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12.1.2 Catching Errors

When an error occurs, it can be detected and handled using the try statement as described in The try Statement. As an example, the following piece of code counts the number of errors that occurs during a for loop. 

     number_of_errors = 0;
     for n = 1:100
       try
         ...
       catch
         number_of_errors++;
       end_try_catch
     endfor

The above example treats all errors the same. In many situations it can however be necessary to discriminate between errors, and take different actions depending on the error. The lasterror function returns a structure containing information about the last error that occurred. As an example, the code above could be changed to count the number of errors related to the ‘*’ operator. 

     number_of_errors = 0;
     for n = 1:100
       try
         ...
       catch
         msg = lasterror.message;
         if (strfind (msg, "operator *"))
           number_of_errors++;
         endif
       end_try_catch
     endfor

— Built-in Function: lasterr = lasterror ()
— Built-in Function: lasterror (err)
— Built-in Function: lasterror ('reset')

Query or set the last error message structure. When called without arguments, return a structure containing the last error message and other information related to this error. The elements of the structure are:

'message'
The text of the last error message
'identifier'
The message identifier of this error message
'stack'
A structure containing information on where the message occurred. This may be an empty structure if the information cannot be obtained. The fields of the structure are:
'file'
The name of the file where the error occurred
'name'
The name of function in which the error occurred
'line'
The line number at which the error occurred
'column'
An optional field with the column number at which the error occurred

The last error structure may be set by passing a scalar structure, err, as input. Any fields of err that match those above are set while any unspecified fields are initialized with default values.

If lasterror is called with the argument 'reset', all fields are set to their default values.

See also: lasterr.

— Built-in Function: [msg, msgid] = lasterr ()
— Built-in Function: lasterr (msg)
— Built-in Function: lasterr (msg, msgid)

Query or set the last error message. When called without input arguments, return the last error message and message identifier. With one argument, set the last error message to msg. With two arguments, also set the last message identifier.

See also: lasterror.

When an error has been handled it is possible to raise it again. This can be useful when an error needs to be detected, but the program should still abort. This is possible using the rethrow function. The previous example can now be changed to count the number of errors related to the ‘*’ operator, but still abort if another kind of error occurs. 

     number_of_errors = 0;
     for n = 1:100
       try
         ...
       catch
         msg = lasterror.message;
         if (strfind (msg, "operator *"))
           number_of_errors++;
         else
           rethrow (lasterror);
         endif
       end_try_catch
     endfor

— Built-in Function: rethrow (err)

Reissue a previous error as defined by err. err is a structure that must contain at least the 'message' and 'identifier' fields. err can also contain a field 'stack' that gives information on the assumed location of the error. Typically err is returned from lasterror.

See also: lasterror, lasterr, error.

— Built-in Function: err = errno ()
— Built-in Function: err = errno (val)
— Built-in Function: err = errno (name)

Return the current value of the system-dependent variable errno, set its value to val and return the previous value, or return the named error code given name as a character string, or -1 if name is not found.

— Built-in Function: errno_list ()

Return a structure containing the system-dependent errno values.