Re: interpreting a strongly typed regexp constant variable as literal regular expression?

[Manuel Collado]

El 30/05/2020 a las 18:28, Craig escribió:
> Ok,
>
> This was originally asked in comp.lang.awk

And subsequently answered in comp.lang.awk.

> For the given examples,  Script 1 generates the desired output.
> The desired output for script 2 should be the same the output
> generated by script 1.
> As provided, in script 2, the 'Yes" is printed as the 4th record
> instead of 2nd record because @/var/ isn't interpreted as an
> actual regular expression, but as a string (as in script 3's
> example).
>
> typeof() shows the script 2 variable as being type regexp.
>
> Thanks,
> Craig
>
> ===== script 1
> awk '{
> switch($0) {
>     case /^a/ :
>        print "Yes"
>     break
>     default :
>        print "NR: " NR
>     break
>     }
> }' << END
> g
> a
> c
> @/a/
> END
>
> ==== script 1 output:
> NR: 1
> Yes
> NR: 3
> NR: 4
>
> ===== script 2
> awk '{ var = @/^a/
> switch($0) {
>     case @/var/ :
>        print "Yes"
>     break
>     default :
>        print "NR: " NR
>     break
>     }
> }' << END
> g
> a
> c
> @/var/
> END
>
> ==== script 2 output
> NR: 1
> NR: 2
> NR: 3
> Yes

El 30/05/2020 a las 21:50, Craig escribió: (private mail)
> I'd be nice to just be able to just add @ before the switch keyword,
> ( aka @switch) to indicate to awk that at runtime the strongly typed
> regular expression(s) in switch block should be updated with constants
> before awk executes the @switch block.

The switch statement requires constant values as case labels. If you 
want them to be evaluated dynamically, just replace the switch with a 
cascade of if/else

{ var = @/^a/
 if ($0 ~ var) {
        print "Yes"
 } else {
        print "NR: " NR
 }
}

--
Manuel Collado - http://mcollado.z15.es