I saw this problem from some blog. The following two loops are given, the question is which one is faster.
for(int i = 100000; i > 0; i--) {}
for(int i = 1; i < 100001; i++) {}
Why the first one is faster than the second one?
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Ryan
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18Benchmarks or it didn't happen. – Mysticial Jul 28 '13 at 22:05
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The differences, if any, would be measured in nanoseconds. – Bohemian Jul 28 '13 at 22:07
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1It probably depends more on what you are doing in the loops than the loops themselves. If you're going down this micro optimisation route did you know that `++i` is faster than `i++`? – Boris the Spider Jul 28 '13 at 22:08
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@Bohemian which means that in Java they wouldn't be measurable due to the 15ms clock resolution. – Boris the Spider Jul 28 '13 at 22:10
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1Possible duplicate - http://stackoverflow.com/questions/4181941/loop-counter-in-java-api – nkukhar Jul 28 '13 at 22:12
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@boris huh? See [`System.nanoTime()`](http://docs.oracle.com/javase/6/docs/api/java/lang/System.html#nanoTime%28%29) – Bohemian Jul 29 '13 at 00:04
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On some processors in code generated by some compilers the first one could be faster because the value is compared to zero. As @DeadMG notes it applies, for example, to x86 processors before 1985.
But it is:
- a premature optimization, don't change first line to the second only because of this reason! Donald Knuth once said and I totally agree with him
We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil.
- a non-portable optimization - it works faster only on some specific processors in some specific cases! And with a high probability it will work not just the same but even slower on other architectures.
You are warned.
sasha.sochka
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1To be more specific, this has only been the case on, for example, x86 processors before 1985. So you might want to be a lot more specific about what processors you think this applies to. – Puppy Jul 28 '13 at 22:14
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And it's a carry over from C, where you can code the condition as simply `i`, ie `for(int i = 100000; i; i--)` – Bohemian Jul 29 '13 at 00:01