unt311's blog

time complexity of subtraction of iterators
By unt311, history, 4 years ago, In English

I couldn't find relevant answers to this online.

If I do int pos = iterator1 - iterator2

would this be an O(n) or a O(1) operation.

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4 years ago, # |
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$$$O(1)$$$

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4 years ago, # |
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To specify, I assume this is C++. For a RandomAccessIterator (for e.g. a vector) this is indeed $$$O(1)$$$. If it's a weaker form of iterator, e.g. a BidirectionalIterator from a set, then first of all it won't compile, you will need int pos=distance(it1, it2) and also it will be $$$O(it_2-it_1)=O(N)$$$.

If you are more innterested, you can have a look at the specifications of iterators here. I think the two iterator types I named are the most common ones in CP (just my feeling).

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4 weeks ago, # |
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for the case of set and map it's $$$O(N)$$$ but there is a work around.

using the __gnu_pbds library

you can create an ordered_map or ordered_set and take advantage of .order_of_key() to find the difference in $$$log(n)$$$ .

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4 weeks ago, # |
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upvote!!! please support newbies!!!

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