[OS X TeX] finding missing braces

Roussanka Loukanova rloukano at stp.lingfil.uu.se
Tue Mar 6 12:40:17 CET 2007


On Mon, 5 Mar 2007, Herbert Schulz wrote:

> On Mar 4, 2007, at 5:05 PM, Roussanka Loukanova wrote:
>>> Howdy,
>>> I believe the problem is intractable. How can a program know what you 
>>> intended? E.g., suppose you leave off a closing brace in a sentence,
>>> \emph{emphasized text ...
>>> how will a program know whether the closing brace goes after the word 
>>> `emphasized' or the word `text', or ...?
>> Right: In cases like the above, \emph{emphasized text ..., the error notice 
>> in the log (e.g., the console in TeXShop) may say that the scope of the 
>> unmatched "{" goes to the last line of the entire tex file.
> Howdy,
> Just as a note: since \emph{...} is meant for short snippets it will stop and 
> complain if you come the end of a paragraph and it hasn't reached a closing 
> brace. I.e., it is a ``short'' command.

A straight note since I did miss to pay attention to this specific feature 
of \emph{...}. I used the word "may" for far more general reasons.
But, the \emph{... is a good example as a reason for a (compiler-like) 
program to leave the possible location of the missing closing brace 
in similar cases unspecified, since this is only up to the user's 

Other than this specific feature of \emph{...}, the error notice may 
point to the end of a scope which encompasses an unmatched brace (or 
a similar delimiter), and that may be the end of a scope which is not the 
narrowest enclosing one.

Detection of mismatched delimiters and error notice, in general, depend 
on what algorithm for well-balanced delimiters ({, }, (, ), $, etc.) has 
been implemented (in this case, in TeX, and also, in TeXShop or Emacs, 
etc. editors, which we use not only to type, but also to get detected 
balanced-unbalanced delimiters, and, ideally, to see their scope, for ex. 
by: the text between matched delimiters is highlighted; the cursor 
jumps to the matching opening brace/parenthesis/$.

There exists no algorithm that would simply count the number of 
occurrences of opening and closing delimiters to check for 
well-matching, just by looking if the two numbers are equal.

A popular simple algorithm uses an auxiliary stack to check for 
matched and unmatched delimiters as follows (loosely described):

Read the characters of the input string (representing the text) from left 
to right

Case 1: the input is not (yet) empty

- for each input "("
    = save a copy of "(" on the top of the stack

- for each input ")"
    = delete the "(" from the top of the stack, if there is such "(", (this 
is the matching step), otherwise
    = i.e., the stack is empty: stop and output "unbalanced )"

Case 2: the input is empty

- the stack is empty: OK, otherwise
- i.e., the stack has a "(" on its top: stop and output "unbalanced ("


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