develooper Front page | perl.perl6.users | Postings from August 2020

Re: Learning the "ff" (flipflop) infix operator? (was Re: Rakuversion of "The top 10 tricks... .")

Thread Previous | Thread Next
From:
William Michels via perl6-users
Date:
August 2, 2020 02:06
Subject:
Re: Learning the "ff" (flipflop) infix operator? (was Re: Rakuversion of "The top 10 tricks... .")
Message ID:
CAA99HCyMtzYr-snkDDR+vy+=exLo4eCHC4U7UxiQUwir2=ppsQ@mail.gmail.com
Hi Yary, Nice code!

The general approach of using an anonymous counter is useful to me. Below
are  examples when I only want to recover the first one or two blocks of
text starting with "Start" and ending with "Mark" (nota bene: I took your
example text and deleted the blank lines):

user@book:~$ raku -ne ' say $_ if ($_ eq "Start" && $++ < 1) fff ($_ eq
"Mark" && $++ < 1);' yary_ff_example2.txt
Start
hi print me
yes!
Mark
user@mbook:~$ raku -ne ' say $_ if ($_ eq "Start" && $++ < 1) ff ($_ eq
"Mark" && $++ < 1);' yary_ff_example2.txt
Start
hi print me
yes!
Mark
user@mbook:~$ raku -ne ' say $_ if ($_ eq "Start" && $++ < 2) fff ($_ eq
"Mark" && $++ < 2);' yary_ff_example2.txt
Start
hi print me
yes!
Mark
Start
We're back!
Mark
user@mbook:~$ raku -ne ' say $_ if ($_ eq "Start" && $++ < 2) ff ($_ eq
"Mark" && $++ < 2);' yary_ff_example2.txt
Start
hi print me
yes!
Mark
Start
We're back!
Mark
Still here!
Start
haha that Start does nothing
going to end it now
Mark
!bye bye don't see me!
user@mbook:~$

I guess I have to say--I'm still a little surprised by the last result
using the "ff" infix operator. I'd appreciate knowing why "ff" and "fff"
behave differently in the last two examples, since the beginning marker
doesn't look anything like the end marker (suggesting they should act
identically). Also, is there a simpler way to write the conditional?

Thx, Bill.



On Sat, Aug 1, 2020 at 4:04 PM yary <not.com@gmail.com> wrote:
>
> This made me want to try a contrived puzzle, use 'fff' to show things
between a "start" and 2nd "mark" line. That is, print any line below not
marked with "!" at the start
>
> $ cat example.txt
>
> !ignore me
>
> Start
>
> hi print me
>
> yes!
>
> Mark
>
> still print me
>
> Mark
>
> !ignore this line
>
> !this line too
>
> Start
>
> We're back!
>
> Mark
>
> Still here!
>
> Start
>
> haha that Start does nothing
>
> going to end it now
>
> Mark
>
> !bye bye don't see me!
>
>
> Let's see...ooh that was easy!!
>
> raku -ne '.say if "Start" ff ($_ eq "Mark" && ++$ %% 2)' example.txt
>
>
> That increments the anonymous state variable $ and then checks if it is
divisible by 2, so that only every 2nd Mark returns True
>
> Don't know if I'll ever need it, fun to have it.
>
> -y
>
>
> On Tue, Jul 28, 2020 at 7:09 PM Brad Gilbert <b2gills@gmail.com> wrote:
>>
>> A regex doesn't have to match the entire string.
>>
>>     'abcd' ~~ / bc /
>>     # 「bc」
>>
>> A string has to match exactly with the smart-match. (`ff` and `fff` do
smart-match)
>>
>>     'abcd' ~~ 'bc' # False
>>     'abcd' ~~ 'abcd' # True
>>
>> A string inside of a regex only makes that a single atom, it does not
make it match like just a string.
>>
>>     'abcd' ~~ / 'bc' /
>>     # 「bc」
>>
>>      'aBaBaB' ~~ / aB+ /
>>     「aB」
>>
>>      'aBaBaB' ~~ / "aB"+ /
>>     「aBaBaB」
>>
>> In fact a string inside of a regex doesn't do much more than square
brackets.
>>
>>      'aBaBaB' ~~ / [aB]+ /
>>     「aBaBaB」
>>
>> If you want the regex to match fully, add a beginning of string and end
of string marker.
>>
>>     'abcd' ~~ / ^ bc $ /
>>     # Nil
>>
>>     'abcd' ~~ / ^ abcd $ /
>>     # 「abcd」
>>
>> ---
>>
>> Since `ff` can begin and end at the same time, the following is turning
on and off at almost every iteration of the loop after it starts.
>>
>>     $ raku -ne '.put if /star {print q[on ]}/ ff /start {print q[off ]}/
;' startling.txt
>>     on star
>>     on off start
>>     on off startl
>>     on off startli
>>     on off startlin
>>     on off startling
>>
>> On Tue, Jul 28, 2020 at 1:43 PM William Michels <wjm1@caa.columbia.edu>
wrote:
>>>
>>> Thank you, Brad and Larry, for explaining the "ff" and "fff" infix
>>> operators in Raku to me!
>>>
>>> I have to admit that I'm still fuzzy on the particulars between "ff"
>>> and "fff", since I am not familiar with the sed function. I can
>>> certainly understand how useful these functions could be to 'pull out
>>> all PGP signatures' from a file (which was the Perl5 example given in
>>> the Oracle Linux Blog). So I can now  pull out the html "head" section
>>> from the page _ https://raku.org/fun/ _ (saved locally as file
>>> "fun.txt") using the following Raku code:
>>>
>>> user@mbook:~$ raku -ne '.put if Q[<head>] ff Q[</head>]' fun.txt
>>> <head>
>>>     <meta charset="utf-8">
>>>     <meta http-equiv="X-UA-Compatible" content="IE=edge">
>>>     <meta name="viewport" content="width=device-width, initial-scale=1">
>>>     <title>Raku is optimized for fun!</title>
>>>
>>>
>>>     <link href="/favicon.ico" rel="shortcut icon" type="image/x-icon">
>>>     <link href="/bootstrap/css/bootstrap.min.css" rel="stylesheet">
>>>     <link href="/bootstrap/css/bootstrap-theme.min.css"
rel="stylesheet">
>>>     <link href="/style.css?v=1" rel="stylesheet">
>>>
>>> </head>
>>> user@mbook:~$
>>>
>>> What I'm less clear on is how the code below is functioning. I first
>>> print out file named "startling.txt" with 'cat':  it's supposed to
>>> stand in for a text delimited linewise by "header 1", "header 2", etc.
>>> After the 'cat' example, I show three examples with Perl(v5.26.3) and
>>> three examples with Raku(2020.06), generally comparing literal vs
>>> regex arguments.
>>>
>>> The first two Perl5 examples returns nothing; the third Perl5 example
>>> returns everything after the "star" line. For the Raku code, the
>>> 'DWIMmiest' output below is the first Raku example, which returns two
>>> lines, "star" and "start". This is what I expected/desired. But I'm
>>> not really understanding what's happening with the other 2 Raku
>>> examples (which return everything after the "star" line):
>>>
>>> user@mbook:~$ cat startling.txt
>>> s
>>> st
>>> sta
>>> star
>>> start
>>> startl
>>> startli
>>> startlin
>>> startling
>>>
>>> user@mbook:~$ perl -nE 'print if "star" .. "start" ;' startling.txt
>>> user@mbook:~$ perl -nE 'print if /"star"/ .. /"start"/ ;' startling.txt
>>> user@mbook:~$ perl -nE 'print if /star/ .. /start/ ;' startling.txt
>>> star
>>> start
>>> startl
>>> startli
>>> startlin
>>> startling
>>> user@mbook:~$ raku -ne '.put if "star" ff "start" ;' startling.txt
>>> star
>>> start
>>> user@mbook:~$ raku -ne '.put if /"star"/ ff /"start"/ ;' startling.txt
>>> star
>>> start
>>> startl
>>> startli
>>> startlin
>>> startling
>>> user@mbook:~$ raku -ne '.put if /star/ ff /start/ ;' startling.txt
>>> star
>>> start
>>> startl
>>> startli
>>> startlin
>>> startling
>>> user@mbook:~$
>>>
>>> I'm all in favor of improving the "ff" and "fff" functions in Raku
>>> over their Perl5 counterparts, but I'm hoping to gain a better
>>> (mnemonic?) way of remembering the expected return values with literal
>>> vs regex arguments.
>>>
>>> Any assistance appreciated, Bill.
>>>
>>>
>>>
>>>
>>>
>>>
>>> On Sun, Jul 26, 2020 at 10:04 AM Larry Wall <larry@wall.org> wrote:
>>> >
>>> > On Sat, Jul 25, 2020 at 04:32:02PM -0500, Brad Gilbert wrote:
>>> > : In the above two cases ff and fff would behave identically.
>>> > :
>>> > : The difference shines when the beginning marker can look like the
end
>>> > : marker.
>>> >
>>> > The way I think of it is this:  You come to the end of "ff" sooner,
so you
>>> > do the end test immediately after passing the start test.  You come
to the
>>> > end of "fff" later, so the end test is delayed to the next iteration
from
>>> > the start test.  (Same mnemonic for .. and ... in Perl, by the way,
since
>>> > ff and fff were modeled on .. and ... (in their scalar form), but we
stole
>>> > .. and ... in Raku for ranges and sequences so we needed something
else.)
>>> >
>>> > I suppose if you're musical you can come up with mnemonics based on
"fff"
>>> > being louder than "ff", so it echoes longer before it stops...  :)
>>> >
>>> > Larry

Thread Previous | Thread Next


nntp.perl.org: Perl Programming lists via nntp and http.
Comments to Ask Bjørn Hansen at ask@perl.org | Group listing | About