c++ sort text file

[raliot]

I need a help , to make this code to sort data from text file.Any help is appreciated!
file: 50 72 10 30 90 4 23

#include<iostream>
#include<conio.h>
void mergesort(int[],int,int);
void merge(int[],int,int,int);
void main()
{
int a[20],p,q,r,i,n;
cout<<"Enter the number of elements";
cin>>n;
p=0;
r=n-1;
cout<<"Enter the array";
for(i=0;i<n;i++)
{
     cin>>a[i];
}
mergesort(a,p,r);
cout<<"The sorted array is:";
for(i=0;i<n;i++)
{
     cout<<"\n"<<a[i];
}
getch();
}

void mergesort(int a[],int p,int r)
{
     if( p < r)
    {
         int q=(p+r)/2;
         mergesort(a,p,q);
         mergesort(a,q+1,r) ;
         merge(a,p,q,r);
     }
}
void merge(int a[],int p,int q,int r)
{
int c[10];
int i=p;
int j=q+1;
int k=p;
while((i<=q)&&(j<=r))
{
if(a[i]<a[j])
{
     c[k]=a[i];
      i=i+1;
     k=k+1;
}
else
{
     c[k]=a[j];
      j=j+1;
      k=k+1;
}
}
while(i<=q)
{
     c[k] =a[i];
     i=i+1;
     k=k+1;
}
while(j<=r)
{
     c[k]=a[j];
     j=j+1;
     k=k+1;
}
int l=p;
while(l<=r)
{
a[l]=c[l];
l=l+1;
}
}

[Maritimo]

I recomend to you to use vector<int> v and sort(v.begin(), v.end()); all your program can be written in less than 10 lines.