A little problem with my assignment on a telephone directory

[arjv]

How do i validate the strings where if someone accidentally types a number in the name category , it says "Try again"
also how do i add a sorting program which if i input "a" , tells me details of people whose name start with "a"

//Telephone directory system

#include<fstream.h>
#include<stdio.h>
#include<conio.h>
#include<string.h>
#include<stdlib.h>

class Directory
{
 char name[20];
 char num[30];
 char address[50];
 public:
 void enter()
  {
   cout<<"Enter name: "<<endl;
   gets(name);
   cout<<"Enter number: "<<endl;
   gets(num);
   cout<<"Enter address: "<<endl;
   gets(address);
  }
void show()
  {
    cout<<"Name - "<<name<<endl;
    cout<<"Number - "<<num<<endl;
    cout<<"Address - "<<address<<endl;
  }
  char *rname()
  {
   return name;
  }
  char *radd()//to return address
  {
   return address;
  }
 };

void create()
  {
   fstream F;
   F.open("phonedirectory.dat",ios::binary|ios::app);
   Directory D; char ch;
   do
   {
    D.enter();
    F.write((char*)&D,sizeof(D));
    cout<<"\nDo you want to enter more(y/n)\n";
    cin>>ch;
   }
   while(ch=='y');
   F.close();
  }
void display()
  {
  fstream F;
  F.open("phonedirectory.dat",ios::binary|ios::in);
  Directory D;
  cout<<"Displaying all the records\n";
  while(F.read((char*)&D,sizeof(D)))
   {
     D.show();
   }
   getch();
   F.close();
  }

void searchn()//search by name
  {
   fstream F;
   Directory D;
   char name[20];
   int found=0;
   F.open("phonedirectory.dat",ios::binary|ios::in);
   cout<<"Enter name to be searched - ";
   gets(name);
   while(F.read((char*)&D,sizeof(D)))
   {
    if(strcmpi(name, D.rname())==0)
      {
       cout<<"Record Found"<<endl<<endl;
       D.show();
       found++;
      }
   }
   if(!found)
    {
    cout<<name<<endl<< "Not found "<<endl;
    }
   F.close();
   getch();
  }
void insrec()
  {
   int choice;
   fstream F;
   F.open("phonedirectory.dat",ios::binary|ios::app);
   Directory D;
   do
   {
    cout<<"Enter the details for new record "<<endl;
    D.enter();
    F.write((char*)&D,sizeof(D));
    cout<<"Do you want to enter more records "<<endl;
    cout<<"1. Yes  "<<"2. No  "<<endl;
    cin>>choice;
   }
   while(choice==1);
   F.close();
   sortname();
  }
void delrec()
  {
   char delname[30];             //for checking the record to be deleted
   fstream F1,F2;
   F1.open("phonedirectory.dat",ios::binary|ios::in);
   F2.open("temp.dat",ios::binary|ios::app);
   Directory D;
   int del=0;//checking if name found
   cout<<"Enter the name of the record you want to Delete - ";
   gets(delname);
   while(F1.read((char*)&D,sizeof(D)))
    {
      if((strcmpi(delname,D.rname()))!=0)
	 {
	  F2.write((char*)&D,sizeof(D));
	 }
       else
       del++;
    }
	 if(del)
	 cout<<"The record has been deleted";

	 F1.close();
	 F2.close();
	 remove("phonedirectory.dat");
	 rename("temp.dat","phonedirectory.dat");
	 sortname();
	 getch();
  }
int main()
  {
   clrscr();
   int choice;
   int t;
   int opt;
   char pass[20];
   cout<<"********* WELCOME *********"<<endl;
   cout<<"********* TO THE *********"<<endl;
   cout<<"********* PHONE DIRECTORY *********"<<endl;
   cout<<endl<<endl<<endl;
   cout<<"SELECT YOUR IDENTITY"<<endl;
   cout<<"1.VIEWER"<<endl;
   cout<<"2.ADMINISTRATOR"<<endl;
   cout<<"CHOICE - ";
   cin>>opt;
   if(opt==1)
     {
	do
       {
	clrscr();
	cout<<endl<<endl;
	cout<<"What would you like to do - "<<endl;
	cout<<"1.Search record"<<endl
	<<"2.Display record"<<endl
	<<"3.Emergency numbers"<<endl
	<<"4.Exit"<<endl;
	cout<<"Enter choice : \n";
	cin>>t;
	clrscr();
	switch(t)
	{
	case 1:searchn();
	break;
	case 2:display();
	cout<<endl<<endl<<endl;
	break;
	case 3: cout<<"Police - 100"<<endl;
	cout<<"Ambulance - 108"<<endl;
	cout<<"Fire Department - 101"<<endl;
	break;
	case 4:
	cout<<"EXITING THE PROGRAM......"<<endl;
	cout<<"Thank You"<<endl;
	default:
	exit(0);
	}//for switch case
       }
      while(t!=3);//end of while
     }
   else if(opt==2)
     {
      do
      {
	clrscr();
	cout<<"What would you like to do - "<<endl;
	cout<<"1.Add"<<endl
	<<"2.Delete"<<endl
	<<"3.Search"<<endl
	<<"4.Sort"<<endl
	<<"5.Display"<<endl
	<<"6.Exit"<<endl;
	cout<<"Enter choice - ";
	cin>>choice;
	clrscr();
	switch(choice)
       {
	case 1:insrec();
	break;
	case 2:delrec();
	cout<<endl<<endl<<endl;
	break;
	case 3:searchn();
	cout<<endl<<endl<<endl;
	break;
	case 4:sortname();
	cout<<endl<<endl<<endl;
	break;
	case 5:display();
	cout<<endl<<endl<<endl;
	break;
	case 6:
	exit(0);
	default: exit(0);
       }//for switch case
     }
      while(t!=3);//end of while
    }
    else
    {
     cout<<"\nExiting...\n";
    }
    getch();
    return 0;
  }

[jonnin]

read input as a string and check that it is all letters.

as for how you break the data up or sort it...
sort your container and you can either iterate to find the records that start with a (brute force)
or
sort your container and use binary search (a little tricky as you have to find the first record starting with a, not just any record with a)
or
store the records in a container of containers by first letter

do you need to find substrings (contains letters) or only starts-with conditions? Is starts-with only one letter or possible a string?