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each or keys?by rdw (Curate)
|on Oct 11, 2000 at 02:16 UTC||Need Help??|
rdw has asked for the wisdom of the Perl Monks concerning the following question:
Having been caught once again by 'each' not resetting, I decided to do some benchmarks to compare the performance of a while loop with 'each' and a foreach loop with 'keys'.
The results surprised me, so I'm seeking enlightenment - can anybody explain this one?Here is the code I tried... (obviously I could have used 'values' in the second case, but this code was constructed just for benchmarking)
And here are the results...
Benchmark: timing 100000 iterations of each, keys... each: 1 wallclock secs ( 0.61 usr + 0.01 sys = 0.62 CPU) @ 161290.32/s (n=100000) keys: 93 wallclock secs (90.03 usr + 0.77 sys = 90.80 CPU) @ 1101.32/s (n=100000) Rate keys each keys 1101/s -- -99% each 161290/s 14545% --
...but... (and this is the weird bit) if I comment out the line which breaks out of the loop ('last if...') then the results are completely different... (I dropped the number of iterations - hope this hasn't changed too much - I got bored waiting for it)
Benchmark: timing 1000 iterations of each, keys... each: 2 wallclock secs ( 2.77 usr + 0.00 sys = 2.77 CPU) @ 361.01/s (n=1000) keys: 3 wallclock secs ( 2.36 usr + 0.00 sys = 2.36 CPU) @ 423.73/s (n=1000) Rate each keys each 361/s -- -15% keys 424/s 17% --
So it seems that 'each' is much quicker, but only if you plan on leaving the loop early - which is when the not resetting gotcha kicks in.
Now I understand that 'keys' would have to generate a list which might be expensive I guess, but I'm still a bit puzzled by the results. Perhaps there is something else going on.