Re: Interpreter for old basic programs from the line number era

On 7 Mrz., 20:38, ArarghMail803NOS...@[EMAIL PROTECTED]
 wrote:
> On Fri, 7 Mar 2008 17:12:49 +0000 (UTC), "Auric__"
>
>
>
> <not.my.r...@[EMAIL PROTECTED]
> wrote:
> >On Thu, 06 Mar 2008 20:05:46 GMT,  wrote:
>
> <snip>
> >> The log file of a very small example program:
>
> >>   100 PRINT "Hello, world!"
> >>   105 IF I=0THEN 107ELSE 110
> >>   107 IF I=1THEN 110ELSE 110
> >>   110 INPUT "Press return to continue",a$
>
> >Wow. Hard to believe that the lack of spaces in 105 and 107 are valid
> >(or even accepted). Yikes. (I realize that isn't something you came up
> >with -- it works under QBASIC. Yikes yikes yikes.)
>
> Not hard to believe at all.  :-)  The only time a space is reallt
> needed in basic is to seperate 2 tokens that can't otherwise be
> identified.  For example on line 105:
> The space between 'IF' & 'I' is needed because htat would be seen as
> 'IFI'.
> No space is needed between '0' & 'THEN' because a number can not
> contain a 'T'.
>
> If the word following a number started with a 'e' or 'd' then a space
> is probably required because floating point numbers can contain a 'd'
> or 'e'.

What you said is OK when an LL(1) scanner (which uses
a lookup of one character) is used. Since upper and lower
case characters are equivalent and old BASIC programs
contain sequences like "107ELSE' or '107else' I had no
option and had to switch to LL(2) in this case.
When the letter after the 'E' or 'e' is a sign ('+ or '-')
or a digit they belong all to the float literal. In
all other cases the 'E' or 'e' is the first character
of the next symbol.

Greetings Thomas Mertes

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