Entering an email...the correct way!

I was working on a project, and I was wondering if I can make something like a email entering system, probably using a class. I want it to do the following -
1. First perform a prelim check for any invalid special characters.
2. Perform a check for the @ sign (which should be present only once), and atleast one or more periods ('.')
Its easy to check for point 1, but I dont know how to accomplish point 2 simultaneously. Even one @ sign would be easily 'caught' in the first check, and if I make that an exception (by not putting it in the list for checking symbols), then multiple instances of the '@" will be possible...not realistic. :(
Please help

>and atleast one or more periods ('.')

email addresses do not need a period. perspective@net is a valid email address on a top level domain. Most top level domains don't give out email addresses though, but some do.

The best way to validate an email address would be to use a regular expression. Failing that, you can implement you're point two by keeping track of how many '@' symbols you've seen while you do your scan.

Have you considered using a regular expression library such as the PCRE library, or the one provided by Boost?

It is extremely hard, and just might be impossible, to write a regular expression that correctly handles every valid e-mail address as specified by RFCs 2821 and 2822.

The Wikipedia article is pretty good.
http://en.wikipedia.org/wiki/E-mail_address

Using purely your rules as provided:

Code:

---

#include <algorithm>
#include <string>
...
std::string email;    // Get input from user for email address and put here
std::string special;  // Add whatever invalid characters you want here
...
if( std::count(email.begin(),email.end(),'@') != 1 ||
    std::count(email.begin(),email.end(),'.') == 0 ||
    email.find_first_of(special) != std::string::npos )
{
    // Invalid address
}
else
{
    // Valid address
}

---

If you're just trying something simple out then that should work. But yeah, in reality the rules (what's a valid/invalid address) are more complex.

Quote:

---

Originally Posted by CornedBee

It is extremely hard, and just might be impossible, to write a regular expression that correctly handles every valid e-mail address as specified by RFCs 2821 and 2822.

The Wikipedia article is pretty good.
http://en.wikipedia.org/wiki/E-mail_address

---

It is possible. http://ex-parrot.com/~pdw/Mail-RFC822-Address.html

Note quite:

Quote:

---

This regular expression will only validate addresses that have had any comments stripped and replaced with whitespace (this is done by the module).

---

Code:

---

class Email
{
  char email[20];
  int count=0;
 public:
  void enter_email();
};
Email::enter_email()
{
  cout<<"\nEnter email...";
  cin.getline(email,20);
  for(int i=0; email[i]=='\0'; i++)
  {
    if(email[i]=='@')
      count++;
  }
  if(count!=1)
  {
    cout<<"\nInvalid email. Please try again.";
    enter_email(); //I hope this wont cause trouble if I make multiple objects? O.o
    count=0;
  }
}

---

I hope that cuts it, and more simply so o_o

Recursion is a poor solution for this particular type of code, don't you think?

--
Mats

Yep. Enter enough invalid email addresses, (a few thousand or ten thousand, but that's no trouble for a little script) and your app crashes.

Oh, and 19 effective characters is awfully little. My short email address has 18, my long one 29. And I've seen far longer addresses.

Yes, even my short e-mail address [an alias through a friendly web-site] is longer than 20. My work-email is 27 characters [not counting the end-zero], and my "actual" mail-address that the alias leads to is simiarl length. webmaster(at)cprogramming(dot)com would also be well beyond the limit.

Any e-mail that is "[email protected]", given that most names are 3-10 letters each side of the dot would mean that a "common" name would easily reach 30-40 letters. I'd say e-mail addresses should be alloweed to be at least 50 chars, just to cope with unusually long, double-barrel names and such.

--
Mats

Just use a std::string and have it be nearly any length. (But data input is subject to denial of service through out of memory conditions, so limit it at a 1000 characters or so.)

Uh, length shouldnt be a problem at THIS stage, I guess, its just experimental -_-
Whatever is recursion? O.o
And this app is meant for a school project...if I were making something for myself, I'd prolly make it something better and longer and stronger, but I suppose no point getting an army for killing a worm. o.o
And how dyou limit characters in std::string?

Quote:

---

Originally Posted by ultrabot90

Uh, length shouldnt be a problem at THIS stage, I guess, its just experimental -_-
Whatever is recursion? O.o
And this app is meant for a school project...if I were making something for myself, I'd prolly make it something better and longer and stronger, but I suppose no point getting an army for killing a worm. o.o
And how dyou limit characters in std::string?

---

You don't limit it "in std::string", you limit it in getline().
http://www.cppreference.com/cppstring/getline.html

Recursion is when you call your own function - which is fine if:
1. You know that there will not be an "infinite" number of calls.
2. It actually helps solve the problem. In this case, it just avoids using a while/for/do-while loop construction.

In your case, #2 is definitely broken, and #1 is not completely impossible.

--
Mats

you call enter_email from inside enter_email, if a entered string is an invalid email-address (after your definition) to try the next one. thats recursion. silly one, not needed and dangerous, because every new recursion step puts things on the stack of your process and the stack is very limited.
Better transform the algorithm to utilize an outer loop (and therefore make it iterative).

Ahm, I take it back - getline() for strings appears to NOT be able to limit the string - I don't quite know how you'd do that.

--
Mats

matsp, on the link you gave, it says...

Code:

---

string s;
 getline( cin, s );
 cout << "You entered " << s << endl;

---

But I thought string wasn't a keyword? O_O
And Im afraid I did not understand your solutions to the problem (pheres and matsp)...

That reference implicitly assumes that the std namespace is in use, so string is actually std::string, just as getline, cin, cout and endl are actually std::getline, std::cin, std::cout and std::endl respectively. Either way they are not keywords, but names from the std namespace.

Quote:

---

Originally Posted by matsp

Ahm, I take it back - getline() for strings appears to NOT be able to limit the string - I don't quite know how you'd do that.

---

Basically, by implementing your own getline using low-level character input and string's push_back. Not pleasant, but since getline() is lacking, pretty much unavoidable.

Thanks for that one o.o
I wonder how I can do away with the recursion here, though...?

Instead of

Code:

---

get_email()
{
  get input;

  if(error)
  {
    get_email();  // <- recursion. You are calling A from inside A again and again, as long
                          // as invalid formats are entered
  }

}

---

as you are doing now, do

Code:

---


bool success = false;

do
{
  get_email();
  if(error)
  {
    success = false;
  }
  else
  {
      success = true;
  }
}
while(! success)    // <- now call A in a loop, till a valid address is gotten

---