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From: nath@cernvax.cern.ch (Alfred Nathaniel)
Newsgroups: comp.text.tex
Subject: TeX bug: optional spaces are mandatory
Message-ID: <4117@cernvax.cern.ch>
Date: 8 Feb 91 14:51:35 GMT
Organization: CERN, European Laboratory for Particle Physics
Lines: 108

I discovered a bug in TeX3.0. It also shows up in TeX2.9. Going through the 
change records I couldn't find any reference to it. So I suppose that it has 
been there for much longer.

In sequences

     ...\counter=<constant>\ifnum\counter...
and

     ...\counter=<constant>\fi \ifnum\counter...

the \ifnum test fails if <constant> is not followed by a space.

The problem is in this part of scan_int:
-------------------------------------------------------------------------
@<Accumulate the constant...@>=
loop@+  begin if (cur_tok<zero_token+radix)and(cur_tok>=zero_token)and
    (cur_tok<=zero_token+9) then d:=cur_tok-zero_token
  else if radix=16 then
    if (cur_tok<=A_token+5)and(cur_tok>=A_token) then d:=cur_tok-A_token+10
    else if (cur_tok<=other_A_token+5)and(cur_tok>=other_A_token) then
      d:=cur_tok-other_A_token+10
    else goto done
  else goto done;
  vacuous:=false;
  if (cur_val>=m)and((cur_val>m)or(d>7)or(radix<>10)) then
    begin if OK_so_far then
      begin print_err("Number too big");
@.Number too big@>
      help2("I can only go up to 2147483647='17777777777=""7FFFFFFF,")@/
        ("so I'm using that number instead of yours.");
      error; cur_val:=infinity; OK_so_far:=false;
      end;
    end
  else cur_val:=cur_val*radix+d;
  get_x_token;
  end;
done:
-------------------------------------------------------------------------
To look for another digit belonging to <constant> get_x_token is called.
But if the next token is \ifnum the condition is evaluated before scan_int
did realize that <constant> is complete and has to be stored in \counter.

Here is an example where the bug occurs:

-------------------------------------------------------------------------
\tracingcommands=2
%
\count255=0
\count255=999\ifnum \count255=0 \message{[BOGUS: \the\count255<>0]} \fi
%
\count255=0
\ifnum 0=0 \count255=888\fi
\ifnum \count255=0 \message{[BOGUS: \the\count255<>0]} \fi
%
\end
-------------------------------------------------------------------------
This is TeX, C Version 3.0 (format=plain 91.1.28)  7 FEB 1991 16:23
**ifnumbug
(ifnumbug.tex
{vertical mode: \count}
{\count}
{\ifnum}
{true}
{\message}
 [BOGUS: 999<>0]
{blank space  }
{\fi}
{\count}
{\ifnum}
{true}
{\count}
{\fi}
{\ifnum}
{true}
{\message}
 [BOGUS: 888<>0]
{blank space  }
{\fi}
{\end}
 )
No pages of output.
-------------------------------------------------------------------------

Simply replacing get_x_token by get_token fixes this bug but the trip test
falls flat on its face. I would classify it as category S according to
"The Errors of TeX".

A nasty feature is that it is a Heisenbug. If you look at it it goes away.
Computing the absolute value of a macro argument:
      \counter=#1\ifnum\counter<0\counter=-\counter\fi
may fail depending on the initial value of \counter while:
      \counter=#1\showthe\counter\ifnum\counter<0\counter=-\counter\fi
always gives the correct result as does:
      \counter=#1 \ifnum\counter<0\counter=-\counter\fi

The lesson I learned from this that when writing TeX macros optional spaces
are sometimes mandatory.

Is this the right channel for bug reports? If not could somebody who knows
forward it or let me know where to send it to.

Best regards,
Alfred Nathaniel

Internet: nath@cernvax.cern.ch = 128.141.1.74
          nathanie@dxlmun.cern.ch = 128.141.1.121
  Bitnet: nathanie@cernvm