This is Part 4 of the 7-part Exercise 6-0. Click here to see the other parts.

Exercise 6-0 (Part 4 / 7)

Relating to section 6.1.3 of the textbook (page 105-109). Implement and test out the “Finding URLs” program. The program scans a line of input text, and automatically detect and display all the URL addresses within that line of text.

The Project

This section summarises the partitioned program in the form of C++ source and header files.

Source File List

• main.cpp
• find_urls.cpp
• not_url_char.cpp
• url_beg.cpp
• url_end.cpp
• vcout.cpp

Header File List

• find_urls.h
• not_url_char.h
• url_beg.h
• url_end.h
• vcout.h

Source Files

main.cpp

#include <iostream>  // cin, cout, endl
#include <string>  // string
#include <vector>  // vector
#include "find_urls.h"  // find_url
#include "vcout.h"  // vcout

using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;

int main()
{
    cout << "Enter a line. This program automatically find URLs..." << endl;

    // Read a line of input, then find and display URLs.
    string line;
    vector<string> urls;
    while (getline(cin, line)) {
      vector<string> urls = find_urls(line);
      vcout(urls);
    }

    return 0;
}

find_urls.cpp

#include <string>  // string
#include <vector>  // vector
#include "url_beg.h"  // url_beg
#include "url_end.h"  // url_end

using std::string;
using std::vector;

vector<string> find_urls(const string& s)
{
  vector<string> ret;
  typedef string::const_iterator iter;
  iter b = s.begin(), e = s.end();

  // look through the entire input
  while (b != e) {

    // look for one or more letters followed by ://
    b = url_beg(b, e);

    // if we found it
    if (b != e) {
      // get the rest of the URL
      iter after = url_end(b, e);

      // remember the URL
      ret.push_back(string(b, after));

      // advance b and check for more URLs on this line
      b = after;
    }
  }
  return ret;
}

not_url_char.cpp

<br />#include <string>          // string, isalnum
#include <algorithm>       // find

using std::string;

bool not_url_char(char c)
{
  // characters, in addition to alphanumerics, that can appear in a URL
  static const string url_ch = "~;/?:@=&$-_.+!*'(),";

  // see whether c can appear in a URL and return the negative
  return !(isalnum(c) || find(url_ch.begin(), url_ch.end(), c) != url_ch.end() );
}

url_beg.cpp

#include <string>  // string, isalpha
#include <algorithm>  // search
#include "not_url_char.h"  // not_url_char

using std::string;

string::const_iterator
url_beg(string::const_iterator b, string::const_iterator e)
{
  static const string sep = "://";
  typedef string::const_iterator iter;

  // i marks where the separator was found
  iter i = b;

  while ((i = search(i, e, sep.begin(), sep.end() )) != e) {

    // make sure the separator isn't at the beginning or end of the line
    if (i != b && i + sep.size() != e) {

      // beg marks the beginning of the protocol-name
      iter beg = i;
      while (beg != b && isalpha(beg[-1]))
        --beg;

      // is there at least one appropriate character before and after the separator?
      if (beg != i && !not_url_char(i[sep.size()]))
        return beg;
    }

    // the separator we found wasn't part of a URL; advance i past this separator
    i += sep.size();
  }
  return e;
}

url_end.cpp

#include <string>  // string
#include <vector>  // vector
#include <algorithm>  // find_if
#include "not_url_char.h"  // not_url_char

using std::string;

string::const_iterator
url_end(string::const_iterator b, string::const_iterator e)
{
  return find_if(b, e, not_url_char);
}

vcout.cpp

#include <iostream>
#include <string>    // string
#include <vector>    // vector

using std::cout;
using std::endl;
using std::string;
using std::vector;

int vcout(const vector<string>& v)
{
  for (vector<string>::const_iterator i = v.begin(); i != v.end(); ++i)
    cout << (*i) << endl;

  return 0;
}

Header Files

find_urls.h

#ifndef GUARD_FIND_URLS_H
#define GUARD_FIND_URLS_H

#include <vector>
#include <string>

std::vector<std::string> find_urls(const std::string&);

#endif // GUARD_FIND_URLS_H

not_url_char.h

#ifndef GUARD_NOT_URL_CHAR_H
#define GUARD_NOT_URL_CHAR_H

bool not_url_char(char);

#endif // GUARD_NOT_URL_CHAR_H

url_beg.h

#ifndef GUARD_URL_BEG_H
#define GUARD_URL_BEG_H

std::string::const_iterator
url_beg(std::string::const_iterator, std::string::const_iterator);

#endif // GUARD_URL_BEG_H

url_end.h

#ifndef GUARD_URL_END_H
#define GUARD_URL_END_H

#include <string>

std::string::const_iterator
url_end(std::string::const_iterator, std::string::const_iterator);

#endif // GUARD_URL_END_H

vcout.h

#ifndef GUARD_VCOUT_H
#define GUARD_VCOUT_H

#include <string>
#include <vector>

int vcout(const std::vector<std::string>&);

#endif // GUARD_VCOUT_H

Test program

Let’s do some simple tests:

• Submitting the line “Have you tried http://google.co.uk and http://bbc.co.uk ?”, the program should return: “http://google.co.uk” and “http://bbc.co.uk”.
• Submitting the line “Have you tried http:// or google.co.uk or http://google.co.uk ?”, the program should return: “http://google.co.uk”.
• Submitting the line “Have you tried http or :// or http://bbc.co.uk ?”, the program should return: “http://bbc.co.uk “.

Reference

Koenig, Andrew & Moo, Barbara E., Accelerated C++, Addison-Wesley, 2000