C++

Forum

 
While loop not updating User-Defined type object

pieroborrelli (62)
Hello everybody, I'm new to programming and today I'm here because I need help with a simple exercise that I'm trying to solve. The exercise consists of various small parts :

1) Define a struct called Point with two x and y coordinates (double).
2) Prompt the user to enter 7 (x,y) pairs.
3) As we read those pairs we have to store them in a vector of Points called original_points.


So this is my first attempt : 

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
 
struct Point {
	double x; 
	double y; 
};


istream& operator>>(istream& is, Point& p)
{

	char ch1; 
	if (is >> ch1 && ch1 != '(') { 
		is.unget(); 
		is.clear(ios_base::failbit); // oops format error
		return is; 
		
	}

	double v1, v2;
	char ch2, ch4;

	is >> v1 >> ch2 >> v2 >> ch4;
	if (!is || ch2 != ',' || ch4 != ')') error("Bad reading"); // messed up reading 

	p.x = v1; 
	p.y = v2; 

	return is; 

}
void skip_to_character(char character)
{
	if (cin.fail()) {
		cin.clear(); 
		for (char ch; cin >> ch;) {
			if (ch == character) {
				cin.unget(); 
				return; 
			}

		}

	}
	if (cin.eof()) error("No input\n"); 

}

int main()
try {
	
	cout << "Please enter seven (x,y) coordinates : \n"; 

	vector<Point> original_points; 
	for (int i = 0; i < 7; ++i) {
		Point p;
		cin >> p;
		skip_to_character('('); // let's try to recover from a simple format error
		original_points.push_back(p);

	}

}
catch (runtime_error& e) {
	std::cerr << "Error : " << e.what() << '\n'; 

}
catch (...) {
	cerr << "Unexpected error \n"; 

}


If a simple format error occurs, I try to recover by skipping every character until the program reaches a '('. My problem is that if I try to enter a wrong input format such as : 9,99) the code will push_back() both the first correct reading after the error and also the first one before the error occurred. So for example if I try to input such data :

(1,2)
9,99)
(5,6)


I will have three elements in my vector : (1,2), (1,2) again, and (5,6). It seems like when I get into the loop after every condition check the value of P remains the same. Could you help me please with this problem ? Why is this error occurring ? Thank you ?
Last edited on
Duthomhas (13129)
Performing correct input is actually one of the most difficult things to do.

Here is basic (user-style) example that does what you want:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
 
#include <iostream>

// This thing helps us input a desired character, causing stream failure on error
struct expect
{
  char c;
  expect( char c ): c(c) { }
};

std::istream& operator >> ( std::istream& ins, const expect& e )
{
  ins >> std::ws;
  bool ok = (ins.peek() == e.c);
  if (ok) ins.get();
  else    ins.setstate( std::ios::failbit );
  return ins;
}

// Here is your (modified) Point class
struct Point
{
  double x, y;
  Point(): x(), y() { }
  Point( double x, double y ): x(x), y(y) { }
};

std::istream& operator >> ( std::istream& ins, Point& p )
{
  return ins >> expect('(') >> p.x >> expect(',') >> p.y >> expect(')');
}

// And something to check it out
int main()
{
  Point p;
  
  std::cout << "point? ";
  std::cin >> p;
  if (!std::cin)
    std::cout << "fooey!\n";
  else 
    std::cout << "Good job. x = " << p.x << " and y = " << p.y << ".\n";
}

Hope this helps.
Topic archived. No new replies allowed.