Saturday, 18 June 2011

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!

Tuesday, 14 June 2011

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!

Tuesday, 7 June 2011

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:

Thursday, 2 June 2011

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:

Wednesday, 25 May 2011

'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.

Wednesday, 11 May 2011

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:

Tuesday, 3 May 2011

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:

Wednesday, 27 April 2011

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:

Wednesday, 20 April 2011

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.

Tuesday, 12 April 2011

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.

Tuesday, 5 April 2011

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.