Segmentation error but I don't know why - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define MAX_SIZE 100
I am trying to quicksort an array of 2D points based upon their distance from the origin but my code hits a Seg fault after the first scanf.
typedef struct point{
double x;
double y;
double dist;
} point;
void sortpoints(point arr[], int low, int high);
void printpoints(point arr[], int n);
Sortpoints is just a manipulation of a quicksort in ordering an array of Point structs depending on the value of Point.dist
void sortpoints(point arr[], int low, int high){
int piv, i, j;
piv = low;
i = low;
j = high;
point box;
if(low < high){
while(i<j){
while((arr[i].dist)<=(arr[piv].dist) && i<= high)
i++;
}
while((arr[j].dist) > (arr[piv].dist) && j>= low)
j--;
}
if(i<j){
box = arr[i];
arr[i] = arr[j];
arr[j] = box;
}
box = arr[j];
arr[j] = arr[piv];
arr[piv] = box;
sortpoints(arr, low, j-1);
sortpoints(arr, j+1, high);
}
Printpoints just prints the points in order of their distances from the origin
void printpoints(point arr[], int n){
int i; for(i = 0; i <= n; i++){
printf("(%.2lf, %.2lf)\n", arr[i].x, arr[i].y);
}
}
The user enters the number of points and value of the points in the form (point.x, point.y)
int main(){
point pointa;
point pointarray[MAX_SIZE];
int n=0;
printf("how many points would you like to enter?\n");
scanf("%d", &n);
if(n<MAX_SIZE){
int i;
for(i=0; i<n ; i++){
scanf("(%lf,%lf)", &pointa.x, &pointa.y);
pointa.dist = sqrt(pointa.x*pointa.x + pointa.y*pointa.y);
pointarray[i] = pointa;
}
sortpoints(pointarray, 0, n-1);
printpoints(pointarray, n);}
else{
printf("sorry, not a valid array size\n");
}
return 0;
}

Answer 1

I've got stack overflow.
Your program calls function sortpoints, and calls itself too (sortpoints calls sortpoints). But i cant see where sortpoints must stop!
As a result, j-1 goes under null, and arr[j] is not valid at all.

Answer 2

When I changed this line:
scanf("(%lf,%lf)", &pointa.x, &pointa.y);
To this:
scanf("%lf,%lf", &pointa.x, &pointa.y);
The point values were actually being read in, with the parens there nothing gets read in, I don't see anything in the documentation on scanf about this though, is anyone familiar with this?
You sorting code seems to go into infinite recursion and cause a stack overflow however.

Answer 3

Your issue is with your braces. I have reformatted your sortpoints function slightly so it is clearer where each basic block actually is.
void sortpoints(point arr[], int low, int high){
int piv, i, j;
piv = low;
i = low;
j = high;
point box;
if(low < high){
while(i<j){
while((arr[i].dist)<=(arr[piv].dist) && i<= high)
i++;
}
while((arr[j].dist) > (arr[piv].dist) && j>= low)
j--;
}
if(i<j){
box = arr[i];
arr[i] = arr[j];
arr[j] = box;
}
box = arr[j];
arr[j] = arr[piv];
arr[piv] = box;
sortpoints(arr, low, j-1);
sortpoints(arr, j+1, high);
}
It should be evident now that your segfault is the result of accidental infinite recursion. This is a good argument for using { } for your loops, even with a single statement.