How to pass an array of members of a struct as a parameter to a function?

Question

Here is my problem: I have a struct:

struct point
{
    int x;
    int y;
};

and then I have an array:

for (int i = 0;i < n;i++)
{
    arr[i].x=rand() % n + 1;
}

I defined the quicksort function as follows:

 void quicksort(int *a, int left, int right);

and I want to sort the point by X coordinate, so I call the quicksort :

quicksort(arr.x, 0, n-1);

And this is the error message:

error: request for member 'x' in 'arr', which is of non-class type 'point [(((unsigned int)(((int)n) + -0x000000001)) + 1)]'

Sorry if the question is too stupid or badly formulated, the truth is I'm a newbie and I'm really willing to learn as much as possible and I'd be very thankful for your help!

Answer 1

If you always want to sort by x , then you can hard-code it into the sort function, and just pass a pointer to the array to sort:

void quicksort(point * arr, int left, int right) {
    // test points with 
    // if (arr[i].x < arr[j].x) {/* i sorts before j */}
}

quicksort(arr, 0, n-1);

To specify a class member to sort by, you need a pointer-to-member, not a pointer; something like:

void quicksort(point * arr, int point::*member, int left, int right){
    // test points with 
    // if (arr[i].*member < arr[j].*member) {/* i sorts before j */}
}

quicksort(arr, &point::x, 0, n-1);

More generically, you could follow the example of std::sort and accept any comparison functor:

template <typename RandIter, typename Compare>
void quicksort(RandIter begin, RandIter end, Compare compare) {
    // test points with 
    // if (compare(*it1, *it2)) {/* *it1 sorts before *it2 */}
}

quicksort(arr, arr+n, 
    [](point const &lhs, point const &rhs) {return lhs.x < rhs.x;});

And of course, unless you're learning how to implement a sorting algorithm, just use std::sort .

Answer 2

quicksort(arr,0,n-1);

然后在quicksort ，尝试比较arr[i].x

Answer 3

There are a few problems with your code.
1. quicksort accepts int* but you try to pass int value x
2. You try to pass int but you actually call an undefined variable arr.x

What you need to do is either call in the form of &arr[i].x, but to accomplish what you want, you probably want to pass the entire struct as a pointer.

Answer 4

You need to pass arr as the parameter, as that is the array to be sorted. arr.x is meaningless. You are not passing the string "arr.x" as a parameter which can somehow be interpreted as meaning sort on the x field - when the compiler sees this, it is looking for an x element of arr , which doesn't exist, as the error message suggests - only the elements of arr (eg arr[0] ) have x elements (accessed as arr[0].x ).

Answer 5

Assuming this is for academic purposes (why else would you declare your own sorting algorithm instead of using one of the ones already implemented with a custom comparator?), you can do this a few ways:

Array

std::array<point, 10> myArray; // declares an array of size 10 for points
template<size_t N>
void quicksort(std::array<point, N>& arr, ...)
{
    // implement sort operating on arr
}

Vector

std::vector<point> myVector; // declares a dynamic array/vector of points
void quicksort(std::vector<point>& arr, ...)
{
    // implement sort operating on arr
}

If for some god-awful reason, you want to keep it in C:

Legacy

const size_t SIZE = 10;
point arr[SIZE]; // declare an array of 10 points
void quicksort(point* p, const size_t n, ...)
{
    // implement sort operating on elements in p passing in SIZE for n
}

Answer 6

I'd rather defined the function as:

 void quicksort(void *a,int left,int right, size_t size, int (*fp)(void*,void*));

size is the size of one element of array and fp is a compare function which returns true if the two arguments are equal. Now you can pass the call the function as:

quicksort(arr,0,n-1,sizeof(arr)/sizeof(arr[0]), compare);

where function compare is something like:

int compare(void* a, void* b) { return *((int*)a) >= *((int*)b); }  

Rest of the implementation of function is trivial I think.

Answer 7

(almost) never try to fool the system by passing a pointer to a member when you really want to pass a pointer to an object. Do as Grijesh suggested. Passing a member can lead to horrible side effects. For example, quicksort is going to sort all the integers together, regardless of which of them are X's and which are Y's. In milder cases you may get wrong compare criteria, and often hard to debug effects such as incorrect pointer optimization. Just be honest with the compiler and pass the object pointer if you need to pass an object pointer. There are very very very few exceptions, mostly to do with low-level system programming where the "other side' of the function call won't be able to handle the object.