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:

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// 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;
}

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cin.clear()

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:

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

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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. :)

Quote:

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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. :)

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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:

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// 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';
}

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Yes, well, it seems to work if you switch the order of the lines I supplied, and instead use

Code:

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

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Sorry about that. I didn't know that ignore() does not work on streams that are in a fail state.