About Me

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I'm a colonist who has declared war on machines and intend to conquer them some day. You'll often find me deep in the trenches fighting off bugs and ugly defects in code. When I'm not tappity-tapping at my WMD (also, known as keyboard), you'll find me chatting with friends, reading comics or playing a PC game.

Thursday, October 27, 2011

How to erase specific elements from an STL container - the C++ way.

STL iterators are tricky little fellows. One moment they act clean and the very next, they throw their hands up and say something cryptic. Consider this snippet that erases all elements whose values are 20 from a vector of integers.

typedef std::vector<int> IntVectorType;
IntVectorType vInt;

for(IntVectorType::const_iterator iter = vInt.begin();
iter != vInt.end(); ++iter)
if( *iter == 20 )

Lo and behold! The program crashes! This is because when you erase an element from a container all iterators to its position are invalidated. This just happens to be our loop iterator. So, what is an innocent programmer to do?

Well, there are two ways to erase individual elements in an STL container.

Method 1: The crude way ( recommended for cavemen/barbarians/C -programmers ;) )

IntVectorType::iterator iter = vInt.begin();
while(iter != vInt.end())
if( *iter == 20 )
iter = vInt.erase(iter); // Get iterator to next element.
++iter; // Increment if not erased.

The method vector::erase() returns an iterator to the element that subsequently follows the last element that was erased. So, we just use that iterator to continue looping. Good! Problem solved! Is there a better way?

Method 2: The modern C++ way (recommended for rockstars/you/me)

struct EraseFromVector
// Functor
bool operator ()(const int value) const
return (value == 20);

vInt.erase(std::remove_if(vInt.begin(), vInt.end(), EraseFromVector()), vInt.end());

Looks like g(r)eek, you say? Alright then, let me explain. Basically, std::remove_if() is a magical chap. He takes two iterators as a range within which to work. For each element within that range, he then checks the unary predicate(our third argument) to see whether the element should be removed or not. If the predicate returns true, the element is removed.

I did say "removed" but in actuality, remove_if() sends the doomed elements to the back of the container. He doesn't actually remove them. How could he? He doesn't know what kind of container he's working with. Only the container can remove elements.

The nice thing about remove_if() is that he is guaranteed by the standard to preserve the order of the elements that were not removed. So, if our vector contained the elements 10, 20, 30, 20, 40 in that order, it will now contain the elements 10, 30, 40, 20, 20.

Finally, remove_if() returns an iterator to the start of the sequence containing all those miserable elements just sitting there at the back waiting for the axe to fall. We pass this iterator as the first argument to vector::erase() and an iterator to the end of the vector as the second argument. In one full swoop, all elements in the range are erased.

Now, doesn't this seem a lot more fun than a boring old while loop? :)

Saturday, October 8, 2011

R.I.P Steve Jobs

At first, the world saw him as nothing more than a nerd, a high school drop out, a rebellious kid. Then the world saw him as a geek, a competitor, an irrational human. Being fired from Apple was the best thing that could have happened because that's what made him a real entrepreneur.

A man who looked at the potential in smaller companies and brought out the best in them. He believed in beauty and elegance in computers unlike the rest of the world. The world resisted his ideology but never killed it.

He returned and changed the music industry for the better. He went through a near-death experience. He beat it, came back and changed the entire way we perceived a digital world with the release of the iPhone. Now, the world sees him in a different light - Visionary, Creator, Future Technologist.

Requiem De Pace - Steve Jobs. You will sorely be missed.