Hi Bill, When building a range that's an arithmetic or geometric progression, the sequence operator is a little quicker to type. And thus also more likely to be understood more quickly too.
> ('a' .. 'h')[(0..*-1).grep: * %% 2 ] (a c e g) > ('a' .. 'h')[ 0, 2 ... * ] (a c e g) > ('a' .. 'h')[(0..*-1).grep: * % 2 ] (b d f h) > ('a' .. 'h')[1, 3...*] (b d f h) # Geometric example- powers of 2 > ('a' .. 'z')[1, 2, 4...*] (b c e i q) There isn't a simple translation for the is-prime example that I can think of, that is a good use for "grep" Ranges also support arbitrary functions, the doc page shows a Fibonacci number generator https://docs.raku.org/language/operators#index-entry-sequence_operator "This allows you to write say 1, 1, * + * ...^ *>= 100; # OUTPUT: «(1 1 2 3 5 8 13 21 34 55 89)» to generate all Fibonacci numbers up to but excluding 100." -y On Tue, Aug 24, 2021 at 6:36 PM William Michels via perl6-users < perl6-users@perl.org> wrote: > Hi Marc, > > My understanding is that ranges are pretty cheap to construct, and in any > case, the range @x[0..*-1] is just the index of all elements in @x. The > .grep() approach may be most useful if you have a function (e.g. %, %%, and > .is-prime shown below): > > > (0...9) > (0 1 2 3 4 5 6 7 8 9) > > (0...9)[0..*-1] > (0 1 2 3 4 5 6 7 8 9) > > (0...9)[(0..*-1).grep: * ] > (0 1 2 3 4 5 6 7 8 9) > > (0...9)[(0..*-1).grep: * %% 2 ] > (0 2 4 6 8) > > (0...9)[(0..*-1).grep: * % 2 ] > (1 3 5 7 9) > > (0...9)[(0..*-1).grep: *.is-prime ] > (2 3 5 7) > > > > You can find a related example in the docs ( > https://docs.raku.org/routine/grep#class_HyperSeq). Anyway, I'm sure each > approach has its fans, > > Best, Bill. > > On Tue, Aug 24, 2021 at 3:59 AM Marc Chantreux <e...@phear.org> wrote: > >> hello everyone, >> >> I made a mistake while replying to all of us so anwsers never reached >> your boxes. I'll summerize in one answer: >> >> Bill: >> >> > Is it just even/odd elements that you want to separate out? If so, maybe >> > .grep() is your friend here >> >> I don't think it is: 0, 2 ... * seems to be >> >> * closer to what i have in mind when i think about the problem >> (so i invoke readability there) >> * probably more efficient than (0..*).grep(* % 2) that >> * generate twice the number of required elements >> * need to filter the result >> >> Also, trying to play with this version: >> >> my ($a,$b) = >> .[0,2...*], >> .[1,3...*] >> with <AaBbCc>.comb; >> >> just don't work because the lists are squashed into scalar context >> in the process. >> >> So Brian and Fernando made my day with := and the unexpected power of >> the [] operator. >> >> my (@a,@b) := <AaBbCc>.comb[ [0,2...*], [1,3...*] ]; >> >> I really like how declarative it is. Also the use of := now seems >> obvious to me. >> >> Sigils still remains something strange to me desprite all your examples >> but i'll take some time. thanks everyone. >> >> marc >> >>