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

[Peng Yu]

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