iteration of the function.

Hope this helps.

Qapla'