Index problems with partition function in C

Question

I was attempting to create a partition function for a quicksort that I want to run on an array of a Struct that I created. However, during runtime the indexes suddenly act weird and go crazy. (for example after a certain bigger=bigger-1 call, bigger suddenly gets the value 0 for no apparent reason.

This is my code:

int partition(Student studentsArray[], int left, int right)
{
    Student tempArr[right+1 - left];
    int smaller = left; int bigger = right;
    for(int i =left; i<right;i++)
    {
        if(stringComperator(studentsArray[i].name,studentsArray[right].name)>0)
        {
            tempArr[bigger]=studentsArray[i];
            bigger=bigger-1;
        }
        else
        {
            tempArr[smaller]=studentsArray[i];
            smaller=smaller+1;
        }
    }
    tempArr[smaller]=studentsArray[right];
    for(int i=left;i<right-left+1;i++)
    {
        studentsArray[i]=tempArr[i];
    }
    return smaller;
}

edit: updated code: seems to work inside of the method, but it seems like studentsArray won't change it values outside of the function call.

void swap(Student* a, Student* b)
{
    Student temp = *a;
    *a = *b;
    *b = temp;
}

int partition(Student studentsArray[], int left, int right)

{
    Student pivot = studentsArray[right];
    int i = left;
    for(int j = 0; j<right;j++)
    {
        if(stringComperator(studentsArray[j].name,pivot.name)<0)
        {
            swap(&studentsArray[i],&studentsArray[j]);
            i=i+1;
        }
    }
    swap(&studentsArray[i],&pivot);
    return i;
}

Answer 1

Your code has undefined behavior in lines such as:

  tempArr[bigger]=studentsArray[i];

Your tempArr size is right+1 - left, and bigger starts as equals to right. This means that when left > 0, you are out of bounds.

One of the many things that could happen in these cases, is overriding other automatic allocated variables, such as bigger.

---

Few ways to solve it include pedantic checking for sizes. But, imho, the straightforward way is to avoid creating tempArr, and update the values inplace.