list.reverse(): add slice support #19181
Conversation
Hi all,
In Python, I need a tool to reverse part of a list (tail) quickly.
I expected that
nums[start:end].reverse()
would do it inplace with the performance similar to nums.reverse().
However, it doesn't work at all. The fastest way to reverse a part of
the list that I found is like this:
nums[start:end] = nums[end:start-1:-1]
But it is 30 times slower than pure reverse(). The patch below adds
a region support for the reverse(). It works as fast as I expect.
The test script and results are like this:
>>> exec(open('test.py').read())
nums.reverse() 0.006764888763427734
nums = nums[::-1] 0.10066413879394531
nums.reverse(-L/2) 0.003548145294189453
nums.reverse(L/2, L) 0.003538370132446289
nums = nums[:L/2] + nums[L:L/2-1:-1] 0.19934582710266113
nums[L/2:L] = nums[L:L/2-1:-1] 0.11419057846069336
import time
nums = list(range(10000000))
L = len(nums)
LL = int(L/2)
t = time.time()
nums.reverse()
print('nums.reverse()\t\t\t\t', str(time.time() - t))
t = time.time()
nums = nums[::-1]
print('nums = nums[::-1]\t\t\t', str(time.time() - t))
t = time.time()
nums.reverse(-LL)
print('nums.reverse(-L/2)\t\t\t', time.time() - t)
t = time.time()
nums.reverse(LL, L)
print('nums.reverse(L/2, L)\t\t\t', time.time() - t)
t = time.time()
nums = nums[:LL] + nums[L : LL - 1 : -1]
print('nums = nums[:L/2] + nums[L:L/2-1:-1]\t', time.time() - t)
t = time.time()
nums[LL:L] = nums[L:LL-1:-1]
print('nums[L/2:L] = nums[L:L/2-1:-1]\t\t', time.time() - t)
If there is better way to reverse lists, can someone point me at the right
direction? If not, I'll be happy to fix all existing issues and upstream
this approach.
Thanks,
Yury
Signed-off-by: Yury Norov <yury.norov@gmail.com>
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Created: |
Hi all,
In Python, I need a tool to reverse part of a list (tail) quickly.
I expected that
would do it inplace with the performance similar to nums.reverse().
However, it doesn't work at all. The fastest way to reverse a part of
the list that I found is like this:
But it is 30 times slower than pure reverse(). The patch below adds
a region support for the reverse(). It works as fast as I expect.
The test script and results are like this:
import time
nums = list(range(10000000))
L = len(nums)
LL = int(L/2)
t = time.time()
nums.reverse()
print('nums.reverse()\t\t\t\t', str(time.time() - t))
t = time.time()
nums = nums[::-1]
print('nums = nums[::-1]\t\t\t', str(time.time() - t))
t = time.time()
nums.reverse(-LL)
print('nums.reverse(-L/2)\t\t\t', time.time() - t)
t = time.time()
nums.reverse(LL, L)
print('nums.reverse(L/2, L)\t\t\t', time.time() - t)
t = time.time()
nums = nums[:LL] + nums[L : LL - 1 : -1]
print('nums = nums[:L/2] + nums[L:L/2-1:-1]\t', time.time() - t)
t = time.time()
nums[LL:L] = nums[L:LL-1:-1]
print('nums[L/2:L] = nums[L:L/2-1:-1]\t\t', time.time() - t)
If there is better way to reverse lists, can someone point me at the right
direction? If not, I'll be happy to fix all existing issues and upstream
this approach.
Thanks,
Yury
Signed-off-by: Yury Norov yury.norov@gmail.com