高维数组与C ++中的一维数组效率

Question

I'm curious about the efficiency of using a higher dimensional array vs a one dimensional array. Do you lose anything when defining, and iterating through an array like this:

array[i][j][k];

or defining and iterating through an array like this:

array[k + j*jmax + i*imax];

My inclination is that there wouldn't be a difference, but I'm still learning about high efficiency programming (I've never had to care about this kind of thing before).

Thanks!

Answer 1

The only way to know for sure is to benchmark both ways (with optimization flags on in the compiler of course). The one think you lose for sure in the second method is the clarity of reading.

Answer 2

The former way and the latter way to access arrays are identical once you compile it. Keep in mind that accessing memory locations that are close to one another does make a difference in performance, as they're going to be cached differently. Thus, if you're storing a high-dimensional matrix, ensure that you store rows one after the other if you're going to be accessing them that way.

In general, CPU caches optimize for temporal and spacial ordering. That is, if you access memory address X, the odds of you accessing X+1 are higher. It's much more efficient to operate on values within the same cache line.

Check out this article on CPU caches for more information on how different storage policies affect performance: http://en.wikipedia.org/wiki/CPU_cache

Answer 3

If you can rewrite the indexing, so can the compiler. I wouldn't worry about that.

Trust your compiler(tm)!

Answer 4

它可能取决于实现，但是我想说它或多或少等于您的一维数组代码。

Answer 5

Do yourself a favor and care about such things after profiling the code . It is very unlikely that something like that will affect the performance of the application as a whole. Using the correct algorithms is much more important

And even if it does matter, it is most certainly only a single inner loop that needs attention.