How to implement the count function of algorithm ourselves

[tomy12]

Hi,

Exercise 3 of chapter 21 of the book Programming Principles and Practice Using C++ says:
Implement count yourself and test it.

I sought it and found the declaration of that function:

template<typename InputIterator, typename Type>
typename iterator_trains<InputIterator>::difference_type count(
	InputIterator first, InputIterator last, const Type& val)

And then tried to implement it using the simplest and first way that comes in mind.

#include <std_lib_facilities_4.h>
using namespace std;

template<typename InputIterator, typename Type>
typename iterator_trains<InputIterator>::difference_type count(
	InputIterator first, InputIterator last, const Type& val)

{
	int i = 0;
	for (auto it = first; it != last; ++it)
		if (*it == val)
			++i;
	return i;
}

int main()
{
	vector<int> v1 = { 10, 20, 30, 10, 40, 10 };
	cout << "v1 = (";
	for (const auto v : v1)
		cout << v << ' ';
	cout << ")\n";

	vector<int>::iterator::difference_type result;
	result = count(v1.begin(), v1.end(), 10);

	cout << "The number of 10s in v1 = " << result << endl;

	system("pause");
	return 0;
}

I get 14 errors using VS 2017 compiler and it seems all of them refer to the count function used in the code. It's not known for the compiler although I've declared and implemented it above!
Why please?

What does typename iterator_trains<InputIterator>:ifference_type mean, too, please?

[VictorN]

Could you show the exact error messages together with the related code lines?

[2kaud]

#include <std_lib_facilities_4.h>

What does this include contain?

typename iterator_trains<InputIterator>::difference_type count(

I didn't know that c++ involved transport?

[2kaud]

int i = 0;
...
return i;

What is the return type of count() ?

[tomy12]

trains was a mistake in place of the right choice traits!
It's the content of that header file.

Now the errors are different:

1- Severity Code Description Project File Line Suppression State
Error (active) E0308 more than one instance of overloaded function "count" matches the argument list: 23

2- Severity Code Description Project File Line Suppression State
Error C2668 'count': ambiguous call to overloaded function 23

The return type of count is: typename iterator_traits<InputIterator>:ifference_type. This is totally unclear for me and I don't think if it can convert to an integer correctly.

[tomy12]

I changed the whole code to this:

#include <C:\Users\Abbasi\Desktop\std_lib_facilities_4.h>
using namespace std;

template<typename InputIterator, typename Type>
typename iterator_traits<InputIterator>::difference_type my_count(
	InputIterator first, InputIterator last, const Type& val)
{
	int i = 0;
	for (auto it = first; it != last; ++it)
		if (*it == val)
			++i;
	return i;
}

int main()
{
	vector<int> v1 = { 10, 20, 30, 10, 40, 10 };
	cout << "v1 = (";
	for (const auto v : v1)
		cout << v << ' ';
	cout << ")\n";

	vector<int>::iterator::difference_type result = my_count(v1.begin(), v1.end(), 10);

	cout << "The number of 10s in v1 = " << result << endl;

	system("pause");
	return 0;
}

And it's working.
Is it now a correct answer to the exercise?

[2kaud]

return i;

What is the return type of the function?

[tomy12]

The return type of the function is something vague for me, typename iterator_traits<InputIterator> :: difference_type, but what I've returned is of integer (i).

[2kaud]

The return type of the function is something vague for me, typename iterator_traits<InputIterator> :: difference_type, but what I've returned is of integer (i).

Yes. So what should the return type be?

[AvDav]

I guess, it should be of an integer value or either an iterator.

[2kaud]

As the function return type is

typename iterator_traits<InputIterator>::difference_type

then that is what the type of i should be to avoid any implicit conversion. Hence

template<typename InputIterator, typename Type>
typename iterator_traits<InputIterator>::difference_type my_count(
	InputIterator first, InputIterator last, const Type& val)
{
	typename iterator_traits<InputIterator>::difference_type i = 0;
	for (auto it = first; it != last; ++it)
		if (*it == val)
			++i;
	return i;
}

However, if you look at the base type of :: difference_type, on VS2017 this is an int. So in this case it doesn't matter. But in general, the type of the return variable should be the type of the function return.

[tomy12]

Thank you very much.