Блог пользователя PraveenDhinwa

Using Constructors and comparison function in C++
Автор PraveenDhinwa, 13 лет назад, По-английски
  • Using constructors in C++

struct point {
    int x, y;
    point() {} // default constructor
    point (int _x, int _y) {
        x = _x;
        y = _y;
    }
};

Another way


struct point {
    int x, y;
    point() {} // default constructor
    point (int x, int y): x(x), y(y) {}
};
  • Using Comparison function in C++. For information of operator overloading visit link

struct point {
    int x, y;
    point() {}
    point (int x, int y) : x(x), y(y) {}
    
    // overloading of < operator
    bool operator<(const point &rhs) const{
        // your main logic for the comparator goes here
        return make_pair(x,y) < make_pair(rhs.x, rhs.y);
    }
};

Now A Sample Implementation

Thanks to Break-Neck for pointing out the problem with earlier a <= b in the code of cmp function, see the comment of Break-Neck comment for more details and also see the well commented cmp function.
You can also view link to know more about strict weak ordering in stl.


// please write your includes do not forget to include vector or direcly use include bits/stdc++.h

bool cmp(int a,int b) {
    return a < b;
    // please view the first comment so as to understand the concept of strict weak ordering to be used in comparison if you want to use std:sort etc functions. Strict weak ordering means that specify the case when a is strictly less than b to true, all other cases to false.
  
}

int main() {
    int n;
    cin >> n;

    vector a;

    for (int i = 0; i < n; i++) {
        int x;
        cin >> x;
        a.push_back(x);
    }

    sort (a.begin(), a.end(), cmp);

    for (int i = 0; i < a.size(); i++)
        cout << a[i] << " ";


    return 0;
}

PS. Please do comment if the article was useful or needs some changes or more explanation.

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13 лет назад, скрыть # |
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It's very important: NEVER use a <= b in comparators for STL algorithms and data structures. All comparators must be like a < b. If one use comparation from the author's example for std::sort, std::map, std::set, they'll have WA. There is one thing more: in DEBUG mode Visul Studio always check that "less" operation is correct – never a < b and b < a and etc. Globaly, you should get STL less-comparators like "If I want the first parameter to be always before the second one (and never after) I must return true, otherwise false".

Hope this'll help you and you won't have some stupid bugs because of that.

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13 лет назад, скрыть # |
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I think It's 

return (x == rhs.x) ? y < rhs.y : x < rhs.x;

better than make_pair

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13 лет назад, скрыть # |
 
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Use std::tie instead of make_pair

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Usually nowadays I make use of C++11 features to remove some of the boilerplate:

struct point { int x, y; };
// ...
sort(begin(a), end(a), [](const point& a, const point& b) { 
   return tie(a.x,a.y) < tie(b.x,b.y); });

If I want to create an instance of point, I just do { x, y } (possibly with a cast in the rare cases where the type cannot be inferred).

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