How to sort an struct of parabolas after their vertex height, using qsort?

I got a problem where I have to first create a function that calculates the vertex height of a given struct of parabolas and returns 0 for parabola und 1 if not. Then sort them with qsort using the vertex height as parameter which ascends. Also if a == 0 (not a parabola), those will be sorted after the parabolas. Then I have to use a test-main to print example-parabolas and see if they got sorted correctly. Global variables are not allowed.

I think I got the calculation of vertex height, but I have no clue what I have to write into the function for qsort, tutorials say that I have to compare a and b and return 1, -1 or 0, but I don't know how to "translate" that for my case. I'm kinda new to programming and have not quite gotten the concept of pointers or referncing functions in c.

The given header.h

Code:

---

#ifndef header
#define header 1

struct parabola {
    double a;
    double b;
    double c;
};

int vertexheight(struct parabola * p, double *y);

void sort_parabola(struct parabola * p, int n);


#endif

---

my program

Code:

---

#include <stdio.h>
#include <stdlib.h>
#include "header.h"

int compare();


int vertexheight(struct parabola * p, double *y) {
    int rc = 0;
       
    if (p->a == 0) {
       
        rc = 1;
   
    }  else {
   
        *y = p->c - ((p->b*p->b) / (4*p->a));
    }   
     
    return rc;
}

int compare(const void * a, const void * b) {


    //?
   
    return 0;
   
}


void sort_parabola(struct parabola * p, int n) {
   
    qsort(p, n, sizeof(struct parabola), compare); 
   
}

int main() {
       
    struct parabola p[] = {
        {1,2,3},
        {2,5,-19}, {0,-100,-56}, {-967,24,-24}, {36,2,70},
        {5,72,0}, {75,-4,55}, {20,41,7},
        {-1,0,0}
    };
    double y;
    int i, size = sizeof(p) / sizeof(struct parabola);
   
   
    sort_parabola(p, sizeof(p) / sizeof(struct parabola));
 
    for (i = 0; i < size; i++){
       
        //output
       
    }
     
    return 0;
}

---

The sorted output should be something like this:

Code:

---

[0]
{5.000000,72.000000,0.000000},
y=-259.200000


[1]
{-967.000000,24.000000,-24.000000},
y=-23.851086


[2]
{2.000000,5.000000,-19.000000},
y=-22.125000


[3]
{20.000000,41.000000,7.000000},
y=-14.012500


[4]
{-1.000000,0.000000,0.000000},
y=0.000000


[5]
{1.000000,2.000000,3.000000},
y=2.000000


[6]
{75.000000,-4.000000,55.000000},
y=54.946667


[7]
{36.000000,2.000000,70.000000},
y=69.972222

---

Thanks a lot if someone can help me with this! :)

Your compare function would start something like this.

Code:

---

int compare(const void * a, const void * b) {
  const struct parabola *pa = a;
  const struct parabola *pb = b;
  double ha, hb;  // heights
  int ra, rb; // returns
  ra = vertexheight(pa, &ha);
  rb = vertexheight(pb, &hb);
  // now you compare things.

---

Thanks for your answer! If I compare it like this, the output is not sorted properly. Also my compiler
gives me a warning: expected 'struct parabola *' but argument is of type 'const struct parabola *'
I have to use -Wall

Guess I did something wrong?
Also, how do I sort the parabolas with a == 0 at the end of the list? These have no vertexheight, because they are not parabolas.

Code:

---

int compare(const void * a, const void * b) {


    const struct parabola *pa = a;
      const struct parabola *pb = b;
     
    double ha, hb;  // heights
      int ra, rb; // returns
      ra = vertexheight(pa, &ha);
    rb = vertexheight(pb, &hb);
   
      if (ra < rb) return -1;
      if (ra > rb) return 1;
      if (ra == rb)return 0;
   
   
    return 0;
   
}

---

Code:

---

int main() {
       
    struct parabola p[] = {
        {1,2,3},
        {2,5,-19}, {0,-100,-56}, {-967,24,-24}, {36,2,70},
        {5,72,0}, {75,-4,55}, {20,41,7},
        {-1,0,0}
    };
    double y;
    int i, size = sizeof(p) / sizeof(struct parabola);
   
    sort_parabola(p, sizeof(p) / sizeof(struct parabola));
 
    for (i = 0; i < size; i++){
       
        vertexheight(p+i, &y);
        printf("[%i]\n{%lf,%lf,%lf},\ns=%lf\n\n", i, p[i].a, p[i].b, p[i].c, y);
        y = 999;
    }
       
   
     
    return 0;
}

---

Quote:

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Originally Posted by Moepius

Also my compiler
gives me a warning: expected 'struct parabola *' but argument is of type 'const struct parabola *'

---

Your compiler is right: to compute the height, you don't need to modify the struct object, therefore this:

Code:

---

int vertexheight(struct parabola * p, double *y);

---

should have been:

Code:

---

int vertexheight(const struct parabola * p, double *y);

---

since you did not declare vertexheight to be const-correct, when you passed a const struct parabola pointer to it in the compare function, the compiler complained because from its perspective, it is possible that vertexheight might modify the struct object, whereas the const says that the struct object that the pointer points to should not be modified.

Quote:

---

Originally Posted by Moepius

Guess I did something wrong?

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Yes, your compare function is too simple. You're only sorting parabola vs non-parabola, without taking vertex height into account.