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: err = lasterror (err)

Built-in Function: lasterror ('reset')

Returns or sets the last error message. Called without any arguments returns a structure containing the last error message, as well as other information related to this error. The elements of this 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 might be an empty structure if this in the case where this information cannot be obtained. The fields of this 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 err structure may also be passed to lasterror to set the information about the last error. The only constraint on err in that case is that it is a scalar structure. Any fields of err that match the above are set to the value passed in err, while other fields are set to their default values.

If lasterror is called with the argument 'reset', all values take their default values.

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

Without any arguments, return the last error message. With one argument, set the last error message to msg. With two arguments, also set the last message identifier.

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)

Reissues 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.