How can I sort the array of class?

Question

I have made the class having 2d array (4 x 4) and maximum value in 2d array like below:

class B {
public:
    int shape[4][4] = { 0 };
    int maxh = 0;

    B() {};

    void record(int module[4][4]) {
        for (int i = 0; i < 4; i++) {
            for (int j = 0; j < 4; j++) {
                shape[i][j] = module[i][j];
                if (shape[i][j] > maxh) { maxh = shape[i][j]; }
            }
        }
    }
};

If there is a class 'B' array,

B b_arr = new B[30000];

how do I sort the class object array by the maximum value?

I have tried to sort array like below code, but I have a stack overflow error.

int partition(B arr[], int p, int r) {
    int i = p - 1;
    for (int j = p; j < r; j++) {
        int cri = arr[r].maxh;
        if (arr[j].maxh < cri) {
            i++;
            B tmp = arr[i];
            arr[i] = arr[j];
            arr[j] = tmp;
        }
    }
    B tmp = arr[i + 1];
    arr[i + 1] = arr[r];
    arr[r] = tmp;
    return i + 1;
}


void quickSort(B arr[], int p, int r) {

    if (p < r) {
        int q = partition(arr, p, r);
        quickSort(arr, p, q - 1);
        quickSort(arr, q + 1, r);
    }
}

Why dont you use std::sort ?

marcinj
– marcinj

2019-11-09 19:23:56 +00:00
Commented Nov 9, 2019 at 19:23 — marcinj
– marcinj, Commented Nov 9, 2019 at 19:23

Chih-Chen Kao · Accepted Answer · 2019-11-09 21:04:58Z

2

You can define your comparator for std::sort():

See the prototype below:

template< class RandomIt, class Compare >
void sort( RandomIt first, RandomIt last, Compare comp );

For example you can do:

std::sort(
    /*std::begin(b_arr)*/b_arr,
    /*std::end(b_arr)*/b_arr+30000,
    [](const B& left, const B& right){
        return left.maxh < right.maxh;
    }
);

Note that std::begin() and std::end() do not work with pointers to dynamic arrays. In this case you must specify the range by adding the size. I recommend using std::vector or std::array instead.

edited Nov 9, 2019 at 21:04

answered Nov 9, 2019 at 19:25

Chih-Chen Kao

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2 Comments

Remy Lebeau Over a year ago

left.maxh > right.maxh needs to be left.maxh < right.maxh instead. The comparator for std::sort() is required to return true only if left is less than right otherwise return false. You are doing the opposite.

Chih-Chen Kao Over a year ago

thanks. I didn't look closer and intuitively thought he wanted to sort in a reversed order with max at the beginning.

Adrian Mole · Accepted Answer · 2019-11-09 19:34:54Z

2

You could use std::qsort, if you define a comparator function for the class objects:

int bArrCompare(const void* a, const void* b) {
    const B* pa = reinterpret_cast<const B*>(a);
    const B* pb = reinterpret_cast<const B*>(b);
    return (pb->maxh - pa->maxh);
}

int main()
{
    B* b_arr = new B[30000];
    //...
    std::qsort(b_arr, 30000, sizeof(B), bArrCompare);
    //...
    return 0;
}

answered Nov 9, 2019 at 19:34

Adrian Mole

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3 Comments

Remy Lebeau Over a year ago

qsort() is a C function. Use std::sort() in C++.

Adrian Mole Over a year ago

@RemyLebeau std::qsort is available in C++ - and it is easier to use for 'old-style pointer' arrays.

Remy Lebeau Over a year ago

it is not easier to use qsort() vs std::sort(), you can pass an 'old-style pointer' array to std::sort() using fewer parameters than qsort(), and there is no typecasting needed in the std::sort() comparator. So it is actually easier to use std::sort(), and less error-prone since it has no size parameter to get wrong

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