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Timesaving Tips

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Join Date: Aug 2005

Posts: 148

Reputation: Micko is on a distinguished road

Solved Threads: 6

Micko

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Junior Poster

Timesaving Tips

Oct 14th, 2005

Hello,
I've been reading Narue's Timesaving Tips and found this code from Dave Sinkula

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#include <iostream>
#include <vector>
#include <iterator>
 
using namespace std;
 
int main()
{
  int a[] = {1,2,3,4,5};
  vector<int> v(a, a + 5);
 
  vector<int>::const_iterator it, end = v.end();
  for (it = v.begin(); it < end; ++it)
  {
     cout << *it << endl;
  }
}
#include <iostream>
#include <vector>
#include <iterator>

using namespace std;

int main()
{
  int a[] = {1,2,3,4,5};
  vector<int> v(a, a + 5);

  vector<int>::const_iterator it, end = v.end();
  for (it = v.begin(); it < end; ++it)
  {
     cout << *it << endl;
  }
}

This line is interesting:

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for (it = v.begin(); it < end; ++it)
for (it = v.begin(); it < end; ++it)

This works for vectors, but generally it's a bad programmer practice to use
relation operator < with iterators because it won't work with all containers.
Operator < is provided for random access iterators so this will not work with list
container.
I think, in order to write generic code, it's better practice to get used to uniform
approach in for different kind of containers and that's:

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for (it = v.begin(); it != end; ++it)
for (it = v.begin(); it != end; ++it)

And BTW, there is no need to include header <iterator> in this case since iterator class
is nested in vector class.
So, finally, this code example is:

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 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <vector>
 
using namespace std;
 
int main()
{
  int a[] = {1,2,3,4,5};
  vector<int> v(a, a + 5);
 
  vector<int>::const_iterator it, end = v.end();
  for (it = v.begin(); it != end; ++it)
  {
     cout << *it << endl;
  }
}
#include <iostream>
#include <vector>

using namespace std;

int main()
{
  int a[] = {1,2,3,4,5};
  vector<int> v(a, a + 5);

  vector<int>::const_iterator it, end = v.end();
  for (it = v.begin(); it != end; ++it)
  {
     cout << *it << endl;
  }
}

A small bonus:
Did you notice that some programmers use this form of for loop:

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int i;
for ( i = 0; i < 10; i++ )
/*do something*/
int i;
for ( i = 0; i < 10; i++ )
/*do something*/

when i is int,
and this form:

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for ( i = v.begin (); i != v.end (); ++i )
/*do something*/
for ( i = v.begin (); i != v.end (); ++i )
/*do something*/

when i is an iterator?
Notice prefix and postfix ++i and i++. Are these forms same when using in loops?
Answer is depends on what counter i is. If i is an integer then there is no difference,
but if i is iterator, then there could be difference. As you probably know iterator is
often an object i.e. instance of a class and in that case there is difference whether using
i++ or ++i form.
Look at this small example (clasical implementation of operator ++=:

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class Integer
{
      int x;
public:
      Integer ( int i ) : x ( i ) { };
 
      Integer& operator ++ () //prefix
      {
               x++;
               return *this;
      }
 
      Integer operator ++ (int) //postfix
      {
 
              return Integer ( x++ );
      }
};
class Integer
{
      int x;
public:
      Integer ( int i ) : x ( i ) { };
	
      Integer& operator ++ () //prefix
      {
               x++;
               return *this;
      }

      Integer operator ++ (int) //postfix
      {
              
              return Integer ( x++ );
      }
};

As you can see postfix version include creation of new object so we can conclude
that prefix form is faster.
If this issue is discussed before, I apologize but it's good to know.

- Micko

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Join Date: Apr 2004

Posts: 4,334

Reputation: Dave Sinkula has a brilliant future

Solved Threads: 234

Dave Sinkula Dave Sinkula is offline

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long time no c

Re: Timesaving Tips

Oct 14th, 2005

•

Originally Posted by Micko

I've been reading Narue's Timesaving Tips and found this code from Dave Sinkula
Help with Code Tags

C++ Syntax (Toggle Plain Text)
for (it = v.begin(); it < end; ++it)
for (it = v.begin(); it < end; ++it)

Oops. [Damn copy/paste coding habits.] Thanks for spotting that -- I'll fix that post.

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