zeref_dragoneel's blog

Difference in answers in unordered_map and map
By zeref_dragoneel, history, 9 years ago, In English

http://codeforces.net/contest/512/problem/B

solution id : 12300691 and 12300696 http://codeforces.net/submissions/arbit

unordered_map is giving wrong answers while map gives the correct answer.

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9 years ago, # |
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Looks like you use fact, that in map all pairs are sorted. It's true for normal map(because it's based on BST), but wrong for unordered_map(because it's based on hash table and keys are sorted not by value, but by hashs)

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    9 years ago, # ^ |
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    According to the solution, the ordered property is not making any difference.

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      9 years ago, # ^ |
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      Ok, that's my error :(

      Here

      if (it2 == M.end() || it2->second > cost)
	M[fc] = cost;

      You can add element to map. Maybe it can make iterator it1 invalid...

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        9 years ago, # ^ |
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        According to what you are saying, map should give wrong answers however it is the other way round.

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          9 years ago, # ^ |
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          When you change container there are magic with iterators, so it's better just not to do it :) You can never be sure what you will get

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9 years ago, # |
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std::map and std::unordered_map have different iterator invalidation rules. Basically, any std::unordered_map iterator can be invalidated after any insertion or deletion because of rehashing, and you can't control it. On other hand, std::map iterator is guaranteed to be valid until you erase it explicitly. That's why you got 2 different results.

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