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

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From:
yary
Date:
August 1, 2020 23:04
Subject:
Re: Learning the "ff" (flipflop) infix operator? (was Re: Rakuversion of "The top 10 tricks... .")
Message ID:
CAG2CFAaZ=ovbZiTWyCs0DGt6jLdJ=dYRmMAsEhy-shf_o4yyBQ@mail.gmail.com
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
>>
>

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