代码对比
以下两段代码中哪一段应该优先于其他代码？一般应在什么基础上作出决定？
在此函数的各种调用中，MAX_LIMIT 可能从 1000 到 5000 不等。
for (i=0;i<MAX_LIMIT;++i)
{
for (j=0;j<MAX_LIMIT;++j)
{
anObj.setMatrix(i,j,0);
}
}
for (i=0;i<MAX_LIMIT;++i)
{
anObj.setMatrix(i,i,1);
}
VS
for (i=0;i<MAX_LIMIT;++i)
{
for (j=0;j<MAX_LIMIT;++j)
{
if(i==j)
{
anObj.setMatrix(i,j,1);
}
else
{
anObj.setMatrix(i,j,0);
}
}
}
谢谢。
Which one of the following two pieces of code should be preferred over other? and on what basis the decision should be taken generally?
MAX_LIMIT might vary from 1000 to 5000 in various calls of this function.
for (i=0;i<MAX_LIMIT;++i)
{
for (j=0;j<MAX_LIMIT;++j)
{
anObj.setMatrix(i,j,0);
}
}
for (i=0;i<MAX_LIMIT;++i)
{
anObj.setMatrix(i,i,1);
}
vs
for (i=0;i<MAX_LIMIT;++i)
{
for (j=0;j<MAX_LIMIT;++j)
{
if(i==j)
{
anObj.setMatrix(i,j,1);
}
else
{
anObj.setMatrix(i,j,0);
}
}
}
Thanks.
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两者的性能应该渐近相等，因为两者的运行时间都是 O(n^2)。
您可能应该更喜欢最具可读性的一个。
The perfomance of the two should be asymptoticaly equal, as both run in O(n^2).
You should probably prefer the one with most readability.
第二个，它按照它所说的去做。但是，我强烈希望
The second, which does what it says. However, I'd strongly prefer
在我看来，我会选择两者中的第二个。最好限制循环某些内容的次数。由于您正在考虑至少 O(n^2) 运行时间，因此我肯定会将自己限制为只有 2 个循环。
In my opinion I would go with the second of the two. It is best to limit the number of times you have to loop through something. Since you're looking at at least O(n^2) runtime I would definately limity yourself to only 2 loops.
更好用于什么？我认为第二个版本最容易一眼理解，尽管其他人可能有其他意见。
如果您对性能感兴趣，那么对这两种方法进行计时应该很容易。 （它甚至可能是相关的。）
从性能角度来看，我怀疑第二种方法会更快，因为它一次会处理一个缓存行的数组。第一种方法将一次处理一个缓存行的数组，然后继续执行 N 个缓存故障。我的猜测是，这比在循环中添加额外的条件更重要。
因此，如果与其他操作相比，数组初始化需要大量时间，并且这一点很重要，请对每个操作进行计时并查看。如果没有，请选择更具可读性的版本。
Better for what? I think the second version is easiest to understand at a glance, although other people may have other opinions.
If you're interested in performance, it should be easy to time the two methods. (It may even be relevant.)
Taking a look at this for performance, I'd suspect the second method would be faster, because it will work through the array one cache line at a time. The first method will work through the array one cache line at a time, and then go on to execute N cache faults. My guess is that this is more significant than putting an extra conditional in the loop.
So, if array initialization takes a significant amount of time compared to other operations, and this matters, time each one and see. If not, then go with the more readable version.