如何使这些嵌套的for循环更有效？

Question

I have two lists:

1. ActiveExchange = []
2. ActiveStock = []

My code is very sequential in nature and I'm trying to optimize it:

for exchange in ActiveExchange:
    for stock in Activestock:
        ...
        ...
        ...

However, it seems to be highly inefficient due to the for-loop. What can i do to make it run more efficiently?

Answer 1

1. Does it just seem unefficient or did you run it and it took too long? If you just think its unefficient - test it. And don't uptimize a not existing problem. You can use your time more efficiently.

Anything else will be case-dependent. Like:

1. if you are searching for one or tow items in ActiveStack for every elemant in ActiveExchange,make the ActiveStack a hash.
2. If you have if statements, and the statement depends on one variable only, make it the outter one. that way you opyimize brach prediction. This may be done automaticlly by the compiler.
3. If you want to manipulate/read all pairs - you cannot optimize it.

Answer 2

C/C++ and Python are have row-major arrays. If you are stepping through an array, list, vector, etc, be sure to do this:

for i in range(25):
  for j in range(25):
    A[j][i] *= 1

rather than:

for i in range(25):
  for j in range(25):
    A[i][j] *= 1

That can make a difference of 3-6 times in performance.