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

[Peter Brooks]

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


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