Advanced C++ with Examples

On vacation from coding

2011-06-18T02:56:00.000+01:00

I am taking a vacation from coding and as a result, I may not be updating the blog for some time to come.

I will still try my best to moderate the comments and reply to them as far as possible.

If you are interested in C++, I recommend that you join the C++ professional group on Linkedin and there are lots of interesting discussions going on, on a regular basis. There is also the other websites page that links other interesting sites on C++.

For the time being, Adieu!

When true != true

2011-06-14T06:33:00.004+01:00

If we define a boolean variable, say:

bool xyz;

then when we have to do some kind of a comparison we may do this as follows:

if(xyz == true)

When I used to be a new programmer, I used to always do this as follows:

if(true == xyz)

I think the way I did it is more robust and less error prone because you can by mistake do as follows:

if(xyz = true) //Single = instead of ==

The problem with my approach is that the code readability is affected and most people hate to see code like this. Instead, I started doing what normal people would do but making sure that I always use '==' and not '='.

Now some of you may be thinking why not:

if(xyz)

True, in case of Boolean this is a simplest way to do it and I had personally no problem with it until recently. While reviewing some code, I noticed that an engineer had changed a piece of code from:

if(xyz == true) to if(xyz)

Initially I thought that it may be that he did not like the '==' as its redundant. So when I checked his other code, I realised that he may have done this for a reason. Executing his code in the debug mode worked fine. So I changed:

if(xyz) back to if(xyz == true) and lo and behold, the test started to fail.

Putting a breakpoint I can see that while on the statement:

if(xyz == true)

the debugger shows xyz is 'true' but the comparison fails.

As usual digging into the problem further, I found that in one of the constructors, xyz was not initialised and the default garbage value appeared to show it as 'true'.

So its again got me into thinking that it may be better to write if(xyz == true) instead of if(xyz) as it may help in catching uninitialised variables. The best thing of course would be to make sure all variables in the class, whether public, protected or private are initialised.

Opinions welcome!

Multisets and Multimaps

2011-06-07T06:14:00.000+01:00

I was trying to understand where multisets would be used as compared to multimaps and I didnt find any straightforward answer. I found this simple explanation at StackOverflow:

multimap

With ZIP code as a key, all people which have that ZIP code
With account ID as key, all open orders of that person/account
A dictionary, with per keyword various explanations

multiset

is in essence a map with a key and a integer count.

The inventory of a shop, all products have their key and the amount still available is the value
accumulated sales data of a shop, every time a product is sold the product id get's added to the multiset thereby increasing the amount sold

As a result I created this example below. This is probably not the best of examples but I didnt want to change it. The main problem below is that multimap's are not strictly speaking required in the example below. I could have used a map. Multimap would be needed if there was no quantity_ in the class and then I can add the products one by one. I could have then used the count method to get the quantity.

Anyway, program as follows:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Simple example of multiset

#include <iostream>
#include <string>
#include <set>
#include <map>
#include <algorithm>

using namespace std;

class Product //Keeping this simple
{
public:
Product(string name, int code, int price) :
  name_(name), productCode_(code), price_(price)
  {
    quantity_ = 10; //Lets fix this value for simplicity
    }
bool availableForSale()
{
  if(quantity_ > 0)
  {
    quantity_--;
    return true;
  }
  return false;
}
string name_;
int productCode_;
int price_;
int quantity_;
private:
Product(); //Disabled the default constructor
};

void sellProduct(string name, multimap<string, Product*>& inventory, multiset <int> &soldItems)
{
multimap<string, Product*>::iterator it = inventory.find(name);
if(it != inventory.end())
{
  if ((*it->second).availableForSale())
  {
    soldItems.insert((*it->second).productCode_);
  }
}
else
{
  cout << "Unknown product : " << name << endl;
}
}

void soldItemsList(multimap<string, Product*>& inventory, multiset <int> &soldItems)
{
multimap<string, Product*>::iterator it = inventory.begin();
for(it = inventory.begin(); it != inventory.end(); ++it)
{
  int soldCount = soldItems.count((*it->second).productCode_);
  cout<<"Product = " << (*it->second).name_ << ", Quantity Sold = " << soldCount << endl;
}
}

int checkSales(multimap<string, Product*>& inventory, multiset <int> &soldItems)
{
int totalSales = 0;
multimap<string, Product*>::iterator it;
for(it = inventory.begin(); it != inventory.end(); ++it)
{
  int soldCount = soldItems.count((*it->second).productCode_);
  totalSales += soldCount * (*it->second).price_;
}
return totalSales;
}

int main()
{
//There is no special reason to use multimap instead of a map
//If you wanted to add same product and create quantity multimap is required
  multimap<string, Product*> inventory;
Product* prod1 = new Product("product1", 2334, 10);
Product* prod2 = new Product("product2", 4556, 50);
inventory.insert(pair<string, Product*>("product1",prod1));
inventory.insert(pair<string, Product*>("product2",prod2));

multiset <int> soldItems;

sellProduct("product1", inventory, soldItems);
sellProduct("product2", inventory, soldItems);
sellProduct("apple", inventory, soldItems);
sellProduct("product1", inventory, soldItems);
sellProduct("product1", inventory, soldItems);
sellProduct("product2", inventory, soldItems);

soldItemsList(inventory, soldItems);

cout<<"Total sales = " << checkSales(inventory, soldItems) << endl;

delete prod1;
delete prod2;
return 0;
}

The output as follows:

Example of Multisets

2011-06-02T06:08:00.002+01:00

Took this one from Java2s and modified it slightly to show a bit more info.

Example as follows:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Simple example of multiset

#include <iostream>
#include <set>
#include <algorithm>

using namespace std;

int main()
{
 int a[ 10 ] = { 7, 22, 9, 1, 18, 30, 100, 22, 85, 13 };
 int aSize = sizeof(a) / sizeof(int);
 std::multiset< int, std::less< int > > intMultiset(a, a + aSize);

 cout << "Printing out all the values in the multiset : ";
 multiset<int>::iterator it;
 for ( it = intMultiset.begin(); it != intMultiset.end(); ++it)
   cout << "  " << *it;
 cout << endl << endl;

 std::ostream_iterator< int > output( cout, " " );

 cout << "There are currently " << intMultiset.count( 15 )
      << " values of 15 in the multiset\n\n";

 intMultiset.insert( 15 );
 intMultiset.insert( 15 );

 cout << "After two inserts, there are currently " << intMultiset.count( 15 )
      << " values of 15 in the multiset\n\n";

 cout << "Printing out all the values in the multiset : ";
 for ( it = intMultiset.begin(); it != intMultiset.end(); ++it)
   cout << "  " << *it;
 cout << endl << endl;

 return 0;
}

Output as follows:

'auto' and 'register' variables

2011-05-25T06:51:00.003+01:00

One of the questions that is still being asked in the interviews is about 'auto variables' and 'register variables'. I have never come across any practical code that uses them and dont even see any need for them being used but as I say, Interview questions dont reflect how good a coder would be and even if a person answers all questions correctly, how can we judge someone's debugging skills.

Anyway, 'auto' variables are the default local variables. When you define a local variable, it is by default 'auto'. There is no need to use it as its implicit. In the new C++ standard, C++0x, 'auto' serves another purpose and may break any C++ code using auto when compiled using C++0x.

The 'register' keyword was initially used to help the compiler optimise the performance for the variables that were read/written very often. Since the compilers have evolved, nowadays they choose to ignore the requests as they can optimize the performance better than us humans.

The following is from StackOverflow discussion:

Here's an excerpt from Section 6.7.1 (footnote 101) of the C99 standard (pdf):

The implementation may treat any register declaration simply as an auto declaration. However,whether or not addressable storage is actually used, the address of any part of an object declared with storage-class specifier register cannot be computed, either explicitly (by use of the unary & operator as discussed in 6.5.3.2) or implicitly (by converting an array name to a pointer as discussed in 6.3.2.1). Thus, the only operator that can be applied to an array declared with storage-class specifier register is sizeof.

And from Section 7.1.1, Paragraph 3 of the C++ standard (pdf):

A register specifier has the same semantics as an auto specifier together with a hint to the implementation that the object so declared will be heavily used. [Note: the hint can be ignored and in most implementations it will be ignored if the address of the object is taken. —end note]

So, this seems like another case of C and C++ having "identical" features that behave the way you'd expect them most of the time, but diverge and cause confusion other times. In this situation, I think the way C does it makes sense since it

is semantically closest to being correct and;
doesn't do things behind your back.

Fun tidbits about `register`

The C++ group (WG21) wants to deprecate register:

The register keyword serves very little function, offering no more than a hint that a note says is typically ignored. It should be deprecated in this version of the standard, freeing the reserved name up for use in a future standard, much like auto has been re-used this time around for being similarly useless.
Notes from the March, 2009 meeting:
The consensus of the CWG was in favor of deprecating register.

Look what the C99 group (WG14) said about register (pdf) at a meeting:

General agreement to deprecate the “auto” keyword. Should we ask WG21 to go back to the previous use of “register” (no address)? No, this will not fly with WG21.

Sample program as follows:


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

using namespace std;

int main()
{
 int i = 123;
 auto int j = 456;
 register int k = 789;

 cout<<"Address of i = " << &i <<", Value = " << i << endl;
 cout<<"Address of j = " << &j <<", Value = " << j << endl;
 cout<<"Address of k = " << &k <<", Value = " << k << endl;

 return 0;
}

Output as follows:

Note that the address of i, j and k are very close implying that the compiler chose to ignore the 'register' keyword.

Example of set_union and set_intersection

2011-05-11T06:19:00.000+01:00

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

2011-05-03T06:14:00.000+01:00

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

2011-04-27T06:33:00.002+01:00

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

2011-04-20T06:10:00.004+01:00

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()

2011-04-12T06:53:00.003+01:00

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()

2011-04-05T06:37:00.000+01:00

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:

The last two Tom's are part of 'Tomorrow', how can you make sure they are not printed
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

2011-03-29T06:23:00.000+01:00

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

2011-03-22T06:57:00.002Z

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

2011-03-08T06:39:00.000Z

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

2011-03-04T06:59:00.004Z

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

2011-03-01T06:30:00.002Z

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'

2011-02-22T06:18:00.000Z

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

2011-02-15T06:48:00.000Z

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 '+'

2011-02-09T06:24:00.004Z

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

2011-02-02T06:42:00.005Z

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.

Swap two variables without using third and in one line

2011-01-26T06:28:00.001Z

Couple of weeks back I was interviewing a fresh graduate. Even though they are taught programming, I am not sure if they take it seriously and learn or practice it well. One of the questions I asked was to swap 2 numbers without using a temp variable.

Looking back now, I think it may be a bigger challenge to ask to swap numbers without using a temp variable and in one line. Below are my three different approaches but I would advise you to try it yourself before looking at the answer.

//Program to swap 2 numbers without using 3rd variable and in one line
//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>

using namespace std;

void approach1(int& a, int& b)
{
  cout<<"\nApproach 1"<<endl;
  a^=b^=a^=b;
}

void approach2(int& a, int& b)
{
  cout<<"\nApproach 2"<<endl;
  //b=(a+b)-(a=b); - This should work but doesnt, why?
  a =((a = a + b) - (b = a - b)); 
}

void approach3(int& a, int& b)
{
  cout<<"\nApproach 3"<<endl;
  a = ((a = a * b) / (b = a / b));
}



int main()
{
  int a = 13, b = 29;
  cout<<"\nOriginal"<<endl;
  cout<<"a = "<<a<<", b = "<<b<<endl;

  approach1(a, b);
  cout<<"a = "<<a<<", b = "<<b<<endl;

  a = 13, b = 29;
  approach2(a, b);
  cout<<"a = "<<a<<", b = "<<b<<endl;

  a = 13, b = 29;
  approach3(a, b);
  cout<<"a = "<<a<<", b = "<<b<<endl;

  return 0;
}

The output is as follows:

Agreed that the above would be applicable only for integers.

'sizeof' a class

2011-01-19T06:25:00.003Z

Continuation from last week. What is the sizeof of a class? Wrote a simple program as follows:

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

using namespace std;

class A
{
  int a;
};

class B
{
  int b;
  int someFunc(void) {return b;}
};

class C
{
  void someFunc1(void) {};
  void someFunc2(void) {};
};

class D : public A
{
  void someFunc(void) {};
};


int main()
{
  cout<<"Size of class A = "<<sizeof(A)<<endl;
  cout<<"Size of class B = "<<sizeof(B)<<endl;
  cout<<"Size of class C = "<<sizeof(C)<<endl;
  cout<<"Size of class D = "<<sizeof(D)<<endl;
}

The output is as follows:

After finishing this, I found this interesting article here.

Checking the 'sizeof' doubts

2011-01-12T06:57:00.003Z

I noticed sometime back that one of my programs behaved unexpectedly in certain scenarios which was traced to an incorrect use of sizeof. As a result, I made a small program to make sure that my understanding of sizeof is correct. Here is the program:

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

using namespace std;

int main()
{
 char a[]="";
 cout<<"Size of a[] = "<<sizeof(a)<<endl;

 char b[]=" ";
 cout<<"Size of b[ ] = "<<sizeof(b)<<endl;

 char c[10];
 cout<<"Size of c[10] = "<<sizeof(c)<<endl;

 char* d;
 cout<<"Size of d = "<<sizeof(d)<<endl;

 char* e = c;
 cout<<"Size of e = "<<sizeof(e)<<endl;

 char f[] = "123456";
 cout<<"Size of f = "<<sizeof(f)<<endl;

 char g[] = {'1','2','3','4','5','6'};
 cout<<"Size of g = "<<sizeof(g)<<endl;

 return 0;
}

The output is as follows:

A lot of interviewers love the sizeof questions.

Have you come across any interesting sizeof problems? Please feel free to add them in comments.

C++ example of Linked List implementation

2011-01-05T06:50:00.002Z

It is debatable if Linked lists are needed in C++, especially because STL can do a very good job for the same functionality for which Linked Lists are required. Nevertheless sometimes it maybe convenient to know linked lists.

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

using namespace std;

class Node
{
public:
  Node() : next(NULL) {}; //default constructor
  Node(const string& s1) : info(s1), next(NULL) {};
  string info;
  Node* next;
};

Node* getNewNode()
{
  //Lets flush the input buffer first 
  istream& in(cin);
  in.ignore( numeric_limits<std::streamsize>::max(), '\n' );

  //Read the String from console
  cout<<"\tEnter a string: ";
  istreambuf_iterator<char> eos; // end-of-range iterator
  istreambuf_iterator<char> iit (cin.rdbuf()); // stdin iterator
  string s2;
  while (iit!=eos && *iit!='\n') s2+=*iit++;

  //Call the constructor with the input string
  Node* newNode = new Node(s2);

  return newNode;
}

int main()
{
  Node* head = NULL;
  bool contFlag = true;
  char c;

  while(contFlag)
  {
    cout<<"\n\n";
    cout<<"0. Quit"<<endl;
    cout<<"1. Traverse and Print"<<endl;
    cout<<"2. Insert node at the start"<<endl;
    cout<<"3. Insert node at the end"<<endl;
    cout<<"4. Insert after specified number of nodes"<<endl;
    cout<<"5. Remove node from the start"<<endl;
    cout<<"6. Remove node from the end"<<endl;
    cout<<"7. Remove node from a specified number"<<endl;

    cout<<"\nEnter your choice: ";
    cin>>c;

    switch(c)
    {
    case '0':
      contFlag = false;
      break;
    case '1':
      {
        Node* tempNode = head;
        int count = 0;
        while(tempNode)
        {
          cout<<"\tItem "<<count<<" = "<<tempNode->info<<endl;
          tempNode = tempNode->next;
          count++;
        }
      }
      break;
    case '2':
      {
        Node* newNode = getNewNode();
        newNode->next = head;
        head = newNode;
      }
      break;
    case '3':
      {
        Node* newNode = getNewNode();

        Node* endNode = head;
        if(endNode)
        {
          while(endNode->next)
            endNode = endNode->next;
          endNode->next = newNode;
        }
        else //In case its the first element
        {
          head = newNode;
        }
      }
      break;
    case '4':
      {
        Node* tempNode = head;
        if(tempNode)
        {
          unsigned num;
          cout<<"\tInsert after how many nodes? : ";
          cin>>num;
          Node* newNode = getNewNode();

          unsigned count = 1;
          //If less elements are present then insert at the end
          while(tempNode->next && (count < num))
          {
            tempNode = tempNode->next;
            count++;
          }
          newNode->next = tempNode->next;
          tempNode->next = newNode;
        }
        else
        {
          cout<<"\tNo nodes present yet"<<endl;
        }
      }
      break;
    case '5':
      {
        Node* firstNode = head;
        if(firstNode)
        {
          head = firstNode->next;
          delete firstNode;
        }
      }
      break;
    case '6':
      {
        Node* endNode = head;
        if(endNode) //Atleast one element
        {
          Node* endEndNode = endNode->next;
          if(endEndNode) //Atleast 2 elements
          {
            while(endEndNode->next)
            {
              endNode = endEndNode;
              endEndNode = endEndNode->next;
            }
            endNode->next = NULL;
            delete endEndNode;
          }
          else //Only one element
          {
            head = NULL;
            delete endNode;
          }
        }
      }
      break;
    case '7':
      {
        Node* tempNode = head;
        if(tempNode)
        {
          unsigned num;
          cout<<"\tRemove after how many nodes? : ";
          cin>>num;

          unsigned count = 1;
          //If less elements are present then return
          while(tempNode->next && (count < num))
          {
            tempNode = tempNode->next;
            count++;
          }
          if(count == num && tempNode->next && tempNode->next->next)
          {
            Node* nodeToRemove = tempNode->next;
            tempNode->next = nodeToRemove->next;
            delete nodeToRemove;
          }
        }
        else
        {
          cout<<"\tNo nodes present yet"<<endl;
        }
      }
      break;
    default:
      cout<<"Incorrect selection."<<endl;
    }
  }

  return 0;
}

The output is as follows:

You can try more combinations and if you have ideas for improvements please add in the comments section.

Few things to note:

1. This program is for reference and I have not accounted for memory leaks after pressing '0'.

2. For simplicity, getNewNode() is shown as a seperate function. In a real program, it may be better as a method of class Node

3. General Linked Lists use Struct's but in C++, Structs and Classes are nearly the same so its better to use Class.

4. In C++ there is rarely a need to use Linked Lists. Its better to use the STL which already have vectors, stacks, lists, etc.

5. There should be check for memory allocation and try catch blocks that have been ignored for simplicity

String Shift and Print

2010-12-22T06:44:00.002Z

After my last blog post, someone pointed out a similar problem here. The problem goes:

Write a program that generates from the string "abcdefghijklmnopqrstuvwxyz{" the following:

bcb

cdedc

defgfed

efghihgfe

fghijkjihgf

ghijklmlkjihg

hijklmnonmlkjih

ijklmnopqponmlkji

jklmnopqrsrqponmlkj

klmnopqrstutsrqponmlk

lmnopqrstuvwvutsrqponml

mnopqrstuvwxyxwvutsrqponm

nopqrstuvwxyz{zyxwvutsrqpon

Well there is no time limit this time and we dont even need to use google. I think this is slightly trickier than the last one.

Here is my solution:

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

#include<iostream>
#include<string>

using namespace std;

int main()
{
  string str = "abcdefghijklmnopqrstuvwxyz{";
  unsigned strlen = str.length();
  string tempStr;
  int loop = 0;
  while(tempStr.length() < strlen)
  {
    int start1 = loop;
    int count1 = loop + 1;
    tempStr = str.substr(start1,count1);

    count1--;
    for(; count1 > 0; count1--)
      tempStr.append(str.substr(start1+count1-1, 1));

    cout<<tempStr;
    cout<<endl;
    loop++;
  }
  return 0;
}

Advanced C++ with Examples

On vacation from coding

When true != true

Multisets and Multimaps

Example of Multisets

'auto' and 'register' variables

Fun tidbits about register

Example of set_union and set_intersection

Example of 'Set' and operations

Example of Algorithm Copy

An Challenging Interview question with a difference

isNumeric()

string::find_next()

Reading file names from directory

Reading Files into Vector

Switch Case using string

Removing all white-spaces from a string

Removing Multiple White-spaces from a string

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

Debugging Mutex and Locks

Add two unsigned integers without using '+'

Consequences of Ignoring Compiler Warnings

Swap two variables without using third and in one line

'sizeof' a class

Checking the 'sizeof' doubts

C++ example of Linked List implementation

String Shift and Print

Fun tidbits about `register`