newlispfanclub.alh.net

In C programming sometimes to look ahead at the next file character a ch=getc(infile) is done and then if the logic requires that ch not be handled now a ungetc( ch, infile) is used to put the char back for later processing.
How feasible would it be to have the same facility available for (read-char
viz (unread-char ? I tried (write-char back into a file opened as update but that doesn't work. viz
> (set 'aFile (open "myfile.ext" "u"))
1
> (setq ch (read-char aFile))
97
> (write-char aFile ch)
1
> (setq ch (read-char aFile))
113
>

If it's not easy at the C source level for newlisp I'll try a getc & ungetc with defines or macros (anybody got such functions?).

I want getc/ungetc for a fairly 'no thinking please' C to newlisp port of a C program.

Regards
Nigel

Nigel

PS I believe Common Lisp has peek-char to let you look 1 char ahead into a stream - saves doing get then unget - instead doing a peek then a get if needed.

Nigel

Some 'unread-char' function is easy to implement, I will put it on the list.

BTW, I did the commandline switch to limit memory usage, that will be in the next development version.

Lutz

Thanks for the memories
usage check

Nigel

this is the definition of an 'unread-char':

(define (unread-char fle) (seek fle (- (seek fle) 1)))

it returns the decremented file position and is surprisingly fast compared with the 'read-char' on a 1.4Ghz Celeron and WinXP:

read-char => 1.8 micro seconds
unread-char => 2.6 micro seconds

Note, that the function will wrap around to the end of the file when the beginning (position 0) is reached.

Lutz