C / C++ / MFC

Re: Acclelerated C++ Chapter 8 Exercises - Don't Understand Question

26-Apr-08 18:18

I think you are making a wrong assumption that people here have read the C++ book you are reading. Always ask complete question rather than pointing to the book.

If I understand question completely that you are asking how a function which is passed an iterator will operate on vector. Well the iterators point to the vector! Think of iterators as pointers so when you pass an iterator to the vector you are passing a pointer to the vector. Now iterator is not exactly a pointer but it follows pointer semantics i.e. you can deference it *it, increment it ++it, decrement it --it, etc.

If you want to read more about STL then try SGI STL[^] and it has good introduction to iterators[^].

-Saurabh

oliversimon27-Apr-08 2:09

Re: Acclelerated C++ Chapter 8 Exercises - Don't Understand Question [modified]

27-Apr-08 2:09

Hi Saurabh & thanks for replying. Actually, the assumption I was making,probably wrongly, was that only people familiar with the book would reply but perhaps I should have made that more clear; in any case apologies.

To make the question more general - we have a template function that takes a vector as an argument:

template <class T>
T median(vector<T> v)
{...}

The task is this: instead of passing the vector ('v')by value, rewrite the function to operate on iterators.

This is probably v. trivial but has me stumped.

Here's the code I have so far if anyone's interested

#include "stdafx.h"
#include <algorithm>
#include <stdexcept>
#include <iostream>
#include <vector>

using namespace std;

template <class T>
T median(vector<T> v)
{
typedef typename vector<t>::size_type vec_sz;
vec_sz size = v.size();

if (size == 0)
throw domain_error("median of an empty vector");

sort(v.begin(), v.end());
vec_sz mid = size/2;

return size % 2 == 0 ? (v[mid] + v[mid-1]) / 2 : v[mid];
}

int main()
{
vector <float> v1;
vector <int> v2;

for (int i=0; i < 10; i++)
{
v1.push_back(i+5);
v2.push_back(i+5);
}
int size = median(v1);
double sf = median(v1);

cout << "The median of the float vector v1 is " << median(v1) << endl;
cout << "The median of the int vector v2 is " << median(v2)<< endl;
system ("pause");
return 0;
}

I think what I'm asking is that if the vector isn't present in the template function, how can iterators like begin() & end() possibly work on it?

Also, why does C++ have to be so awkward and obscure and frustrating (actually no reply needed to that - I know you all love it and I don't want to start a row).

Thanks
Si

Saurabh.Garg27-Apr-08 2:49

Re: Acclelerated C++ Chapter 8 Exercises - Don't Understand Question

27-Apr-08 2:49

Okay first lets consider an example using pointers. Lets say there is an array of ints.

int array[11];

Now let us pretend that this array is only of size 10. Last element is used to mark as end of the vector.

int* begin = &array[0];
int* end   = &array[10];

Now we can write a mean function as (I choose mean because its easier to show concept of iterators):

int Mean(int* begin, int* end)
{
   int _mean  = 0;
   int _count = 0;
   while(begin != end)
   {
       _mean += *begin;
       _count++;
       begin++;
   }
   return _mean/_count;
}

You will notice two things about this code. First, I have not passed array at all but I am able to compute mean. Second, the last location that we reserved in the array is used to find when to stop iterating an array. Thats it, this is the concept of STL iterators. Just imagine pointer is wrapped in an class and we call it iterator.

We can write mean function using generic container as:

template<typename InputIterator>
typename InputIterator::value_type Mean(InputIterator begin, InputIterator end)
{
   typename InputIterator::value_type _mean  = 0;
   int _count = 0;
   while(begin != end)
   {
       _mean += *begin;
       _count++;
       begin++;
   }
   return _mean/_count;
}

Now lets see the requirement in the type of InputIterator. begin and end are used in 3 places in Mean function - begin != end, *begin, and begin++. So the iterator class should should only support these operators. Notice all these three operations are supported by pointer as well. So you can call this mean function for arrays we well.

double points[11] = {0};
double mean = Mean(&points[0], &points[10]);

You can also do this:

std::vector<int> points;
points.push_back(1.0);
points.push_back(2.0);
points.push_back(3.0);
.
.
.
points.push_back(10.0);

double mean = Mean(points.begin(), points.end());

Hope this makes things clear.

-Saurabh

modified on Sunday, April 27, 2008 8:55 AM

oliversimon27-Apr-08 4:16

Re: Acclelerated C++ Chapter 8 Exercises - Don't Understand Question

27-Apr-08 4:16

Hi Saurabh,

Yes indeed that makes things crystal clear! - thank you very much for a brilliant answer. In my head I somehow equated iterators with variables. But, then, I am an idiot so that's not particularly surprising. What do iterators do? They iterate, of course: 1,2,3,4,etc. etc. So if iterators are passed you aren't passing fixed values but the ability to process through a range that they contain. How could I be so thick??

Thanks again for your patience and a really clear and concise answer.

Si

Saurabh.Garg27-Apr-08 6:54

Re: Acclelerated C++ Chapter 8 Exercises - Don't Understand Question

27-Apr-08 6:54

Hey Oliver,

You are most welcome. You got it correctly. Iterators iterate over a container and more importantly they iterate through a given range. So using iterators you can process only a part of array which is not possible when passing array itself and its size without extra arguments.

-Saurabh

oliversimon27-Apr-08 7:12

Re: Acclelerated C++ Chapter 8 Exercises - Don't Understand Question

27-Apr-08 7:12

Hi Saurabh, Smile | :)

OK here's the code I've knocked together as a solution:

#include "stdafx.h"
#include <algorithm>
#include <stdexcept>
#include <iostream>
#include <vector>
using namespace std;

template<typename inputiterator>
typename InputIterator::value_type median(InputIterator begin, InputIterator end)
{
vector<typename> v;
typedef typename vector<typename>::size_type vec_sz;

//After the while loop, below, begin == end so can't use the while loop again.
//So chuck everything in a vector and work on that
//Does this save processing over simply passing a vector into
// the template?
//Is this the way it should be done??
//Who knows - all I know is it works and I can finally move on
//Some people do this for fun, you know...

while (begin != end)
{
v.push_back(*begin);
begin++;
}

vec_sz size = v.size();
if (size == 0)
throw domain_error("median of an empty vector");

sort(v.begin(),v.end());
vec_sz mid = size/2;
return size % 2 == 0 ? (v[mid] + v[mid-1]) / 2 : v[mid];
}

int main()
{
vector <int> v2;
v2.push_back(3);v2.push_back(13);v2.push_back(7);v2.push_back(5);v2.push_back(21);v2.push_back(23);
v2.push_back(23);v2.push_back(40);v2.push_back(23);v2.push_back(14);v2.push_back(12);
v2.push_back(56);v2.push_back(23);v2.push_back(29);

//median of v2 shuld be 22. See http://www.mathsisfun.com/median.html
cout << "The median of the int vector v2 is " << median(v2.begin(),v2.end())<< endl;
system ("pause");
return 0;
}

The only niggle is that after using the while loop to get the size, 'begin' equalled 'end' and so I couldn't iterate through again to find the mid point. In the end I created a vector and put everything in there. This kind of made sense but I can't help feeling I'm missing something by using another vector... perhaps there is a way to reset the iterators after they have been used once (i.e. in the while loop)

Cheers

Si

Saurabh.Garg27-Apr-08 7:57

Re: Acclelerated C++ Chapter 8 Exercises - Don't Understand Question

27-Apr-08 7:57

Okay try this code.

template<typename InputIterator>
typename InputIterator::value_type median(InputIterator begin, InputIterator end)
{
	size_t _size = end - begin;
	if(_size == 0)
	{
		throw std::domain_error("median of an empty vector");
	}
	
	sort(begin, end);
	size_t mid = _size/2;
	return _size % 2 == 0 ? (*(begin+mid) + *(begin+mid-1)) / 2 : *(begin+mid);
}

sort function in STL needs that InputIterator be a model of RandomAccessIterator i.e. ith element in the array can be accessed using array[i]. Thus Inputiterator for Median should also be RandomAccessIterator. For random iterators we can determine size of array simply by subtracting last iterator from the first iterator (they behave exactly like pointers to array) Smile | :)

Rest all is just like an array, simple pointer arithmetic. This function has a serious side effect that it sorts the original v2. It you don't want to sort it then try nth_element function in the <algorithm class="">.

-Saurabh

oliversimon27-Apr-08 8:15

Re: Acclelerated C++ Chapter 8 Exercises - Don't Understand Question

27-Apr-08 8:15

Of course - begin and end reference the place in the vector as well as the contents of that place. Superb - I think that covers that question pretty completely.

Thanks once again, Saurabh - you've saved me a whole load of time trying to sort this one out.

Question 8-4 next...what's the betting I'll be back on here tomorrow Big Grin | :-D

!!
Cheers,
Si

Saurabh.Garg27-Apr-08 8:25

change the program's entry point function

27-Apr-08 8:25

You are most welcome Oliver.

-Saurabh

LiYS26-Apr-08 3:56

LiYS

26-Apr-08 3:56

Hi,

I was just trying to get one of the sample programs compiled. It has customized entry point function configured using directive

#pragma comment (linker, "/entry:\"something\"")

which the compiler didn't seem to recognize, it returned something like "error LNK2019: unresolved external symbol _main referenced in function ___tmainCRTStartup LIBCMTD.lib". How can I get this right? I'm using VS 2005.

Thanks,

Re: change the program's entry point function

Saurabh.Garg26-Apr-08 18:30

26-Apr-08 18:30

I think the problem is in the syntax. There is no need for quotes for the function name. Try this.

#pragma comment(linker, "/entry:something")

-Saurabh

font problem

trioum26-Apr-08 1:19

trioum

26-Apr-08 1:19

how to write other language font(text) in edit control not english

Trioum

Hamid_RT26-Apr-08 4:16

Hamid_RT

26-Apr-08 4:16

See http://www.codeproject.com/KB/cs/multilanguageapplications.aspx[^] does helpful?

trioum27-Apr-08 22:01

trioum

27-Apr-08 22:01

I need code in c++ not in .net

Trioum

phanindra varma28-Apr-08 0:33

phanindra varma

28-Apr-08 0:33

Hi...

Install new font(what u would like to see in u r edit control)

1) Create variable for Edit control m_font.
CEdit m_ctrlEdit;

2) Create varible for CFont
CFont m_font;
3)Initialize with new Font..
m_font = CreateFont(,.....,....,m_fontName) /* see in MSDN
m_edit.SetFont(&m_font);

Try this..

I am able to display "xxxxxxx.ttf" with this appproach..

Hamid_RT28-Apr-08 5:04

Hamid_RT

28-Apr-08 5:04

Oh it was my mistake Wink | ;)

anyway you can use of CFont class of MFC and then use of SetFont.

Is MSDN wrong?

zengkun10026-Apr-08 1:09

zengkun100

26-Apr-08 1:09

Here is a code snippet about CListCtrl from MSDN. And I don't think MSDN give correct code.

   // Get indexes of the first and last visible items in <br />
   // the listview control.<br />
   int index = m_myListCtrl.GetTopIndex();<br />
   int last_visible_index = index + m_myListCtrl.GetCountPerPage();<br />
   if (last_visible_index > m_myListCtrl.GetItemCount())<br />
       last_visible_index = m_myListCtrl.GetItemCount();<br />
<br />
   // Loop until number visible items has been reached.<br />
   while (index <= last_visible_index)<br />
   {<br />
       //……<br />
<br />
       // Get the next item in listview control.<br />
       index++;<br />
   }

IMO, variable last_visible_index is more than one. So the While statement should be written like this:
while (index < last_visible_index)
Am I wrong? Or MSND?

Thank you all! Smile | :)

A Chinese VC++ programmer

Mukesh Kumar26-Apr-08 2:26

Mukesh Kumar

26-Apr-08 2:26

Your Question:

Is MSDN wrong?

You asked:

Am I wrong? Or MSND?

What is wrong? you...? Laugh | :laugh:

Anyway, Did you implement this code?

Mukesh Kumar
Software Engineer

zengkun10026-Apr-08 2:42

zengkun100

26-Apr-08 2:42

Yes, I didn't run this code. But look at this :
last_visible_index = m_myListCtrl.GetItemCount();
The max index of a CListCtrl is m_myListCtrl.GetItemCount() - 1 not m_myListCtrl.GetItemCount().
Any way ,thank you!

A Chinese VC++ programmer

ThatsAlok1-Jul-09 0:02

ThatsAlok

1-Jul-09 0:02

Mukesh Kumar wrote:
What is wrong? you...?

Nope thats why asking the question!

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow
Never mind - my own stupidity is the source of every "problem" - Mixture

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
Support CRY- Child Relief and You

CPallini26-Apr-08 2:48

CPallini

26-Apr-08 2:48

Could you please, post the link to the MSDN page?

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

zengkun10026-Apr-08 2:54

zengkun100

26-Apr-08 2:54

Thank you

http://msdn2.microsoft.com/zh-cn/library/kt4by313(VS.80).aspx

Look at the sample code.

A Chinese VC++ programmer