与动态内存分布有关的编程题，求解！

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

struct student
{
    int num;
    char name[20];
    int sex;
    float score;
};

int main()
{
    struct student* stu;
    int n, i, j = 0, first = 0,dot=0;
    int m;
    char str[256];
    float score[2];
    int num[2];
    memset(score, 0, sizeof(float) * 2);
    memset(num, 0, sizeof(int) * 2);

    printf("请输入学生个数:");
    scanf("%d", &n);
    getchar();

    stu = (struct student*)malloc(sizeof(struct student) * n);

    for(j=0;j<n;j++)
    {
        memset(str,0,256);
        fgets(str,256,stdin);
        first = 0;

        i = 0;
        while (str[i] != ';')
        {
            if (first == 0)
            {
                (stu + j)->num = str[i] - '0';
                first = 1;
            }
            else
                (stu + j)->num = (stu + j)->num * 10 + str[i]-'0';
            i++;
        }

        m = i+1;
        while (str[++i] != ';');
        memset((stu + j)->name, 0, 20);
        memcpy((stu + j)->name, &str[m],i-m);

        first = 0;
        while (str[++i] != ';')
        {
            if (first == 0)
            {
                (stu + j)->sex = str[i] - '0';
                first = 1;
            }
            else
                (stu + j)->sex  = (stu + j)->sex * 10 + str[i] - '0';
        }

        first = 0;
        dot = 0;
        while (str[++i] != ';')
        {
            if (first == 0)
            {
                (stu + j)->score = (float)(str[i] - '0');
                first = 1;
            }
            else
            {
                if (str[i] == '.')
                {
                    dot = 1;
                }
                else
                {
                    if (dot)
                    {
                        (stu + j)->score = (stu + j)->score+ (str[i] - '0')/(float)pow(10,dot++);
                    }
                    else
                    {
                        (stu + j)->score = (stu + j)->score * 10 + str[i] - '0';
                    }
                }
            }
        }

        printf("%d,%s,%d,%f\n", (stu + j)->num, (stu + j)->name, (stu + j)->sex, (stu + j)->score);
    }

    for (j = 0; j < n; j++)
    {
        if ((stu + j)->sex == 0)
        {
            score[0] += (stu + j)->score;
            num[0]++;
        }
        else
        {
            score[1] += (stu + j)->score;
            num[1]++;
        }
    }

    printf("女 %.2f\n", score[0] / num[0]);
    printf("男 %.2f\n", score[1] / num[1]);

    return 0;
}