On vacation from coding

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

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

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

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

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

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.