Loren on the Art of MATLAB

February 1st, 2006

exist can be called with one argument (a string) to test if that specific name is known and available in MATLAB in any form. If we program carefully and check the output result of exist, then we can be sure we know whether we are dealing with a MATLAB variable, a Simulink model, etc.

I often see user's calling exist with a single input argument, a name. This answers whether or not that name is somehow in the MATLAB environment. Very often, however, we really want more specific information, such as whether or not that name refers to a variable. A careful check of the output value (not just >0) can supply this information, though I rarely see code check for this.

Using the two input syntax for exist, we are able to write code that is both more robust, more readable, and frequently more efficient.

For example, if we have a program in which we ask the user to type in a variable name to process, we may then want to verify that the variable exists or else prompt the user again for a valid variable name. To do so, we might write code like this:

varname = '';
while isempty(varname)
     varname = input('Which variable would you like to process?','s');
     if ~exist(varname,'var')
         varname = '';
     end
end

When we run this code, since we are only checking for variables, MATLAB does not need to go look on the file system for all the other possible items MATLAB might be interested in such as M-Files, etc. This can result in a tremendous speed gain! And, by confining the search to variables, the code is more robust since it can't get misled if the name refers to an M-File.

11:51 UTC | Posted in Efficiency, Robustness | Permalink |

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9 Responses to “Existence in MATLAB”

1. david replied on February 4th, 2006 at 21:09 UTC :
   

   Hello.

   Great blog. It is really helpfull with some aspects of Matlab.
   I would like to ask you if it is possible to control a device with matlab or “data acquisition toolbox” trough the serial port RS-232.

   thank you.

2. loren replied on February 6th, 2006 at 07:31 UTC :
   

   You can in MATLAB or the Instrument Control Toolbox. A good place to look for this sort of information is The MathWorks support page. You can search for the function serial and read the documentation beyond the reference pages. There are also some illustrative demo pages.

3. peter replied on October 4th, 2006 at 19:49 UTC :
   

   Yes, but what about

   myFile = [path filesep ‘foo.mat’];
   if ~exist(’myFile’, ‘file’)
   %create the file
   end

   This fails, because Matlab returns 0, thinking that myFile is a var (which it is, but it’s also a file). Hardcoding the string, of course, works.

4. Saadia Iftikhar replied on January 22nd, 2008 at 12:29 UTC :
   

   How to deal with problem which Peter addressed? If File is in MAT format and want to check its existence? How to do this?

   peter replied on October 4th, 2006 at 7:49 pm :

   Yes, but what about

   myFile = [path filesep ‘foo.mat’];
   if ~exist(’myFile’, ‘file’)
   %create the file
   end

5. Loren replied on January 22nd, 2008 at 16:09 UTC :
   

   Saadia and Peter-

   The problem is, you have a created a variable called myFile so asking if it exists gets things in a mess. And there probably isn’t a file itself named ‘myFile’ but the contents of myFile. You need to do this:

   if exist(myFile,’file)
   …

   which will use the contents of the variable myFile

   or you might be able to use which, but in functional form

   which(myFile)

   and see if it returns empty or not (though I don’t think which will work for something not on the matlabpath).

   –Loren

6. Hannah replied on October 3rd, 2008 at 08:37 UTC :
   

   Hi,

   Is there any way to check for the existence of structure elements?

   For example…

   >> c
   c =
   cheese: ‘gruyere’
   wine: ’sauvignon blanc’
   >> d
   d =
   wine: ’sauvignon blanc’
   >> if (exist(’c.cheese’)) disp(c.cheese); else disp(’no cheese here’); end
   no cheese here
   >> if (exist(’d.cheese’)) disp(d.cheese); else disp(’no cheese here’); end
   no cheese here

7. Loren replied on October 3rd, 2008 at 08:51 UTC :
   

   Hannah-

   If you are looking to see if a value of a field is set, use one of the string comparison functions, e.g.,

   if strcmp(c.cheese,'gruyere')
      disp('Thank you')
   else
      disp('How disappointing!')
   end
   

   and if you are looking to see if the field exists, use isfield

   if isfield(c,'cheese')
      if strcmp(c.cheese,'gruyere')
         disp('Thank you')
      else
         disp('How disappointing!')
      end
      disp('no cheese here')
   end
   

   –Loren

8. Sung Soo replied on February 28th, 2009 at 19:07 UTC :
   

   What a cool feature. As an old MATLAB user, I wanted this feature for a long time, but I’ve not noticed that it’s been already implemented long time ago. I had to find your blog earlier…

9. Aramendi replied on October 6th, 2009 at 17:00 UTC :
   

   Peter & Saadie, i had the same problem: how checking existence of a MAT file, and I have to say that

   exist([filepath,’filename.mat’])

   works for me, returning a 2 (not a 0).

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