QuickSort not sorting properly - c

Question

I have a this program to receive a struct, store it and then sort it. I have tried to use Shell Sort, but then I went for the Quick Sort Algorithm. However, when I try to print the array after sorting, it still returns it unsorted. Bear in mind im trying to sort it by 'num_aluno'.
CODE
typedef struct ALUNO
{
char primeiro_nome[15];
char segundo_nome[15];
int num_aluno;
float nota;
}ALUNO;
void swap(ALUNO* a, ALUNO* b)
{
ALUNO t=*a;
*a=*b;
*b=t;
}
int partition(ALUNO *studentList, int low, int high)
{
int pivot= studentList[high].num_aluno;
int i=(low-1);
int j;
for(j=low;j<=high-1;j++)
{
if(studentList[j].num_aluno<=pivot);
{
i++;
swap(&studentList[i], &studentList[j]);
}
}
swap(&studentList[i+1], &studentList[high]);
return(i+1);
}
void quickSort(ALUNO *studentList, int low, int high)
{
if(low<high)
{
int pi=partition(studentList, low, high);
quickSort(studentList, low, pi-1);
quickSort(studentList, pi+1, high);
}
}
int main()
{
ALUNO *studentList=NULL;
int currentPos, studentListSize=1;
//float grade_sum=0;
studentList=(ALUNO*)calloc(studentList, studentListSize*sizeof(ALUNO));
printf("Insira o numero de alunos \n");
scanf("%d", &studentListSize);
studentList=(ALUNO*)realloc(studentList, studentListSize*sizeof(ALUNO));
for(currentPos=0;currentPos<studentListSize;currentPos++)
{
newStudent(studentList, currentPos);
}
quickSort(studentList, 0, studentListSize);
for(currentPos=0;currentPos<studentListSize;currentPos++)
{
printStudent(studentList,currentPos);
}
free(studentList);
return 0;
}
Any help would be appreciated

Answer 1

The reason it doesn't sort the list is, i and j are always the same value in the partition function.
You should start j from high -1, not from low.
Also, you are not considering the values of studentList[i]. You must guarantee that it is larger than pivot, otherwise there may be a value which is smaller than the pivot, yet at the left part of the array.
Here, corrected it.
int partition(ALUNO*studentList, int low, int high)
{
int pivot = studentList[high].num_aluno;
int i=low ;
int j;
for(j=high-1;j>=i;)
{
while(studentList[i].num_aluno < pivot)
i++;
while(studentList[j].num_aluno > pivot)
j--;
if(i<=j){
swap(&studentList[i], &studentList[j]);
i++;
j--;
}
}
swap(&studentList[i], &studentList[high]);
return(i+1);
}
An additional advice in case you have not heard of it, do not choose the first or last values as pivots. Instead, use the median of three strategy.
Median of three strategy: Pick the first, middle and last elements in the unsorted array. Change their locations according to their values in a sorted manner. Then take the middle value and use it as a pivot. This way you are avoiding the worst time complexity(O(n^2)) of QuickSort.