try/catch and error with empty string

Question

I'm working with someone else's code and I am unfamiliar with try/catch so I made a small, similar example. On line 11, if I write error(''), it doesn't seem to catch the error and increase the index j. However, writing error(' ') or error('bad!') does.

So does having an error with an empty string ignore the error, or am I doing something wrong?

% Just a file to understand the Matlab command try/catch

M = 3;
j = 1;
k = [Inf, 5, 4];

while M>0
    try
        M = M-1
        u = k(j)
        if (isinf(u)||isnan(u)), error(''), end;
    catch
        j = j+1
    end
end

chappjc · Accepted Answer · 2013-12-04 19:39:18Z

3

Yes, error('') and error([]) and error(struct([])) all do not actually display an error message and abort running code. I personally consider the use of the single string argument version of error to be bad practice in any real code. You should use always use both a 'MSGID' and a 'ERRMSG' when writing errors for your functions, e.g.

error('FunctionName:SubFunctionName:ErrorMSGID','Error message to be printed.')

Alternatively, you can use MException objects in conjuction with throw, rethrow, and throwAsCaller, which allow you to reuse error information. More here.

edited Dec 4, 2013 at 19:39

chappjc

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answered Dec 4, 2013 at 18:52

horchler

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2 Comments

Catalina Over a year ago

Thank you! It's good to have a confirmation for this. In the documentation for error it says: "All string input arguments must be enclosed in single quotation marks. If msgString is an empty string, the error command has no effect." I should have searched for "error" instead of "try/catch"! Thank you. :)

chappjc Over a year ago

Rolled back since I was mistaken... sorry.

chappjc · Accepted Answer · 2013-12-04 18:54:34Z

2

It is odd, but it's in the documentation for error, for the error('msgString') syntax:

All string input arguments must be enclosed in single quotation marks. If msgString is an empty string, the error command has no effect.

Similarly, if using the error(msgStruct) syntax:

If msgStruct is an empty structure, no action is taken and error returns without exiting the function.

answered Dec 4, 2013 at 18:54

chappjc

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Comments

Vuwox · Accepted Answer · 2013-12-04 18:55:00Z

0

if you have a look to the try documentation you can have an example.

Else want you want for your code it :

M = 3;
j = 1;
k = [Inf, 5, 4];

while M>0
    try
        M = M-1
        u = k(j)
        if (isinf(u)||isnan(u)), error(''), end;
    catch
        disp('I catch an error!');
        j = j+1
    end
end

Because If you never get an error in your code, it will never go in the catch. So by including error('');, it just to say, go execute the statement in the catch.

But you can just modify your code by replacing the error() by the statements into your catch like this :

while M>0
        M = M-1
        u = k(j)
        if (isinf(u)||isnan(u)), j = j+1, end;
end

EDIT

If you take a look in the documentation, you can found this :

%   ERROR(MSGSTRUCT) reports the error using fields stored in the scalar
%   structure MSGSTRUCT. This structure can contain these fields:
%
%       message    - Error message string
%       identifier - See MESSAGE IDENTIFIERS, below
%       stack      - Struct similar to the output of the DBSTACK function
%  
%   If MSGSTRUCT is an empty structure, no action is taken and ERROR
%   returns without exiting the program. If you do not specify the
%   stack, the ERROR function determines it from the current file and line.

So no action is taken as you can read. And nothing, so catch don't get any informations.

edited Dec 4, 2013 at 18:55

answered Dec 4, 2013 at 18:41

Vuwox

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2 Comments

Catalina Over a year ago

My question is about using error('') versus error('nonempty string'). The top case still doesn't go to the catch (for me) unless there is a nonempty string in the error. I was wondering if it's standard for error('') to make errors be ignored. I like the second example. Maybe I will have to edit the code to work like this, but right now I am trying to understand a code that is more complicated than my example (but similar).

Vuwox Over a year ago

@Catalina, Oh! You're right. I never experiment that. Because normally when I throw error, I use message or use it like this error();. Let me have a look on that. I'll come with another answer.

yuk · Accepted Answer · 2013-12-04 18:58:52Z

0

Not sure why you need it, but here is how it works.

error function does not throw an error with empty string or empty vector ([]) as an argument.

If you don't specify argument at all the error function itself generates the error "Not enough arguments". So it will go to catch.

Another way is to specify an empty structure as an argument.

s = struct();
error(s)

In this case, the error will be generated, but the code will not stop and in general flow you will hear no beep. In your case it should go to catch.

answered Dec 4, 2013 at 18:58

yuk

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4 Comments

chappjc Over a year ago

That's not an empty struct, it's a struct with no fields. You need to do s = struct([]); for a 0x0 struct array. Then it will not error.

yuk Over a year ago

I thought the question was to throw the error but have no output. This is exactly what error(struct()) do. error(struct([])) will not throw the error, same as error('').

chappjc Over a year ago

Oh, I think the OP was just curious why error('') didn't throw an error, hence the discussions about empty structs being the same way. But since you point it out, I'd say you have a nice way of tripping an error with no message. :)

yuk Over a year ago

Although it does not make any sense to me. :))) I mean a purpose to do it.

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