in reply to printing every 2nd entry in a list backwards

$ cat in.txt 
1 22 3 -4

a
b c
d e f
g h i j
k l m n o
$ perl -anle 'print "@F[map $#F-$_*2, 0..$#F/2]"' in.txt 
-4 22

a
c
f d
j h
o m k

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But seriously, there are lots of ways to do this. Just one of many:

while (<>) {
    my @fields = split;
    for (my $i=$#fields; $i>=0; $i-=2) {
        print "$fields[$i] ";
    }
    print "\n";
}

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Update: As for your code, I'd probably have used for instead of map, but other than that it's a decent solution. Just for fun, a different way to write that might be: ++$i&1 and print "$_ " for reverse split; although that might be getting a little too clever ;-) This is a fun exercise in TIMTOWTDI! Update 2: Changed wording a bit.

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Re^2: printing every 2nd entry in a list backwards
by Anonymous Monk on May 19, 2017 at 12:25 UTC

    Thank you kindly for your reply.

    Your first solution is indeed exuding the knowledge of a monk, sadly it is rather slow.

    Your second solution is actually what I tried first but it is also slower than my current solution. I am explicitly looking for a fast solution to beat the solution written in the language that also starts with "P" and must not be named.

    I apologize for not being clear in the first place.

[reply]
      Your second solution is actually what I tried first but it is also slower than my current solution.

      How exactly did you measure this - code, sample input, etc.? Because at least under the following conditions, my code suggestion appears to be faster than yours. (Note I inserted use warnings; use strict; at the top of my script.)

      $ perl -wMstrict -le 'print join(" ",map {int(rand(100))-50} 0..rand(7
+)+3) for 1..1000000' >in.txt
$ wc -l in.txt 
1000000 in.txt
$ head -5 in.txt 
-49 43 0 -35 0 -20 -49 5 46 -11
-14 39 39 -24 -49 36 -7 -36 -43
15 30 5 -4 11
37 -25 27 -49 21
49 33 -15 -16 17 10 32 -14 -30
$ time perl 1190600.pl in.txt >out.txt

real    0m1.869s
user    0m1.863s
sys    0m0.004s
$ time perl 1190602.pl in.txt >out.txt

real    0m1.539s
user    0m1.521s
sys    0m0.016s

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[d/l]
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        Apologies for not mentioning how I measure performance, please refer to my other reply for more information

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