Thread: Why it can't read in a number?

Hi, I can't figure out why it can't read in the number N... Can you please help?

Code:

// Exercise 8 - Chapter 5 ("Programming: Principles and Practice using C++")
#include "std_lib_facilities.h"

class out_of_range_error{};

int main()
try {
	vector <int> numbers;
	int num, N, sum;
	
	cout << "Enter some numbers (press '|' at prompt to stop):";
	while (cin>>num) 
		numbers.push_back(num);

	cout << "Enter how many of the first numbers you want to sum: \n";
	cin >> N; // PROBLEM HERE!!! - It cannot read in N!	
	cout << "The sum of the first " << N << " numbers: ";
	sum = 0;
	for (int i=0; i<N; ++i) {
		sum += numbers[i];
		cout << numbers[i] << ", ";
		if (i==(N-1)) cout << " and";
	}
	cout << " is " << sum << endl;
}

catch (out_of_range_error) {
	cout << "Range error!\n";
	return 1;
}

catch (...) {
	cerr << "Exception: Something went wrong\n";
	return 2;
}

cin.clear()

Originally Posted by dwks

When you try to parse a number out of '|', that character will remain in the input stream, and the stream (cin) will enter an error state. You need to remove the '|' from the stream and clear the error state as well, before you can use it for anything else (for example, reading another number).

cin.clear() will clear the error state; but that leaves the offending '|' still in the input stream. std::cin.ignore() and std::cin.clear()
To deal with both problems, you might consider using

Code:

cin.ignore(1000, '\n');  // ignore lots of characters, up until a newline
cin.clear();  // clear error state

There's a better constant to use than 1000, of course. I leave it up to you to search the boards and find it if you're curious.

Thank you for the hint. Does this mean I need to add these two statements as in the following? It still doesn't work somehow :-<

Code:

// Exercise 8 - Chapter 5 ("Programming: Principles and Practice using C++")
#include "std_lib_facilities.h"

int main()
try {
	vector <int> numbers;
	int num, sum;
	unsigned int N = -1;
	
	cout << "Enter some numbers (press '|' at prompt to stop):";
	while (cin>>num) 
		numbers.push_back(num);
	
	cin.ignore(1000, '\n');  // ignore lots of characters, up until a newline
	cin.clear();  // clear error state	

	cout << "Enter how many of the first numbers you want to sum: \n";
	cin >> N; // PROBLEM HERE!!! - It cannot read in N!	
	if (N>numbers.size()) error("Too many numbers than are available in the series", numbers.size());
	if (N<1) error("The number must be at least 1");
	cout << "The sum of the first " << N << " numbers: ";
	sum = 0;
	for (unsigned int i=0; i<N; ++i) {
		sum += numbers[i];
		cout << numbers[i] << ", ";
		if (i==(N-1)) cout << " and";
	}
	cout << " is " << sum << endl;
}

catch (runtime_error e) {	// this code is to produce error messages
	cout << e.what() << '\n';
}