質問
To check an int within range [1, ∞) or not, I can use the following ways (use #1, #2 a lot):
https://stackoverflow.com/questions/21176492
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Russianif (a>=1)if (a>0)if (a>1 || a==1)if (a==1 || a>1)Is there any difference that I should pay attention to among the four versions?
解決
Functionally there is no difference between the 4 ways you listed. This is mainly an issue of style. I would venture that #1 and #2 are the most common forms though, if I saw #3 or #4 on a code review I would suggest a change.
Perf wise I suppose it is possible that some compiler out there optimizes one better than the other. But I really doubt it. At best it would be a micro-optimization and nothing I would ever base my coding style on without direct profiler input
他のヒント
I don't really see why you would use 3 or 4. Apart from being longer to type, they will generate more code. Since in a or condition the second check is skipped if the first is true, there shouldn't be a performance hit except for version 4 if value is not 1 often(of course hardware with branch prediction will mostly negate that).
1. if (a>=1)
2. if (a>0)
3. if (a>1 || a==1)
4. if (a==1 || a>1)
On x86, options 1 and 2 produce a cmp instruction. This will set various registers. The cmp is then followed by a condition branch/jump based on registers. For the first, it emits bge, for the second it emits bgt.
Option 3 and 4 - in theory - require two cmps and two branches, but chances are the compiler will simply optimize them to be the same as 1.
You should generally choose whichever (a) follows the conventions in the code you are working on (b) use whichever most clearly expresses the algorithm you are implementing.
There are times when explicitly writing "if a is equal to one, or it has a value greater than 1", and in those times you should write if (a == 1 || a > 1). But if you are just checking that a has a positive, non-zero, integer value, you should write if (a > 0), since that is what that says.
If you find that such a case is a part of a performance bottleneck, you should inspect the assembly instructions and adjust accordingly - e.g. if you find you have two cmps and branches, then write the code to use one compare and one branch.
Nope! They all are the same for an int. However, I would prefer to use if (a>0).
