在结构数组中合并排序

Question

I have an array of structs, where each object is a student with a name and a grade, and I'm trying to merge sort this array (I want to sort them by grade in ascending order). Here is the Student Struct:

struct Student
{
    char grade[42];
    char city[42];
};

grade is a char because I assign the grade value by getting an input from the user with fgets and sscanf. I don't think it's necessary to put my whole code of the merge-sort algorithm but I want to share this part which I think might be problematic?

    int firstHalfSize = midElement - firstElement + 1;
    int secondHalfSize = lastElement - midElement;
    struct Student firstHalfArray[firstHalfSize];
    struct Student secondHalfArray[secondHalfSize];
    char *p;
    char *s;
    int index1 = 0;
    int index2 = 0;
    int mergedArrIndex = firstElement;

    while (index1 < firstHalfSize && index2 < secondHalfSize)
    {
        if (strtol(firstHalfArray[index1].grade, &p, 10) <= strtol(secondHalfArray[index2].grade, &s, 10))
        {
            arr[mergedArrIndex] = firstHalfArray[index1];
            index1++;
        }
        else
        {
            arr[mergedArrIndex] = secondHalfArray[index2];
            index2++;
        }
        mergedArrIndex++;
    }

the part where I sort the student object by comparing the grade is by converting the char grade into a long with strtol which I think I did good so it might not be a problem. My problems is that I initialize my array of structs like the following:

struct Student students[5501];

and when I get a new input of the user I just add it into the array like the following:

struct Student aStudent;
int lineCounter = 0;
students[lineCounter] = aStudent;

and increase lineCounter by 1 every time I get a new input. ( aStudent is changed when I get a new input) Here I call the merge-sort function and get weird results:

mergeSort(students, 0, 5501); // 5501 is the size of the array
printArray(students, 5501);

Here is the printArray function created just to see if I get the grade in ascending order

void printArray(struct Student A[], int size)
{
    char *p;
    int i;
    for (i=0; i < size; i++)
        printf("%ld", strtol(A[i].grade, &p, 10));

}

but I keep getting that printed:

00000000000000000000000000000000000000000000000000000000000000000000000000000000000000...

and I don't know why. Also how can I do that If the user adds 3 students object in the array students I only treat it as like an array of 3 elements and not an array of 5501 elements? Thanks. And sorry if it's a little long I really tried to be concise without having a loss of essential informations.

Answer 1

Since the user can input N numbers of students I recommend you to use a Linked List instead and put the node (struct Student) in his right position depending on the grade.

You can learn more about Linked Lists here .