Menu
Is there any any faster way to swap two list elements in Python than
L[a], L[b] = L[b], L[a] or would I have to resort to Cython or Weave or the like?
572
Given a list in Python and provided the positions of the elements, write a program to swap the two elements in the list. Since the positions of the elements are known, we can simply swap the positions of the elements.
To swap two list elements by index i and j , use the multiple assignment expression lst[i], lst[j] = lst[j], lst[i] that assigns the element at index i to index j and vice versa.
The standard solution to swap two elements in a List is using the Collections. swap() method, which swaps the elements at the specified positions in a list.
Looks like the Python compiler optimizes out the temporary tuple with this construct:
import dis def swap1(): a=5 b=4 a, b = b, a def swap2(): a=5 b=4 c = a a = b b = c print 'swap1():' dis.dis(swap1) print 'swap2():' dis.dis(swap2) swap1(): 6 0 LOAD_CONST 1 (5) 3 STORE_FAST 0 (a) 7 6 LOAD_CONST 2 (4) 9 STORE_FAST 1 (b) 8 12 LOAD_FAST 1 (b) 15 LOAD_FAST 0 (a) 18 ROT_TWO 19 STORE_FAST 0 (a) 22 STORE_FAST 1 (b) 25 LOAD_CONST 0 (None) 28 RETURN_VALUE swap2(): 11 0 LOAD_CONST 1 (5) 3 STORE_FAST 0 (a) 12 6 LOAD_CONST 2 (4) 9 STORE_FAST 1 (b) 13 12 LOAD_FAST 0 (a) 15 STORE_FAST 2 (c) 14 18 LOAD_FAST 1 (b) 21 STORE_FAST 0 (a) 15 24 LOAD_FAST 2 (c) 27 STORE_FAST 1 (b) 30 LOAD_CONST 0 (None) 33 RETURN_VALUE Two loads, a ROT_TWO, and two saves, versus three loads and three saves. You are unlikely to find a faster mechanism.
163
If you could post a representative code sample, we could do a better job of benchmarking your options. FWIW, for the following dumb benchmark, I get about a 3x speedup with Shed Skin and a 10x speedup with PyPy.
from time import time def swap(L): for i in xrange(1000000): for b, a in enumerate(L): L[a], L[b] = L[b], L[a] def main(): start = time() L = list(reversed(range(100))) swap(L[:]) print time() - start return L if __name__ == "__main__": print len(main()) # for shedskin: # shedskin -b -r -e listswap.py && make # python -c "import listswap; print len(listswap.main())" If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
