Re: [bug-gawk] Efficient way to reverse a string

[arnold]

See https://www.gnu.org/software/gawk/manual/html_node/Function-Example.html
for an example of a function that reverses a string in memory.

It should be efficient enough.  I am not going to discuss this further
and would appreciate bringing the thread to a close.

Arnold

Peng Yu <address@hidden> wrote:

> I don???t need to print the reversed string but get the reversed string in
> memory.
>
> On Thu, Apr 25, 2019 at 7:01 AM Peter Brooks <address@hidden>
> wrote:
>
> > Using printf instead of print - leaving out building the buffer in x is
> > quicker:
> >
> > $ time (echo short | awk '{ for(i=length;i!=0;i--)x=x
> > substr($0,i,1);}END{print x}' >/dev/null)
> >  real 0m0.324s
> > user 0m0.003s
> > sys 0m0.007s
> > $ time (echo short | awk '{ for(i=length;i!=0;i--)printf
> > "%c",substr($0,i,1);}' >/dev/null)
> > real 0m0.028s
> > user 0m0.003s
> > sys 0m0.003s
> >
> > I'm quite impressed at how much awk is faster than rev.
> >
> > On Thu, 25 Apr 2019 at 12:50, Peng Yu <address@hidden> wrote:
> >
> >> So there is not a way to efficiently reverse a string. I think the
> >> problem is that awk strings are not mutable. Wouldn't it make sense to
> >> allow users to allocate a string with enough space and fill up the
> >> content character by character. This sounds to be the only to do an
> >> efficient string reverse.
> >>
> >> On 4/25/19, david kerns <address@hidden> wrote:
> >> > I think he's asking in the context of an awk program...
> >> >
> >> > I don't think you can get away from concat w/out using printf on each
> >> char,
> >> > maybe buffering will save you, but intuitively, I would expect better
> >> > performance out of concat on a string with one print at the end...
> >> >
> >> > $ echo abcdefg 123 456 ABC | gawk '{n=split($0, a, "");for
> >> > (i=n;i>0;i--)printf("%s",a[i]);printf("\n")}'
> >> > CBA 654 321 gfedcba
> >> > $ echo abcdefg 123 456 ABC | gawk
> >> > 'BEGIN{FS=""}{for(i=NF;i>0;i--)printf("%s",$(i));printf("\n")}'
> >> > CBA 654 321 gfedcba
> >> >
> >> > I would expect these concat versions to perform better, but I leave
> >> that as
> >> > an exercise for the student ;)
> >> >
> >> > $ echo abcdefg 123 456 ABC | gawk
> >> 'BEGIN{FS=""}{x="";for(i=NF;i>0;i--)x=x
> >> > $(i);print x}'
> >> > CBA 654 321 gfedcba
> >> >
> >> > $ echo abcdefg 123 456 ABC | gawk '{n=split($0,a,"");for(i=n;i>0;i--)x=x
> >> > a[i];print x}'
> >> > CBA 654 321 gfedcba
> >> >
> >> > in addition to the substr() example in your referenced post
> >> >
> >> >
> >> >
> >> > On Wed, Apr 24, 2019 at 10:57 PM Peter Brooks <
> >> address@hidden>
> >> > wrote:
> >> >
> >> >> Why not just use 'rev' - as an embedded command if you want the result
> >> in
> >> >> awk:
> >> >>
> >> >> $ echo the quick brown fox jumps over the lazy dog|rev
> >> >>
> >> >> god yzal eht revo spmuj xof nworb kciuq eht
> >> >>
> >> >>
> >> >> On Thu, 25 Apr 2019 at 05:06, Peng Yu <address@hidden> wrote:
> >> >>
> >> >> > Hi,
> >> >> >
> >> >> > The following way to reverse a string seems to be inefficient as it
> >> >> > involves multiple concatenations.
> >> >> >
> >> >> >
> >> >> >
> >> >>
> >> https://www.unix.com/shell-programming-and-scripting/223077-awk-reverse-string.html
> >> >> >
> >> >> > Is there a more efficient way to reverse a string? Thanks.
> >> >> >
> >> >> > --
> >> >> > Regards,
> >> >> > Peng
> >> >> >
> >> >> >
> >> >>
> >> >> --
> >> >> Peter Brooks
> >> >>
> >> >>
> >> >> Mobile: +27 82 717 6404
> >> >> Skype:  Fustbariclation
> >> >> Twitter: Fustbariclation
> >> >> Author Page: amazon.com/author/peter_brooks
> >> >>
> >> >
> >>
> >>
> >> --
> >> Regards,
> >> Peng
> >>
> >
> >
> > --
> > Peter Brooks
> >
> >
> > Mobile: +27 82 717 6404
> > Skype:  Fustbariclation
> > Twitter: Fustbariclation
> > Author Page: amazon.com/author/peter_brooks
> >
> -- 
> Regards,
> Peng