This is a reply to an old comment https://news.ycombinator.com/item?id=44452679 https://news.ycombinator.com/item?id=44452679 (since I cannot reply in the origi

This is a reply to an old comment https://news.ycombinator.com/item?id=44452679 https://news.ycombinator.com/item?id=44452679 (since I cannot reply in the original thread) > Even assuming python's foreach loop in these cases get optimized down to a very bare for loop, the operations being performed are dominated by the looping logic itself, because the loop body is so simple. > Each iteration of a for loop performs one index update and one termination comparison. For a simple body that is just an XOR, that's the difference between performing 5 operations (update, exit check, read array, XOR with value, XOR with index) per N elements in the one loop case versus 7 operations (update, exit, read array, XOR with value, then update, exit, XOR with index) in the two loop case. So we're looking at a 29% savings in operations. > It gets worse if the looping structure does not optimize to a raw, most basic for loop and instead constructs some kind of lazy collection iterator generalized for all kinds of collections it could iterate over. > The smaller the loop body, the higher the gains from optimizing the looping construct itself. Let's test your claims import random import time n = int(1e7) A = list(range(1,n+1)) random.shuffle(A) print("Removed:", A.pop()) t = time.time() result = 0 for idx,val in enumerate(A): result ^= idx+1 result ^= val result ^= n print("1-loop:", time.time() - t) print("Missing:", result) t = time.time() result = 0 for value in range(1, n + 1): result ^= value for value in A: result ^= value print("2-loop:", time.time() - t) print("Missing:", result) A sample run gives: Removed: 2878763 1-loop: 1.4764018058776855 Missing: 2878763 2-loop: 1.1730067729949951 Missing: 2878763 And after swapping the order of the code blocks just to ensure there's nothing strange going on: Removed: 3217501 2-loop: 1.200080156326294 Missing: 3217501 1-loop: 1.5053350925445557 Missing: 3217501 So indeed we have about a 20% speedup, only in the complete opposite direction that you claimed we'd have. Perhaps it's best not to assume when talking about performance.