DSA student program

#include <iostream>
#include <algorithm>
#include <iomanip>
using namespace std;

int n, i;

struct student
{
int rno;
string name;
float mark;
}st[100];

void average()
{
float sum = 0.00;
for(i=0; i<n; i++)
{
if(st[i].mark!=-1)
sum+=st[i].mark;
}
float avg = sum/n;
cout<<"Average marks of the class: " <<avg<<endl;
}

void hl_mark()
{
float a[n];
int j=0;
for(i=0; i<n; i++)
{
if(st[i].mark!=-1)
{
a[j++] = st[i].mark;
}
}
sort(a, a+j);
cout<<"Highest mark: "<<a[j-1]<<endl;
cout<<"Lowest mark: "<<a[0]<<endl;
}

void max_mark()
{
float new_arr[n];
int j=0;
for(i=0; i<n; i++)
{
if(st[i].mark!=-1)
{
new_arr[j++] = st[i].mark;
}
}
sort(new_arr, new_arr+j);
float prev = new_arr[0];
int cnt = 1;
for(int i=1; i<j; i++)
{
if(new_arr[i]==prev) cnt++;
else
{
cout<<cnt<<" students scored "<<prev<<" marks."<<endl;
prev = new_arr[i];
cnt = 1;
}
}
cout<<cnt<<" students scored "<<prev<<" marks."<<endl;
}

void absent()
{
int sno = 1;
cout<<"\nAbsent Students"<<endl;
cout<<"Sr. No."<<setw(15)<<"Roll No."<<setw(20)<<"Name of Student"<<endl;
for(int i=0; i<n; i++)
{
if(st[i].mark == -1)
{
cout<<sno++<<setw(20)<<st[i].rno<<setw(20)<<st[i].name<<endl;
}
}
}

int main()
{
int ch;
cout<<"Enter number of students (If a student was absent, enter -1): ";
cin>>n;
for(i=0; i<n; i++)
{
cout<<"\nName: ";
cin>>st[i].name;
cout<<"Roll no.: ";
cin>>st[i].rno;
cout<<"Marks (out of 50): ";
cin>>st[i].mark;
}
cout<<"\nRoll No."<<setw(20)<<"Name of Student"<<setw(15)<<"Marks"<<endl;
for(i=0; i<n; i++)
cout<<st[i].rno<<setw(20)<<st[i].name<<setw(20)<<st[i].mark<<endl;
while(1)
{
cout<<"\n1. Average Marks of the class"<<endl;
cout<<"2. Highest and lowest marks of the class"<<endl;
cout<<"3. Marks scored by most of the students"<<endl;
cout<<"4. Students absent in the test"<<endl;
cout<<"5. Exit"<<endl;
cout<<"Enter your choice: ";
cin>>ch;
cout<<endl;
switch(ch)
{
case 1: average();
break;
case 2: hl_mark();
break;
case 3: max_mark();
break;
case 4: absent();
break;
case 5: exit(0);
break;
default: cout<<"Invalid choice"<<endl;
break;
}
}
return 0;
}