## digitalmars.D.learn - How to use lowerBound and upperBound effectively?

- A. Bressan (30/30) Jul 02 2019 Hi, I am translating the following C++ code to D and I have some
- =?UTF-8?Q?Ali_=c3=87ehreli?= (4/7) Jul 02 2019 I don't understand the specific problem but can 'trisect' be useful?
- A. Bressan (43/52) Jul 02 2019 I try to clarify the problem, sorry for not being clearer.

Hi, I am translating the following C++ code to D and I have some trouble to achieve it using ranges. #include <vector> #include <algorithm> typedef std::vector<int>::const_iterator iter; std::pair<iter,iter> getSubVector(iter beg, iter end, int val, int shift) { return {std::upper_bound(beg,end,val)-shift,end}; } My best result is the following convoluted code, is there a better way? auto getSubVector(T)(const ref T vec, int val, int shift) if (isInstanceOf!(SortedRange, T)) { auto begin=vec.length-assumeSorted!"a>b"(retro(vec)).lowerBound().length-shift; return vec[begin..$]; } The difficulty I have is that, contrary to C++, lowerBound and upperBound give the same piece of information because they return complementary sub-ranges. To get the elements that are equal to val outside of the upperBound I need to reverse the range. I tried to use lowerBound on a range sorted by "a<=b", but it triggers an error that does not seem to be relevant to binary search strategies: std/algorithm/sorting.d(178): Predicate for isSorted is not antisymmetric. Both pred(a, b) and pred(b, a) are true for certain values.

Jul 02 2019

On 07/02/2019 02:27 AM, A. Bressan wrote:contrary to C++, lowerBound and upperBound give the same piece of information because they return complementary sub-ranges.I don't understand the specific problem but can 'trisect' be useful? https://dlang.org/library/std/range/sorted_range.trisect.html Ali

Jul 02 2019

On Tuesday, 2 July 2019 at 17:07:25 UTC, Ali Ã‡ehreli wrote:On 07/02/2019 02:27 AM, A. Bressan wrote:I try to clarify the problem, sorry for not being clearer. Given a value 'v', 'std::lower_bound' returns an iterator pointing to the first element 'a' for which 'a<v' is false. 'std::upper_bound' returns an iterator to the first element 'a' between a pair of iterators for which 'a<=v' is false. Thus the two C++ functions find a different point. The D function 'lowerBound' returns a range containing all elements '<v'. It is possible to recover the position of the first element '>=v' by using length. 'upperBound' returns a range containing all elements '>=v'. Again it is possible to recover the position of the first element '>=v' by subtracting the length of the returned range from the original length. Thus 'lowerBound' and 'upperBound' provide the same piece of information: the position of the first element '>=v'. I need the position of the first element '>v'. v=5 data [1,2,3,4,5,5,6,7,8,9] std::lower_bound ^ std::upper_bound ^ lowerBound _______ upperBound ___________ My current solution is to reverse the range and the ordering so that I can pin-point the correct location. v=5 data [9,8,7,6,5,5,4,3,2,1] std::lower_bound ^ std::upper_bound ^ lowerBound _______ upperBound ___________ My question is motivated by the fact that the search policies (except the linear scan) are variants of the bisection method https://en.wikipedia.org/wiki/Bisection_method and can find both the first element '>=v' or the first element '>v' by changing the predicate '>=' for '>'. It seems to me that there must be a simpler way and that I am overlooking something. The 'trisect' method, provides both the position of first element '>=v' and that of the first element '>v'. So it provides more than what I need, but it is nicer to read and maybe faster than my solution. Thanks Andreacontrary to C++, lowerBound and upperBound give the same piece of information because theyreturncomplementary sub-ranges.I don't understand the specific problem but can 'trisect' be useful? https://dlang.org/library/std/range/sorted_range.trisect.html Ali

Jul 02 2019