Working with std::algorithm functions

[Frank14]

Hi,
Here is the drills of chapter 21 of the book Programming Principles and Practice Using C++ 2nd Ed:
https://s7.postimg.org/s585rhtor/Capture.png

I'm on the drill 6 and have written the code below for that:

#include <std_lib_facilities_4.h>
using namespace std;

struct Item {	
	bool operator()(const Item& Ix, const Item& Iy) const
	{
		double vx = Ix.value;
		double vy = Iy.value;
		if (vx <= vy) return true; // vx <= vy
	    return false; 
	}

	string name;
	int iid;
	double value;
};

//***************************

int main()
{
	vector<Item> vi;
	ifstream rfile("input.txt");

	if (!rfile) {
		cout << " Failar on opening the file!\n\n";
		system("pause");
		return 1;
	}

	string s;
	int id;
	double value;

	for (size_t i = 0; i < 10; ++i) {
		rfile >> s >> id >> value;
		vi.push_back({ s, id, value });
	}

	sort(vi.begin(), vi.end(), Item());
	reverse(vi.begin(), vi.end());
	
	vi.insert(vi.begin(), { "horse shoe", 99, 12.34 });
	vi.insert(vi.begin(), { "Canon S400", 9988, 499.95 });

	for (auto i = vi.begin(); i < vi.end(); ++i)
		if (i->name == "Tom") {
			vi.erase(i);
			break;
		}
	for (auto i = vi.begin(); i < vi.end(); ++i)
		if (i->name == "Ted") {
			vi.erase(i);
			break;
		}

	for (const auto& v : vi)
		cout << v.name << "  " << v.iid << "  " << v.value << endl;

	cout << "\n";

	system("pause");
	return 0;
}

Please note the two for loops I've used for finding the two items for erasing. It works but I feel that there might be some better way instead of those loops. I tried using the find-if function but it didn't support the '==' operator.

How would you do that drill please?
The solution can also be used for the next drill (7) too.

[Thomas1965]

I tried using the find-if function but it didn't support the '==' operator

You need to overload the == operator in your Item class.

Also it would be a good idea to post your input file.

[Frank14]

Thanks for the reply.
This is the content of the input.txt file:

Tom 12 87.2
Jerry 18 65.5
Frank 17 88.12
Jeison 9 83.32
Kate 5 90.5
Loius 3 92.14
Crise 7 94.67
Abbasi 2 99.25
James 1 54.36
Ted 8 75.24

[Thomas1965]

You can actuall remove both items in one loop:

for (auto it = vi.begin(); it != vi.end(); ++it)
{
  if(it->name == "Tom" || it->name == "Ted")
  {
    it = vi.erase(it);
    if (it == vi.end()) // check for last elem
      break;
  }
}

[cire]

It would probably be better to use the less error prone erase-remove idiom:
https://en.wikipedia.org/wiki/Erase%E2%80%93remove_idiom

[Frank14]

And I think I can use a function object instead of is_odd here:
v.erase( std::remove_if(v.begin(), v.end(), is_odd), v.end() );
with an overloaded operator '=='! Rather complicated as before!

Thanks.

[coder777]

You may use lambda functions:

v.erase( std::remove_if(v.begin(), v.end(), [](const Item &i) { return (i.name == "Tom" || i.name == "Ted"); }), v.end() );

[Frank14]

Thank you. It's very handy.

[Frank14]

What if I use an std::list instead of the std::vector in the code?
I used it and got 5 errors on algorithm::sort()!

algorithm::sort() takes Random Access Iterators as its arguments.

Are the errors because std::vector::begin and std::vector::end return Random Access Iterators and std::list::begin and std::list::end return Bidirectional Iterators?

[Thomas1965]

Are the errors because std::vector::begin and std::vector::end return Random Access Iterators and std::list::begin and std::list::end return Bidirectional Iterators?

Yes, sort requires Random Access Iterators

To sort a list you can use the sort function of the list.

[Frank14]

So it's easier than converting Bidirectional Iterators to Random Access Iterators, yeah?

[Thomas1965]

You can't convert Bidirectional Iterators to Random Access Iterators AFAIK
list.sort is the way to go.

[Frank14]

I used this code but it doesn't work as expected:

#include <std_lib_facilities_4.h>
using namespace std;

struct Item {	
	bool operator()(const Item& Ix, const Item& Iy) const
	{
		double vx = Ix.value;
		double vy = Iy.value;
		if (vx <= vy) return true; // vx <= vy
	    return false; 
	}

	string name;
	int iid;
	double value;
};

//***************************

int main()
{
	list<Item> li;
	ifstream rfile("input.txt");

	if (!rfile) {
		cout << " Failar on opening the file!\n\n";
		system("pause");
		return 1;
	}

	string s;
	int id;
	double value;

	for (size_t i = 0; i < 10; ++i) {
		rfile >> s >> id >> value;
		li.push_back({ s, id, value });
	}
	
	li.sort(Item());
	reverse(li.begin(), li.end());
	
	li.insert(li.begin(), { "horse shoe", 99, 12.34 });
	li.insert(li.begin(), { "Canon S400", 9988, 499.95 });

	cout << "Size = " << li.size() << "\n\n";

	  // Using lambda expression and the remove_if helper function
	li.erase(remove_if(li.begin(), li.end(), 
		[](const Item& i) { return (i.iid == 3 || i.iid == 5); })
		, li.end() );    

	for (const auto& l : li)
		cout << l.name << "  " << l.iid << "  " << l.value << endl;

	cout << "\nSize = " << li.size() << "\n\n";

	system("pause");
	return 0;
}

I don't know where the problem is! :(

[coder777]

I used this code but it doesn't work as expected:

Hm, what do you expect and what is the actual outcome?

However, your sort function is wrong. See:

http://www.cplusplus.com/reference/list/list/sort/

It is required to use the operator< not operator<=

Also having a member function (operator()) that does not use member variables is not good. On line 40 you have an unnecessary temporary. So change to:

#include <std_lib_facilities_4.h>
using namespace std;

struct Item {	
	bool operator<(const Item& i) const
	{
	    return (value < i.value); // Note: < not <=
	}

	string name;
	int iid;
	double value;
};

//***************************

int main()
{
	list<Item> li;

...
	
	li.sort();
...

You might consider to overload the operator>> as well.

[Frank14]

Thanks for the explanation.