Re^2: printing every 2nd entry in a list backwards

[Anonymous Monk]

You are indeed correct with your observations about performance. However this can be easily mended by creating a file with 1_000_000 entries, each line having 10 numbers and then running the script against it

Doing this on my machine, the despicable snake language comes out ahead

$ time perl list.pl in >/dev/null

real    0m2.150s
user    0m2.136s
sys     0m0.004s
$ time python list.py in >/dev/null

real    0m1.426s
user    0m1.420s
sys     0m0.000s

I am looking to the monks for help on this one

[shmem]

Doing this on my machine, the despicable snake language comes out ahead

Even a snake can do what other beasts can't. No need to worry. Perl doesn't have a stepping range operator, it doesn't even have a descending one. So the indices must be built every line through. OTOH, knowing that the input lines always contain 10 items, it is trivial to beat the snake:

qwurx [shmem] ~> time perl -lne 'BEGIN{$,=" "}print+(split)[9,7,5,3,1]
+' in.txt >/dev/null

real    0m1.589s
user    0m1.588s
sys    0m0.004s
qwurx [shmem] ~> time python list.py in.txt >/dev/null

real    0m2.204s
user    0m2.188s
sys    0m0.016s

And your snake code does - according to the spec Given a line of numbers from a file, print every 2nd number starting from the back - get it right only for an even number of integers:

qwurx [shmem] ~> python
Python 2.7.9 (default, Jun 29 2016, 13:08:31) 
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> line = "1 2 3 4"
>>> line.split()[::-2]
['4', '2']
>>> line = "1 2 3 4 5"
>>> line.split()[::-2]
['5', '3', '1']
>>> ^D

So, what does every 2nd number starting from the back mean? If we start from the back and take every 2nd, the output for the even example should be ['3','1'], and for the odd example ['4','2']. If we count every 2nd from the beginning, the output ought to be ['4','2'] for both cases.

Because you presented

Example:

1 22 3 -4 ==> -4 22

almost all code examples in this thread assumed that every 2nd meant counting from the beginning, but outputting in reverse order. Tell the snake to do that, and compare again.