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I have a error with sort algorithm of c++:
I don't know where my mistake is. Help me please.Code:struct point{ //define point
}
bool cmp(point a, point b){ //some line of code
}
typedef vector<point> polygon;
polygon Convex_hull(polygon&p){
sort(p.begin,p.end,cmp); //error here: NO MATCHING FUNCTION
thanks :)
This:
should be:Code:sort(p.begin,p.end,cmp);
Incidentally, if you only have one way to compare point objects for a "less than" comparison, then it may make sense to overload operator< instead of defining this cmp function.Code:sort(p.begin(), p.end(), cmp);
Oh, thanks so much, I have use define and forget '()'
Yes, I'm trying to use overloading because my comparison need more parameters. (above code just my test to find why it fall).Code:#define ALL(a) (a).begin(),(a).end //WRONG HERE
But after I used that, I have meet error:
Maybe some problems with my code again :( I try to use advanced feature of class/struct and meet so many error. Can you help me, please :(Code:struct cmp_hull{
point pivot; //this is a third parameter for my comparison so I must use struct to replace cmp function above
bool operator()(point p, point q){ //someline of code to comparison)
}
};
polygon Convex_hull(polygon&p){
cmp_hull comp; //ERROR: synthesized method cmp_hull::cmp_hull() first request here.
//...
sort(ALL(p),comp);
}
thanks :)
I should have complained earlier, grr... please indent your code properly.
Also, it helps if you provide the smallest and simplest program that you expect would compile but which demonstrates the compile error.
Oh, I'm sorry so much.
I have indented my code again. I'm sorry because I type above code in web field, so use 'tab' is so difficult :( I still cannot paste my code in my IDE to browser to, because it's always different computer. (my computer that I use to program not have network :( )
But next time, I will make my code clearly. :)
thanks :)
Once again, it helps if you provide the smallest and simplest program that you expect would compile but which demonstrates the compile error.
this is my code with some shorten (struct point just make my program don't have error, you might not see that struct :D. the program I meet from struct comp_hull and function convex_hull_grahamscan()
I found that, if I close IDE and open again, my program will run (although maybe something don't right). but if I change something, this error come again and I restart IDE and....Code:#include<stdio.h>
#include<math.h>
#include<vector>
#include<algorithm>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR((i,0,n)
#define ALL(a) (a).begin,(a).(end)
#define SZ(a) ((int)a.size())
inline int cmp(int a,int b){return (a<=b)?(a<b)?-1:0:1;}
struct point{
int x,y;
point(){x=y=0;}
point(int x, int y):x(x),y(y){}
point operator+(point q){return point(x+q.x,y+q.y);}
point operator-(point q){return point(x-q.x,y-q.y);}
int operator*(point q){return x*q.x+y*q.y;}
int operator%(point q){return x*q.y-y*q.x;}
};
inline int ccw(point a, point b, point c){return cmp((a-b)%(c-b),0);}
typedef vector<point> polygon;
struct cmp_hull{
point pivot;
bool operator()(point p, point q)
{
int R=ccw(p,pivot,q);
if(R) return R<0;
int a=(p-pivot)*(p-pivot), b=(q-pivot)*(q-pivot);
return cmp(a,b);
}
};
polygon Convex_Hull_GrahamScan(polygon&p)
{
int j=0; polygon r(SZ(p));
if(SZ(p)) return r;
cmp_hull comp; //ERROR HERE AS I SAY BEFORE
//synthesized method 'cmp_hull::cmp_hull()' first required here |
//....
}
Help me please :( this problem make me headache.
thanks for ton :)
sort requires that it's function argument return a Boolean: true, if the first argument to the function comes before the second.
Are you from C? It's not like qsort.
As the latest code that I have posted, it has error on line declare:not because sort function.Code:cmp_hull comp;
I have typed again and still met that error :( I really don't know why.
Please help me explain this, thanks ;)
Try and create an empty default constructor for your structure.
Code:struct cmp_hull{
cmp_hull() {} // add this line.
point pivot;
bool operator()(point p, point q)
{
int R=ccw(p,pivot,q);
if(R) return R<0;
int a=(p-pivot)*(p-pivot), b=(q-pivot)*(q-pivot);
return cmp(a,b);
}
};
Jim
You still should not write the cmp function the way that you have.Quote:
Originally Posted by hqt
What whiteflags is telling you is that the cmp funtion would instead need to be defined like this:However, as you can see the definition of it is so trivial that you can just use the less-than operator directly and remove the function altogether.Code:bool cmp(int a, int b) { return a<b; }
You also probably want a non-default constructor:
This allows you to easily pass in the pivot when constructing the cmp_hull object:Code:struct cmp_hull{
cmp_hull(point pivot) : pivot(pivot) {} // add this line.
point pivot;
Code:point my_pivot(some_x, some_y);
cmp_hull comp(my_pivot);
Compared objects should also be passed into your function as const references. The member function should itself be const as well:
When your code is compiled and it sees you trying to compare objects like this, compilers will search for an appropriate function that is const rather than non-const and has the compared objects being passed as const. If none is found they will sometimes complain that no matching function exists. Logically, why would a comparison test affect the state of the objects... so, why would the objects need to passed as anything other than const?Code:struct cmp_hull{
...
bool operator()(const point& p, const point& q) const
{
...
}
};
[edit]
Incidentally, your belief that you need a struct because of the "third parameter" requirement is not true. An overloaded less-than operator can have an internal point object just like the one your struct makes use of.
[/edit]