{"id":19215,"date":"2022-11-05T21:22:21","date_gmt":"2022-11-05T15:52:21","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-why-does-this-solution-works-in-javascript-but-takes-too-long-in-c-dynamic-programming\/"},"modified":"2022-11-05T21:22:21","modified_gmt":"2022-11-05T15:52:21","slug":"solved-why-does-this-solution-works-in-javascript-but-takes-too-long-in-c-dynamic-programming","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-why-does-this-solution-works-in-javascript-but-takes-too-long-in-c-dynamic-programming\/","title":{"rendered":"[Solved] Why does this solution works in JavaScript but takes too long in C++? (Dynamic Programming)"},"content":{"rendered":"

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A difference between your JavaScript and C++ code samples is: the C++ function has just two parameters instead of 3, the map object being managed as a global entity.<\/p>\n

In some sense, having just 2 parameters is \u201cThe Right Thing<\/em>\u201d. The unordered map is about some internal necessity of the algorithm. Why should user code have to know about that ?<\/p>\n

And should you some day decide to use instead an ordered<\/em> map or a set or a bit vector, why should that force user code to change the list of header files it has to #include ?<\/p>\n

So having just 2 parameters is fine, however managing the map as an external global object<\/strong> is not that good. In C++ programming, global objects are generally frowned upon. In your case, it puts an undue burden on user code, such as having to reset the map object between two calls to canSum()<\/code> ? Such a precaution is all too easily forgotten.<\/p>\n

To solve the problem, you could use two C++ functions: an external one, and the internal one.<\/p>\n

The external one takes care (internally) of the life cycle of the map object. The internal one just passes a pointer to the map object around.<\/p>\n

C++ code for the internal<\/em> function:<\/strong><\/p>\n

#include  <vector>\n#include  <unordered_map>\n#include  <iostream>\n\nusing  MyMapType = std::unordered_map<int, bool>;  \/\/ ad hoc map type\nusing  std::vector;\n\nbool canSumWithMap(int goal, const vector<int>& vec, MyMapType& myMap)\n{\n    if (goal < 0)   return false;\n    if (goal == 0)  return true;\n\n    if (myMap[goal])\n        return myMap[goal];\n\n    for (auto& ell : vec)\n    {\n        if (canSumWithMap(goal-ell, vec, myMap)) \n        {\n            myMap.insert({goal, true});\n            return true;\n        }\n    }\n    myMap.insert({goal, false});\n    return false;\n}\n<\/code><\/pre>\n
Please note that both the map and the vector are passed by reference<\/em>, with a ‘&’ character, in order to avoid unnecessary copying during function calls.<\/p>\n
C++ code for the external<\/em> function, plus main program:<\/strong><\/p>\n
bool canSum(int goal, const vector<int>& vec)\n{\n    MyMapType  myMap;  \/\/ new map object initialized as empty\n\n    bool rc = canSumWithMap(goal, vec, myMap);\n\n    return rc;\n    \/\/ myMap automagically deallocated here\n}\n\n\nusing  std::cout;\nusing  std::endl;\n\n\nint main()\n{\n    cout << std::boolalpha;  \/\/ want to print true or false rather than 0 or 1\n    cout << canSum(7, {2,3})      << endl;\n    cout << canSum(7, {5,3,4,7})  << endl;\n    cout << canSum(7, {2,4})      << endl;  \/\/ no, can only do multiples of 2\n    cout << canSum(8, {2,3,5})    << endl;\n    cout << canSum(300, {7,14})   << endl;  \/\/ no, can only do multiples of 7\n\n    return 0;\n}\n<\/code><\/pre>\n
The above code runs successfully on my semi-vintage Intel x86-64 machine in 50 seconds, GNU C++ v10.2, with -O2 option.<\/p>\n
Program output:<\/strong><\/p>\n
$ g++ --version\ng++ (GCC) 10.2.1 20201125 (Red Hat 10.2.1-9)\nCopyright \u00a9 2020 Free Software Foundation, Inc.\n  ...  \n$ \n$ g++ -O2 q66720598.cpp  -o q66720598.x\n$ time q66720598.x\ntrue\ntrue\nfalse\ntrue\nfalse\n\nreal   0m49,986s\nuser   0m49,841s\nsys    0m0,003s\n$ \n\n<\/code><\/pre>\n
Performance measurements:<\/h2>\n
On my machine, your JavaScript codes runs in 19 seconds. And C++ with unordered maps takes 50 seconds, which is a bit disappointing.<\/p>\n
Switching from unordered maps to plain (ordered) maps decreases the C++ time to 36 seconds, that’s still slower than JavaScript.<\/p>\n
It takes a bit vector<\/em>, defined like this:<\/p>\n
    std::vector<bool>  myMap(goal+1, false);\n<\/code><\/pre>\n
to restore proper hierarchy \ud83d\ude42 and make C++  3 times faster<\/em> than JavaScript, with a wall time of only 6 seconds.<\/p>\n
So this is one of those situations where map objects, though functionally quite powerful and versatile, can be way slower than some ad hoc<\/em> data structure.<\/p>\n<\/p><\/div>\n
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            1<\/span> <\/p><\/div>\n<\/div>\n
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solved Why does this solution works in JavaScript but takes too long in C++? (Dynamic Programming) <\/p>\n","protected":false},"excerpt":{"rendered":"
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