Help me look at the code (I'm just a freshman, so I only know C). Why the bill The data in txt will always become 0 (after each command 1 is executed and more than one order is entered, only the last order in txt is saved, and all the others become 0) (o

#include <stdio.h>
#include<stdlib.h>
#include<string.h>
#define N 200

typedef struct date
{
int year;
int month;
int day;
}DATE;

typedef struct bill
{
char billName[100];
double money;
DATE time;
long long int billID;
char billSort[20];
char note[100];
}BILL;

void web(void);
void PrintMessage(BILL stu[],int i);
void Exchange(BILL temp[],BILL stu[],int i,int j);
void WritetoFile(BILL stu[],int z);
int ReadfromFile(BILL stu[]);

int main()
{
BILL temp[200];
int i=0;int z=0;int n,j;
int count=0;

DATE timech;int x=0;double Sum[13]={
   
   
   
   0,0,0,0,0,0,0,0,0,0,0,0,0}; double S=0;float P[100]={};double sum1=0;double sum2=0;double sum3=0;double sum=0;float p1,p2,p3; long int max,min;BILL stu [N];

WEB: web();
int SLN;
scanf("%d",&SLN);

switch(SLN){    case 1:z=0;printf(" Please start entering the bill .\n"); char a;do {    printf("\n  Bill summary ：");     getchar();    gets(stu[i].billName);    printf("  amount of money ：");    scanf("%lf",&stu[i].money);    printf("  date ：");    scanf("%d",&stu[i].time.year);    scanf("%d",&stu[i].time.month);    scanf("%d",&stu[i].time.day);    printf("  The order number ：");    scanf("%lld",&stu[i].billID);    printf(" *** Please from “ consumption ”“ Loan repayment ”“ Human relations ” Choose among the three .***\n");    printf("  Bill classification ：");    scanf("%s",stu[i].billSort);    printf("  remarks ：");    scanf("%s",stu[i].note);    z++;    printf("\n Enter next item （Y/N）\n");    scanf(" %c",&a);    while(a!='Y'&&a!='N')// Prevent illegal import      {    printf(" illegal input , Please enter the correct instruction \n");    getchar();    scanf("%c",&a);    }

}while(a=='Y');// Refuse to enter the next item
WritetoFile(stu,z);
goto WEB;

    case 2: n=ReadfromFile(stu); count=n;// Chronological array  for(j=0;j<count-1;j++)// Annual ranking   {     for(i=j+1;i<count;i++)     {     if(stu[j].time.year>stu[i].time.year)     {         Exchange(temp,stu,i,j);     }    } } for(j=0;j<count-1;j++)// Monthly sorting   {     for(i=j+1;i<count;i++)     {     if(stu[j].time.month>stu[i].time.month)      {         Exchange(temp,stu,i,j);     }    } } for(j=0;j<count-1;j++)// Daily sorting   {     for(i=j+1;i<count;i++)     {     if(stu[j].time.year>stu[i].time.year)      {        Exchange(temp,stu,i,j);     }    } } for(i=0;i<count;i++) {     printf("\n----------------------------------- \n");    PrintMessage(stu,i);    printf("\n");      } goto WEB;            case 3:    n=ReadfromFile(stu);    // Query material information by date     printf(" Please enter the date you want to find :\n");    scanf("%d%d%d",&timech.year,&timech.month,&timech.day);     for(i=0;i<n;i++){     if(timech.year==stu[i].time.year&&timech.month==stu[i].time.month&&timech.day==stu[i].time.day)     {    printf("\n----------------------------------- \n");    PrintMessage(stu,i);    printf("\n");     x++;    }  }    if(x==0)    printf(" There is no bill corresponding to this date ");    x=0;// Empty x Value to find again        goto WEB;                       case 4:    n=ReadfromFile(stu);                for(i=0;i<n;i++)   {           for(j=1;j<=12;j++)           {         if(stu[i].time.month==j)        {             Sum[j]=Sum[j]+stu[i].money;        }         }          S=S+stu[i].money;   }         for(i=1;i<=12;i++)        {             P[i]=Sum[i]*1.0/S;             printf("%d The proportion of monthly consumption is ：%0.1f%%\n",i,P[i]*100);        }    goto WEB;        case 5:        n=ReadfromFile(stu);        for(i=0;i<n;i++)   {         if(strcmp(stu[i].billSort," consumption \0")==0)         {             sum1=sum1+stu[i].money;         }        if(strcmp(stu[i].billSort," Loan repayment \0")==0)         {             sum2=sum2+stu[i].money;         }         if(strcmp(stu[i].billSort," Human relations \0")==0)        {             sum3=sum3+stu[i].money;         }                  sum=sum+stu[i].money;   }        p1=sum1*1.0/sum;        p2=sum2*1.0/sum;        p3=sum3*1.0/sum;        printf(" The proportion of consumption is ：%0.1f%%\n",p1*100);        printf(" The proportion of loan and repayment is ：%0.1f%%\n",p2*100);        printf(" The proportion of human relations is ：%0.1f%%\n",p3*100);      goto WEB;        case 6: n=ReadfromFile(stu);  max=stu[0].money; min=stu[0].money; for(i=1;i<n;i++) {     if(stu[i].money>max)     max=stu[i].money;     if(stu[i].money<min)     min=stu[i].money; } for(i=0;i<n;i++) {     if(stu[i].money==max)     {     printf("\n------------------------------\n");     printf(" The highest record is :\n");    PrintMessage(stu,i);    }    if(stu[i].money==min)    {     printf("\n------------------------------\n");     printf(" The lowest record is :\n");    PrintMessage(stu,i);        }} goto WEB;        default :            printf(" Your input is wrong ！\n"); goto WEB;        case 0:        exit(0);        }return 0;     

}

void web(void)
{
printf("\n Alipay billing information management system \n");
printf(" \n 1. Bill information entry \n");
printf(" 2. Sort and output bill information by date \n");
printf(" 3. Query bill information by date \n");
printf(" 4. Statistics of consumption amount by month \n");
printf(" 5. Statistical analysis by bill category \n");
printf(" 6. Query the bills with the least amount and the most amount \n");
printf(" 0. Exit the system \n");
printf(" \n Tips ： Please enter the numerical sequence number and select the corresponding operation ");
}

void PrintMessage(BILL stu[],int i)
{
printf(" Bill summary ：%s\n",stu[i].billName);
printf(" amount of money ：%.1f\n",stu[i].money);
printf(" date ：%3d-%02d-%02d\n",stu[i].time.year,stu[i].time.month,stu[i].time.day);
printf(" The order number ：%I64d\n",stu[i].billID);
printf(" Bill classification ：%s\n",stu[i].billSort);
printf(" remarks ：%s\n",stu[i].note);
}

void Exchange(BILL temp[],BILL stu[],int i,int j)
{
temp[i]=stu[i];
stu[i]=stu[j];
stu[j]=temp[i];

}

void WritetoFile(BILL stu[],int z)
{
FILE*fp;
if((fp=fopen("bill.txt","a"))==NULL)
{
printf(" The file was not found ！\n");
exit(0);
}
fwrite(stu,sizeof(BILL),z,fp);
fclose(fp);
}

int ReadfromFile(BILL stu[])
{
FILE*fp;
int i;
if((fp=fopen("bill.txt","r"))==NULL)
{
printf(" The file was not found ！\n");
exit(0);
}

for(i=0;!feof(fp);i++){    fread(&stu[i],sizeof(BILL),1,fp); } fclose(fp); return i-1; 

}