(octave.info)Errors

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2.5 How Octave Reports Errors
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Octave reports two kinds of errors for invalid programs.

   A “parse error” occurs if Octave cannot understand something you have
typed.  For example, if you misspell a keyword,

     octave:13> function y = f (x) y = x***2; endfunction

Octave will respond immediately with a message like this:

     parse error:

       syntax error

     >>> function y = f (x) y = x***2; endfunction
                                   ^

For most parse errors, Octave uses a caret (‘^’) to mark the point on
the line where it was unable to make sense of your input.  In this case,
Octave generated an error message because the keyword for exponentiation
(‘**’) was misspelled.  It marked the error at the third ‘*’ because the
code leading up to this was correct but the final ‘*’ was not
understood.

   Another class of error message occurs at evaluation time.  These
errors are called “run-time errors”, or sometimes “evaluation errors”,
because they occur when your program is being “run”, or “evaluated”.
For example, if after correcting the mistake in the previous function
definition, you type

     octave:13> f ()

Octave will respond with

     error: `x' undefined near line 1 column 24
     error: called from:
     error:   f at line 1, column 22

This error message has several parts, and gives quite a bit of
information to help you locate the source of the error.  The messages
are generated from the point of the innermost error, and provide a
traceback of enclosing expressions and function calls.

   In the example above, the first line indicates that a variable named
‘x’ was found to be undefined near line 1 and column 24 of some function
or expression.  For errors occurring within functions, lines are counted
from the beginning of the file containing the function definition.  For
errors occurring outside of an enclosing function, the line number
indicates the input line number, which is usually displayed in the
primary prompt string.

   The second and third lines of the error message indicate that the
error occurred within the function ‘f’.  If the function ‘f’ had been
called from within another function, for example, ‘g’, the list of
errors would have ended with one more line:

     error:   g at line 1, column 17

   These lists of function calls make it fairly easy to trace the path
your program took before the error occurred, and to correct the error
before trying again.