map

问题 I have a hashmap with byte[] keys. I'd like to sort it through a TreeMap. What is the most effective way to implement the comparator for lexicographic order? 回答1: Using Guava, you can use either of: UnsignedBytes.lexicographicalComparator() SignedBytes.lexicographicalComparator() The UnsignedBytes comparator appears to have an optimized form using Unsafe that it uses if it can. Comments in the code indicate that it may be at least twice as fast as a normal Java implementation. 回答2: Found this

问题 I have a hashmap with byte[] keys. I'd like to sort it through a TreeMap. What is the most effective way to implement the comparator for lexicographic order? 回答1: Using Guava, you can use either of: UnsignedBytes.lexicographicalComparator() SignedBytes.lexicographicalComparator() The UnsignedBytes comparator appears to have an optimized form using Unsafe that it uses if it can. Comments in the code indicate that it may be at least twice as fast as a normal Java implementation. 回答2: Found this

问题 This question already has answers here : Copy std::map into std::vector of pairs (3 answers) Closed last year . What's the best way in C++ to copy a pair from a map to vector? I'm doing this so I can subsequently sort the vector. 回答1: This should do what you want: #include <iostream> #include <vector> #include <map> #include <algorithm> #include <iterator> using namespace std; bool cmp(const pair<int, int> &p1, const pair<int, int> &p2) { return p1.second < p2.second; } int main() { map<int,

问题 I'm getting an error with the following: class Test { std::map<std::string,Test> test; }; The error is "Field has incomplete type 'Test'". I read a few threads with suggested this might be a bug in the version of libcxx which ships with xcode, but it wouldn't surprise me at all if I just have to change it to: class Test { std::map<std::string,std::shared_ptr<Test>> test; }; I just wanted to double check that this is definitely a correct error and not a bug. Cheers! 回答1: The standard requires

问题 I did these so far: EDIT--------------- steps=@ (m) 2*randi([0,1],[1,m])-1; Walk1D =@ (n) [0,cumsum(steps(n))]; findend=@ (x) x(end); LastPoint1D=@(n) findend(Walk1D(n)); nsteps=200; nsq=floor(sqrt(nsteps)); MeanSquareDistance1D= @ (n,m) m.*sum((LastPoint1D(n)).^2)./m; r2D=MeanSquareDistance1D(100,1000) data=[ ]; for i=10:20:90 data=[data; i , MeanSquareDistance1D(i,2000)] end The only problem now,is that the 2nd column of "data" must give me values around 10 30 50 70 90 but not exactly.Only

问题 I did these so far: EDIT--------------- steps=@ (m) 2*randi([0,1],[1,m])-1; Walk1D =@ (n) [0,cumsum(steps(n))]; findend=@ (x) x(end); LastPoint1D=@(n) findend(Walk1D(n)); nsteps=200; nsq=floor(sqrt(nsteps)); MeanSquareDistance1D= @ (n,m) m.*sum((LastPoint1D(n)).^2)./m; r2D=MeanSquareDistance1D(100,1000) data=[ ]; for i=10:20:90 data=[data; i , MeanSquareDistance1D(i,2000)] end The only problem now,is that the 2nd column of "data" must give me values around 10 30 50 70 90 but not exactly.Only

问题 This question is about construction of instances of custom allocator during insertion into a std::map. Here is a custom allocator for std::map<int,int> along with a small program that uses it: #include <stddef.h> #include <stdio.h> #include <map> #include <typeinfo> class MyPool { public: void * GetNext() { return malloc(24); } void Free(void *ptr) { free(ptr); } }; template<typename T> class MyPoolAlloc { public: static MyPool *pMyPool; typedef size_t size_type; typedef ptrdiff_t difference

问题 For both std::map and std::tr1::unordered_map , I see from the standard that: References to elements in the unordered_map container remain valid in all cases, even after a rehash. How are they doing that ( implementation-wise )? Are they maintaining all the entries as a kind of linked list and then the hash-table just stores pointers to the elements? 回答1: Yes, linked lists are involved, although not quite in the way you suggest. The 2011 standard says (23.2.5 para 8), "The elements of an

问题 I've got a map containing some keys (Strings) and values (POJOs) I want to iterate through this map and alter some of the data in the POJO. The current code I've inherited removes the given entry, and adds it back in after making some changes to the POJO. This doesn't work well, since you shouldn't be modifying a map whilst your iterating through it (method is synchronised, but ConcurrentModificationException still appears) My question is , if I need to iterate over a map and change values,

问题 I have a method that returns a map defined as: public Map<String, ?> getData(); The actual implementation of this method is not clear to me, but, when I try to do: obj.getData().put("key","value") I get following compile time error message: The method put(String, capture#9-of ?) in the type Map is not applicable for the arguments (String, String) What is the problem? Is String not of type anything? Thanks in advance. 回答1: The wildcard means "the value type parameter could be anything" - it