Scala标准库中ArrayBuffer实现的效率可能较低

Question

While studying programming, I think I stumbled upon an inefficiency in Scala, given that the description in our material is correct. The possibility for a lighter implementation was confirmed by a friend of mine who has got Silver and Bronze medals in International Olympiad in Informatics.

In our learning material it reads: "For example, for an ArrayBuffer it is very efficient to append elements at the end: as the internal array always doubles in size, the allocation of and copying to the new array occurs rather seldomly. In contrast, prepending an element at the beginning is time-consuming because a new internal array is allocated and the elements copied to it every time the method is called."

Couldn't there be another array storing the indices so that it would always add the elements to the end?

Answer 1

There could be a second array containing the indices, but it would be less efficient for a number of reasons:

1. The array of indices would require additional storage

2. Accessing an element would require at least two memory reads rather than one

3. If indices are not kept sorted then each access requires a search of the array of indices

4. If the indices are kept sorted then you have exactly the same problem with this array that you were trying to avoid for the original array.