string::find_next()

I met this new starter who has come from Java background. He was upset because he couldn't find string::find_next() as C++ function. He said Java is much more flexible this way. In reality, its not that bad if you look at the definition of find carefully.

size_t find ( const string& str, size_t pos = 0 ) const;

size_t find ( const char* s, size_t pos, size_t n ) const;

size_t find ( const char* s, size_t pos = 0 ) const;

size_t find ( char c, size_t pos = 0 ) const;

The first one works as find_next as I show in the example below:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy

#include <iostream>
#include <string>

using namespace std;
 
int main()
{
  string someString("Tom is not in. Infact Tom is not going to be in Tomorrow or day after Tomorrow");
  string findTheString("Tom");
 
  unsigned position = 0;
 
  position = someString.find(findTheString);
  cout<<"First position of " << findTheString << " is " << position << endl;
 
  //We want to find all the next positions which is after position + findTheString.length()
  while(position != string::npos)
  {
    position = someString.find(findTheString, (position + findTheString.length()));
    cout<<"Next position of " << findTheString << " is " << position << endl;
  }
 
  return 0;
}

The output is as follows:

Exercise for new programmers:

1. The last two Tom's are part of 'Tomorrow', how can you make sure they are not printed
2. The last Tom which is 4294... is equal to -1 or string::npos. How can you stop that being printed without making another check for string::npos

Please dont post answers as they should be trivial exercise and you should be able to figure out without much problems.

Reading file names from directory

The other day I had to read all the file names from a directory and I found it really difficult to write a simple program to do that. While searching I ended up at Stack Overflow and the following example is taken from here.

Note that the 'dirent.h' is not available as standard windows file and the best option is to download it from here and add it in your include directory of the project.

The program as follows:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>

#pragma warning( disable : 4996 )
#include "dirent.h"
#pragma warning( default : 4996 )

using namespace std;

int main(int argc, char *argv[])
{
if(argc != 2)
{
  cout<<"Usage: "<<argv[0]<<" <Directory name>"<<endl;
  return -1;
}

DIR *dir;
dir = opendir (argv[1]);
if (dir != NULL)
{
  cout<<"Directory Listing for "<<argv[1]<<" : "<<endl;
  struct dirent *ent;
  while ((ent = readdir (dir)) != NULL)
  {
    cout<<ent->d_name<<endl;
  }
}
else
{
  cout<<"Invalid Directory name"<<endl;
  return -1;
}

return 0;
}

Note that in this program we put Command-line arguments argc and argv. What this means is that the final .exe is run using command prompt and the first parameter will be exe file name and the second parameter will be the directory path or you can add the directory path in Debugging->Command arguments of the properties

You may also encounter the following warning

warning C4996: 'strcpy': This function or variable may be unsafe. Consider using strcpy_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.

to get rid of it, I have used the #Pragma supressor.

You may also get the following error:

error C2664: 'FindFirstFileW' : cannot convert parameter 1 from 'char *' to 'LPCWSTR'

to get rid of it, go to properties->general->Character set - default is 'Use Unicode Charachter set', change it to 'Use Multi-Byte Character Set'

The output is as follows:

Reading Files into Vector

Thought of this while trying to create a parser. The intention is to read a complete text file into a vector and then use this vector for other operations. This program shows how to read from a text file into vectors.

The input file is as follows:

Line num 1

Another line

Line number 3

4th Line!

**%** Last line **%**

Program as follows:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include <iostream>
#include <vector>
#include <fstream>
#include <string>

using namespace std;

int main()
{
 vector<std::string> lines;
 lines.reserve(5000); //Assuming that the file to read can have max 5K lines

 string fileName("test.txt");

 ifstream file;
 file.open(fileName.c_str());

 if(!file.is_open())
 {
   cerr<<"Error opening file : "<<fileName.c_str()<<endl;
   return -1;
 }

 //Read the lines and store it in the vector
  string line;
 while(getline(file,line))
 {
   lines.push_back(line);
 }

 file.close();

 //Dump all the lines in output
  for(unsigned int i = 0; i < lines.size(); i++)
 {
   cout<<i<<". "<<lines[i]<<endl;
 }

 return 0;
}

Output as follows:

Switch Case using string

When I started thinking of this example, I also faced some of the other problems that I have encountered in my initial days of C++. One of these problems was how to convert String to multiple Ints and how to split strings easily, etc. Hopefully you will find the example useful.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>
#include<string>
#include<sstream>
#include<map>

using namespace std;

void split(const string& input, int& num1, int& num2, string& operation);
int calculate(int num1, int num2, string operation);

int main()
{
 string s[5] = {"2 + 2", "4 - 1", "4 * 6", "18 / 3", "12 ^ 2"};

 for(int i = 0; i < 5 ; i++)
 {
   int num1=0, num2=0;
   string op;
   split(s[i], num1, num2, op);
   int retVal = calculate(num1, num2, op);
   cout<<s[i]<<" = "<<retVal<<endl;
 }

 return 0;
}

void split(const string& input, int& num1, int& num2, string& operation)
{
 istringstream iss(input);
 iss >> num1;
 iss >> operation;
 iss >> num2;
}

int calculate(int num1, int num2, string operation)
{
 enum Operators {unknown, add, sub, mul, div};
 map<string, Operators> mapOfOperators;
 mapOfOperators["+"] = add;
 mapOfOperators["-"] = sub;
 mapOfOperators["*"] = mul;
 mapOfOperators["/"] = div;

 switch(mapOfOperators[operation])
 {
 case add:
   return num1 + num2;
 case sub:
   return num1 - num2;
 case mul:
   return num1 * num2;
 case div:
   return num1 / num2;
 default:
   cout<<"Unrecognised Operator "<<operation<<endl;
 }
 return 0;
}

The output is as follows:

See Also: Codeguru article on 'Switch on Strings in C++'.

Removing all white-spaces from a string

Continuing on the same theme as the last post. What if all the spaces need to be stripped out from the input string:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>
#include<string>

using namespace std;

string removeAllSpaces(const string& s)
{
 string newStr(s);
 bool spacesLeft = true;

 while(spacesLeft)
 {
   int pos = newStr.find(" ");
   if(pos != string::npos)
   {
     newStr.erase(pos, 1);
   }
   else
     spacesLeft = false;
 }

 return newStr;
}

int main()
{
 string aString("This  string   has        multiple spaces     problem!");

 cout<<"Original : "<<aString<<endl;
 cout<<"Modified : "<<removeAllSpaces(aString)<<endl;

 return 0;
}

The output is as follows:

Removing Multiple White-spaces from a string

A simple program to remove an arbitrary number of white spaces in between words. Program as follows:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>
#include<string>

using namespace std;
 
bool removeDoubleSpaces(string& s)
{
  bool found = true;
 
  int i = s.find("  "); //Search for 2 spaces
  if(i != string::npos)
  {
    s.replace(i, 2, " ");
  }
  else
    found = false;
 
  return found;
}
 
string removeMultipleSpaces(const string& s)
{
  string newStr(s);
  bool found = true;
 
  while(found)
  {
    found = removeDoubleSpaces(newStr);
  }
 
  return newStr;
}
 
int main()
{
  string aString("This  string   has        multiple spaces     problem!");
 
  cout<<"Original : "<<aString<<endl;
  cout<<"Modified : "<<removeMultipleSpaces(aString)<<endl;
 
  return 0;
}
 

The output is as follows:

A generic sort program with 'functors' and 'templates'

Picked up this question from here and made a program out of it. It may be a good idea to quickly brush functors and templates if required.

The program sorts the input Vector provided regardless of the type.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

using namespace std;

class Int
{
public:
  Int() {x_ = 0;}
  Int(const int &x) {x_ = x;}
  int getID (void) {return x_;}
  int x_;
};

class Str
{
public:
  Str() {x_ = "";}
  Str(const string &x) {x_ = x;}
  string getID (void) {return x_;}
  string x_;
};

template <typename Object> class Comparator {
  public:
    bool operator()(const Object &o1, const Object &o2) const 
    {
      return (const_cast<Object&>(o1).getID() < const_cast<Object&>(o2).getID());
    }

    bool operator()(const Object *o1, const Object *o2) const {
      return (o1->getID() < o2->getID());
    }
};

template <typename VecObject> void Display(VecObject v)
{
  VecObject::iterator it;
  for(it = v.begin(); it != v.end(); ++it)
  {
    cout<<it->getID()<<", ";
  }
  cout<<endl;
}

int main()
{
  vector<Int> objects1;
  objects1.push_back(Int(3));
  objects1.push_back(Int());
  objects1.push_back(Int(77));

  //print the output
  cout<<"objects1 before sort = ";
  Display(objects1);
  std::sort(objects1.begin(), objects1.end(), Comparator<Int> ());
  cout<<"objects1 after sort  = ";
  Display(objects1);

  std::vector<Str> objects2;
  objects2.push_back(Str("Hello Hello"));
  objects2.push_back(Str("apple?"));
  objects2.push_back(Str());
  objects2.push_back(Str("1 Jump"));

  //print the output
  cout<<"objects2 before sort = ";
  Display(objects2);
  std::sort(objects2.begin(), objects2.end(), Comparator<Str> ());
  cout<<"objects2 after sort  = ";
  Display(objects2);

  return 0;
}

The output is as follows:

Debugging Mutex and Locks

When there are multiple Mutex's in the program, it may be required to find if a particular thread is locked far longer than necessary. This can cause problems and the output may not be what is expected.

To get round this, I took an old example from here and modified it to help me print some debugging info.

I modified the printSomething() in Singleton.h to add a sleep as follows:

  void printSomething(char *name, int count)
 {
   Lock guard(mutex_);
   Sleep(10);
   Lock guard2(mutex_);
   std::cout << name << " loop " << count << std::endl;
 }

and I modified the Mutex.h as follows:

//Example from http://www.relisoft.com/Win32/active.html
#if !defined _MUTEX_H_
#define _MUTEX_H_

class Mutex
{
   friend class Lock;
public:
   Mutex () { InitializeCriticalSection (& _critSection); }
   ~Mutex () { DeleteCriticalSection (& _critSection); }
private:
   void Acquire ()
   {
     DWORD start = GetTickCount();
     EnterCriticalSection (& _critSection);
     DWORD elapsed = GetTickCount() - start;
     if (elapsed > 15)
     {
       //Debugging Info
        std::cout<<"Debugging Info: Waited at mutex for "<<elapsed<<std::endl;
     }
   }
   void Release ()
   {
       LeaveCriticalSection (& _critSection);
   }

   CRITICAL_SECTION _critSection;
};

#endif 

The output is as follows:

The small problem with the above approach is that if a Thread is deadlocked, we may not get the debug output as we would have to kill the process.

Add two unsigned integers without using '+'

I found this very interesting discussion (and the program) to add two numbers without the use of the arithmetic operator '+'. The obvious guess would be to use the Bitwise operators.

The logic behind the addition operation using the bitwise operators is as follows:

integer1 =  3  = 0011b
integer2 =  5  = 0101b

    first operation        second operation      third operation
        0011                0011                 shift by one
        0101                0101                   
       ______              ______
 XOR    0110           AND  0001  ------>        <<1   0010

    Again...  
                   first operation   second operation   third operation
previous XOR        0110                0110            shift by one
previous <<1        0010                0010
                   ______              ______
               XOR  0100            AND 0010  ------>  <<1   0100

    Again...  
                   first operation  second operation    third operation
previous XOR        0100                0100            shift by one
previous <<1        0100                0100
                   ______              ______
                XOR 0000           AND  0100  ------>  <<1   1000

    Again...  
                   first operation   second operation   
previous XOR        0000               0000            
previous <<1        1000               1000
                    ______            ______
                XOR 1000          AND  0000  
When AND iguals 0 the result of XOR is iqual to the sum of the two integers

The program is as follows:

//Program to add two numbers by using boolean operators
//Ref: http://www.daniweb.com/forums/thread84950.html
//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>

using namespace std;

unsigned long add(unsigned integer1, unsigned integer2)
{
  unsigned long xor, and, temp;

  and = integer1 & integer2; /* Obtain the carry bits */
  xor = integer1 ^ integer2; /* resulting bits */
    
  while(and != 0 ) /* stop when carry bits are gone */
  {
    and <<= 1; /* shifting the carry bits one space */
    temp = xor ^ and; /* hold the new xor result bits*/
    and &= xor; /* clear the previous carry bits and assign the new carry bits */
    xor = temp; /* resulting bits */
  }
  return xor; /* final result */
}

int main()
{
  int num1 = 13, num2 = 27;

  cout << num1 << " + " << num2 << " = " <<add(num1, num2) << endl;

  return 0;
}

The output is:

13 + 27 = 40

See the original discussion here.

Consequences of Ignoring Compiler Warnings

Though this is a fictional program that I have picked up from here, I have seen similar problems in real life.

Lets say my program (below) was expected to return this

but instead returned:

Program as follows:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>

using namespace std;

class Bclass
{
public:
 Bclass() {}
 virtual void func()
 {
   cout<<"In Bclass::func()"<<endl;
 }
};

class Dclass : public Bclass
{
public:
 Dclass() {}
 virtual void func()
 {
   cout<<"In Dclass::func()"<<endl;
   Bclass:func();
 }
};

int main()
{
 Dclass d;
 d.func();
 //...
  return 0;
}

The clue of the problem was given by the compiler that generated the following warning:

main.cpp(23) : warning C4102: 'Bclass' : unreferenced label

pointing to the line

Bclass:func();

The problem lies in the fact that due to a single : rather than :: the line above is being treated as label and since its an unreferenced label it goes back to the start of the same function.

You can read the complete discussion here. Make sure to check the warnings the next time your program behaves unexpectedly.