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

[Peter Brooks]

Sorry  - my timing shows, of course, that the awk algorithm is the fastest
- then using 'rev' - then calling 'rev' from awk.


On Thu, 25 Apr 2019 at 10:26, Peter Brooks <address@hidden>
wrote:

> Actually, though, the awk version is considerably faster than calling
> 'rev' from awk - even though slower than 'rev' itself. Here are some
> timings:
>
> $./test_time.sh
> awk_rev:  min: 0.024 max: 0.030 mean: 0.02496 std: 0.00096871
> pure_rev:  min: 0.004 max: 0.005 mean: 0.00406 std: 0.000237487
> pure_awk:  min: 0.003 max: 0.004 mean: 0.00301 std: 9.94987e-05
>
>
>
>
> Timing script ( test_time.sh ):
>
> #!/bin/bash
> awk_script=$(mktemp /tmp/awk_script.XXXX)
> cat >$awk_script <<"@@END"
> BEGIN {
> min=9999999999999999999999;
> max=-min;
> }
>     {
>     if ($0 < min ) min = $0;
>     if ($0 > max ) max = $0;
>     count++;
>     sum+=$0;
>     sq_sum += $0*$0;
>     }
> END {
>     if ( min == max )
> {
> mean = min;
> std = 0;
> variance=0;
> }
>     else
> {
> mean = sum / count;
> variance = sq_sum / count - mean * mean;
> std = sqrt(variance);
> }
>     print FILENAME ":  min: " min " max: " max " mean: " mean " std: " std;
>     }
> @@END
> rm -f awk_rev pure_rev pure_awk
> for i in {1..100}
> do
> (time (cat short|awk ' {rev="echo " "\""$0"\"" "|rev";rev|getline
> backwards;close(rev);print backwards;}' - >/dev/null) ) 2>&1| grep sys|sed
> 's/sys //g'|sed 's/0m//'|sed 's/s//' >>awk_rev
> (time (cat short|rev >/dev/null)  )2>&1| grep sys|sed 's/sys //g'|sed
> 's/0m//'|sed 's/s//' >>pure_rev
> (time (echo short | awk '{ for(i=length;i!=0;i--)x=x
> substr($0,i,1);}END{print x}' >/dev/null) ) 2>&1| grep sys|sed 's/sys //g'|sed
> 's/0m//'|sed 's/s//' >>pure_awk
> done
> for i in awk_rev pure_rev pure_awk
>     do
>     awk -f $awk_script $i
>     done
> rm awk_rev pure_rev pure_awk
> rm $awk_script
>
>
>
>
>
>
>
>
>
> On Thu, 25 Apr 2019 at 09:28, Peter Brooks <address@hidden>
> wrote:
>
>> Yes, using it in awk is simple:
>>
>> echo "The quick brown fox jumps over the lazy dog"|awk ' {rev="echo "
>> "\""$0"\"" "|rev";rev|getline backwards;close(rev);print backwards;}' -
>>
>>
>>
>>
>> On Thu, 25 Apr 2019 at 08:21, 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
>>>>
>>>
>>
>> --
>> Peter Brooks
>>
>>
>> Mobile: +27 82 717 6404
>> Skype:  Fustbariclation
>> Twitter: Fustbariclation
>> Author Page: amazon.com/author/peter_brooks
>>
>
>
> --
> Peter Brooks
>
>
> Mobile: +27 82 717 6404
> Skype:  Fustbariclation
> Twitter: Fustbariclation
> Author Page: amazon.com/author/peter_brooks
>


-- 
Peter Brooks


Mobile: +27 82 717 6404
Skype:  Fustbariclation
Twitter: Fustbariclation
Author Page: amazon.com/author/peter_brooks