Example of set_union and set_intersection

Program as follows:

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

using namespace std;
 
int main()
{
  int setOne[] = {5, 10, 15, 20, 25};
  int setTwo[] = {50, 40, 30, 20, 10, 11, 21, 31, 41, 51};
 
  int setOneSize = sizeof(setOne) / sizeof(int);
  int setTwoSize = sizeof(setTwo) / sizeof(int);
 
  //Its necessary to sort if not already sorted
  sort(setTwo, setTwo + setTwoSize);
 
  vector<int> unionSetVector(setOneSize + setTwoSize); 
  set_union(setOne, setOne + setOneSize, setTwo, setTwo + setTwoSize, unionSetVector.begin());
  
  cout<<"\n1. unionSetVector : ";
  copy(unionSetVector.begin(), unionSetVector.end(), ostream_iterator<int>(cout, " "));
  cout<<endl;
 
  vector<int> intersectionSetVector(min(setOneSize, setTwoSize)); 
  set_intersection(setOne, setOne + setOneSize, setTwo, setTwo + setTwoSize, intersectionSetVector.begin());
 
  cout<<"\n1. intersectionSetVector : ";
  copy(intersectionSetVector.begin(), intersectionSetVector.end(), ostream_iterator<int>(cout, " "));
  cout<<endl;
 
  cout<<endl;
  return 0;
}

The output is as follows:

Example of 'Set' and operations

Example of Set and operations as follows:

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

using namespace std;
 
int main()
{
  int someInts[] = {3, 6, 9, 12, 15, 18};
  int someIntsSize = sizeof(someInts) / sizeof(int);
 
  set<int> someSet (someInts, someInts + someIntsSize);
 
  cout<<"\n1. someSet : ";
  copy(someSet.begin(), someSet.end(), ostream_iterator<int>(cout, " "));
  cout<<endl;
 
  //Insert some more elements
  someSet.insert(0);
  someSet.insert(10);
  someSet.insert(20);
 
  cout<<"\n2. someSet : ";
  copy(someSet.begin(), someSet.end(), ostream_iterator<int>(cout, " "));
  cout<<endl;
 
  //Upper Bound means the next greater number 
  set<int>::iterator it = someSet.upper_bound(5);
  cout << "\nupper_bound(5) = " << *it << endl;
 
  it = someSet.upper_bound(10);
  cout << "\nupper_bound(10) = " << *it << endl;
 
  //Lower bound means either equal or greater number
  it = someSet.lower_bound(5);
  cout << "\nlower_bound(5) = " << *it << endl;
 
  it = someSet.lower_bound(10);
  cout << "\nlower_bound(10) = " << *it << endl;
 
  //Equal Range returns a pair
  pair<set<int>::iterator,set<int>::iterator> retIt;
  retIt = someSet.equal_range(5);
  cout<<"\nequal_range(5) - lower bound = " << *retIt.first << 
    " upper bound = " << *retIt.second << endl;
 
  retIt = someSet.equal_range(10);
  cout<<"\nequal_range(10) - lower bound = " << *retIt.first << 
    " upper bound = " << *retIt.second << endl;
 
  cout<<endl;
  return 0;
}

The output is as follows:

Example of Algorithm Copy

Sometimes its useful to know the 'copy' method in algorithm class. The following is an example:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
 
//OutputIterator copy ( InputIterator first, InputIterator last, OutputIterator result );

#include<iostream>
#include <algorithm>
#include <vector>

using namespace std;
 
int main()
{
  int someInts[] = {10, 20, 20, 40, 50};
  int someMoreInts[] = {100, 200, 300, 400, 500, 600, 700};
 
  vector<int> someVector;
  vector<int>::iterator someIterator;
 
  //Get the total size of all elements
  int totalSize = (sizeof(someInts) + sizeof(someMoreInts)) / sizeof(int);
 
  //Resize someVector to right size
  someVector.resize(someVector.size() + totalSize);
 
  //Get the number of elements of someInts
  int tempSize = sizeof(someInts) / sizeof(int);
 
  vector<int>::iterator finalElementIt = copy(someInts, someInts + tempSize, someVector.begin());
 
  cout<<"\n1. someVector contains :";
  for(someIterator = someVector.begin(); someIterator != someVector.end(); ++someIterator)
    cout<<"  "<<*someIterator;
  cout<<endl;
 
  //Get size of someMoreInts
  tempSize = sizeof(someMoreInts) / sizeof(int);
 
  copy(someMoreInts, someMoreInts + tempSize, finalElementIt);
 
  cout<<"\n2. someVector contains :";
  for(someIterator = someVector.begin(); someIterator != someVector.end(); ++someIterator)
    cout<<"  "<<*someIterator;
  cout<<endl;
 
  return 0;
}

The output is as follows:

An Challenging Interview question with a difference

The following program is provided:

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

using namespace std;

//This class can be designed as you wish
class SR
{
public:
SR(int i) {}
};

int main()
{
int sum = 0;
for (int i = 1; i < 100; i++)
{
  SR ii(i);
  while (i--)
    sum += i;
}
cout<<"Expected value of sum = 161700" << endl;
cout<<"Returned value of sum = " << sum << endl;

return 0;
}

As already mentioned above, the expected output of 161700 is provided. The class 'SR' could be written as one wishes. How to do write the class 'SR' to get the desired output?

.

.

.

.

Give it a try before looking at the solution.

.

.

.

.

.

The modified program with the correct 'SR' class as follows:

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

using namespace std;

//This class can be designed as you wish
class SR
{
public:
 SR(int& i):ref(i)
 {
   var = i;
 }
 ~SR()
 {
   ref = var;
 }
private:
 int var;
 int &ref;
};

int main()
{
 int sum = 0;
 for (int i = 1; i < 100; i++)
 {
   SR ii(i);
   while (i--)
     sum += i;
 }
 cout<<"Expected value of sum = 161700" << endl;
 cout<<"Returned value of sum = " << sum << endl;

 return 0;
}

Source: Modified from here. Another similar question is available here.

isNumeric()

For some reason many C++ programmers believe there is an isNumeric() function available as part of the language. In fact I was searching for the header file to include to get a piece of code to work.

Its not difficult to create a function that will be able to check if the string is numeric or not. Here is the code.

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

using namespace std;
 
bool isNumeric1(string stringToCheck)
{
  bool numeric = false;
 
  if(stringToCheck.find_first_not_of("0123456789.") == string::npos)
    numeric = true;
 
  return numeric;
}
 
bool isNumeric2(string stringToCheck)
{
  bool numeric = true;
  unsigned i = 0;
 
  while(numeric && i < stringToCheck.length())
  {
    switch (stringToCheck[i])
    {
    case '0': case '1': case '2': case '3': case '4': case '5': 
    case '6': case '7': case '8': case '9': case '.':
      //do nothing
      break;
    default:
      numeric = false;
    }
    i++;
  }
 
  return numeric;
}
 
bool isNumeric3(string stringToCheck)
{
  stringstream streamVal(stringToCheck);
  double tempVal;
 
  streamVal >> tempVal; //If numeric then everything transferred to tempVal

  if(streamVal.get() != EOF)
    return false;
  else
    return true;
}
 
int main()
{
  string str1("123"), str2("134.567"), str3("12AB");
  cout << "Approach 1" << endl;
  cout << str1 << (isNumeric1(str1) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str2 << (isNumeric1(str2) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str3 << (isNumeric1(str3) ? " is Numeric" : " is Not Numeric") << endl;
 
  cout << "\nApproach 2" << endl;
  cout << str1 << (isNumeric2(str1) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str2 << (isNumeric2(str2) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str3 << (isNumeric2(str3) ? " is Numeric" : " is Not Numeric") << endl;
 
  cout << "\nApproach 3" << endl;
  cout << str1 << (isNumeric3(str1) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str2 << (isNumeric3(str2) ? " is Numeric" : " is Not Numeric") << endl;
  cout << str3 << (isNumeric3(str3) ? " is Numeric" : " is Not Numeric") << endl;
 
  return 0;
}
 

Output is as follows:

There are few problems in the program above:

1. It does not cater for negative numbers

2. It will return wrong result when you have incorrect string like 12.34.56.78

I have intentionally left it as an exercise.

Other approaches are possible as well. Why not try and think of an approach yourself.

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: