compress string of data

[Terry Reedy]

"Josiah Carlson" <jcarlson at nospam.uci.edu> wrote in message
news:bv3bdd$a0j$3 at news.service.uci.edu...
> Piet van Oostrum wrote:
>
> >>>>>>Gerrit Holl <gerrit at nl.linux.org> (GH) wrote:
> >
> >
> >>>How i could compress string of data?.
> >>>
> >>>>>import zlib
> >>>>>zlib.compress("Hello, world")
> >
> > GH> 'x\x9c\xf3H\xcd\xc9\xc9\xd7Q(\xcf/\xcaI\x01\x00\x1b\xd4\x04i'
> >
> > GH> http://www.python.org/doc/current/lib/module-zlib.html
> >
> > Waw, that compresses it from 12 bytes to 20 bytes :=)
>
>
> There is no such thing as universal compression

Only if you specify invertibility (losslessness).

f(s) == '' compressess everything except '' itself ;-)

and that len(f(s)) < len(s) for some s and not just <= for all s (which
allows identity 'compression' f(s) == s).

> - some compressions  result in expansion.

Which is to say that if a 1-1 function maps some strings to smaller
strings, it must correspondingly map others to larger strings.  An
interesting CS tidbit that people keep discovering by stumbling over.

Lossless universal expansion is possible: f(s) == ' '+s.  But it is into
rather than onto, so only strings starting with ' ' can be inversely
mapped.


Terry J. Reedy