Using qsort with a define struct

Question

I'm learning C and I'm solving this challenge, I'm not planning to submit it to the uva platform and the reason I'm coding this exercise is for leaning, maybe is not the best approach to the problem but I'm trying.

The input I print in my terminal is the following:

4
3
20 30 40 50 30 40
Res: 2
4 
20 30 10 10 30 20 40 50
Res: 4
3
10 30 20 20 30 10
Res: 2
4
10 10 20 30 40 50 39 51
Res: 3

The answers from each input test are incorrect and I believe the reason is the qsort function. I'm confuse about how to use the qsort function using a structure, I'm calling my structure that is called array, followed by the size of my input, then using sizeof(int) but do I need to use int or sizeof my structure, lastly I'm calling my compare function. My code is:

#include <stdio.h>
#include <string.h>

struct Dolls{
  int w;
  int h;
}array[20005];

int cmp(struct Dolls a, struct Dolls b){
  if(a.w==b.w){
    return a.h < b.h;
  }else{
    return a.w > b.w;
  }
}

int arr[20005];
int dp[20005];
int n;

int bSearch(int num, int k){
  int low=1;
  int high = k;
  int mid;
  while(low<= high){
    mid = (low+high)/2;
    if(num>=dp[mid]){
      low=mid+1;
    }else{
      high=mid-1;
    }
  }
  return low;
}

int res_dolls(){
  int k=1;
  int i,pos;
  dp[i]=arr[1];
  for(i=2;i<=n;i++){
    if(arr[i]>=dp[k]){
      dp[++k] = arr[i];
    }else{
      pos = bSearch(arr[i],k);
      dp[pos] = arr[i];
    }
  }
  return k;
}

int main(){
  int t,j;
  scanf("%d",&t);
  while(t--){
    memset(array,0,sizeof(array));
    scanf("%d",&n);
    for(j=1;j<=n;j++){
      scanf("%d %d",&array[j].w, &array[j].h);
    }
    qsort(array,n,sizeof(int),cmp);
    for(j=1;j<=n;j++){
      arr[j] = array[j].h;
    }
    printf("%d\n",res_dolls());
  }
  return 0;
}

Answer 1

Your cmp function needs to be defined as int (*)(const void *, const void *) for it to work for qsort.

The way you're performing the comparisons is also not correct. From the man page for qsort:

The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be respec- tively less than, equal to, or greater than the second. If two members compare as equal, their order in the sorted array is undefined.

The comparisons you're doing return the result of the < or > operators, which is either 0 or 1. You need to explicitly check each case and return the proper value.

int cmp(const void *va, const void *vb){
  const struct Dolls *a = va;
  const struct Dolls *b = vb;

  if(a->w > b->w) {
      return 1;
  } else if(a->w < b->w){
      return -1;
  } else if(a->h > b->h) {
      return 1;
  } else if(a->h < b->h){
      return -1;
  } else {
    return 0;
  }
}

As for the call to qsort, you need to give it the size of an array element, i.e. the whole struct, not the size of the subfield:

qsort(array,n,sizeof(struct Dolls),cmp);

EDIT:

Fixed the error with the parameter names. Also changed how the sort is performed to conform to how the comparison function must behave.