#### 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

Alternatively, the output of the `lasterror` function can be found in a variable indicated immediately after the `catch` keyword, as in the example below showing how to redirect an error as a warning:

    try
      …
    catch err
      warning(err.identifier, err.message);
      …
    end_try_catch

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, error, lastwarn.

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, error, lastwarn.

It is also possible to assign an identification string to an error. If an error has such an ID the user can catch this error as will be shown in the next example. To assign an ID to an error, simply call `error` with two string arguments, where the first is the identification string, and the second is the actual error. Note that error IDs are in the format `"NAMESPACE:ERROR-NAME"`. The namespace `"Octave"` is used for Octave’s own errors. Any other string is available as a namespace for user’s own errors.

The next example counts indexing errors. The errors are caught using the field identifier of the structure returned by the function `lasterror`.

    number_of_errors = 0;
    for n = 1:100
      try
        …
      catch
        id = lasterror.identifier;
        if (strcmp (id, "Octave:invalid-indexing"))
          number_of_errors++;
        endif
      end_try_catch
    endfor

The functions distributed with Octave can issue one of the following errors.

`Octave:invalid-context`

  Indicates the error was generated by an operation that cannot be executed in the scope from which it was called. For example, the function `print_usage ()` when called from the Octave prompt raises this error.

`Octave:invalid-input-arg`

  Indicates that a function was called with invalid input arguments.

`Octave:invalid-fun-call`

  Indicates that a function was called in an incorrect way, e.g., wrong number of input arguments.

`Octave:invalid-indexing`

  Indicates that a data-type was indexed incorrectly, e.g., real-value index for arrays, nonexistent field of a structure.

`Octave:bad-alloc`

  Indicates that memory couldn’t be allocated.

`Octave:undefined-function`

  Indicates a call to a function that is not defined. The function may exist but Octave is unable to find it in the search path.

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.