Introduction to Cryptography: C Program for Caesar Cipher

//Caesar cipher
#include<iostream.h>
#include<stdio.h>
#include<conio.h>
#include<ctype.h>
int readfile(char myarray[])
{
 FILE *fp;
 int i = 0;
 char c;
 fp=fopen("abc.txt","r");
 while(! feof(fp))
 {
  c=fgetc(fp);
  myarray[i]=c;
  i++;
 }
 fclose(fp);
 cout<<myarray;
 getch();
 return i;
}
void main()
{
 FILE *ft;
 char myarray[50];
 char cipher[50];
 char key;
 int pos,n;
 clrscr();
 cout<<"The text from disk is :"<<endl;
 n=readfile(myarray);
 cout<<"\nEnter the encryption  key :";
 cin>>key;
 pos =tolower(key)-96;
 for(int i =0;i<n;i++)
 {
   if(myarray[i]>='a' && myarray[i]<='z')
  {
   cipher[i]=myarray[i]+pos;
   if(cipher[i]>122 )
    cipher[i]=cipher[i]-26;
  }
  else if(myarray[i]>='A' && myarray[i]<='Z')
  {
   cipher[i]=myarray[i]+pos;
   if(cipher[i]>90)
    cipher[i]=cipher[i]-26;
  }
  else
   cipher[i]=myarray[i];
 }
 cout<<cipher;
 ft=fopen("xyz.txt","w");
 for(i=0;i<n;i++)
 {
  putc(cipher[i],ft);
 }
 getch();
}