C ++. The program skips the line of code (read variable)

Question

Good day. There was a problem with C ++. In short - in one line does not occur reading using the input stream cin. Code below. The place where the error occurs is indicated by a comment. Tried to clear the stream before reading user_num_to_sum with cin.clear () - don't help.

#include <iostream> //cin, cout #include <conio.h> //_getch() #include <vector> //vector using std::cout; using std::cin; using std::vector; int main () { vector<int> numbers; // вектор из целых чисел int num = 0; // число которое заносим в конец вектора int user_num_to_sum = 0; // число элементов вектора для суммирования(пример: если число 3, то суммируем первые 3 элемента) cout << "Write a few numbers: \n"; while(cin >> num) { numbers.push_back(num);// заносим числа в вектор } cout << "How much elements of vector do you want to sum?\n"; cin >> user_num_to_sum; // ERROR: программа пропускает данную строчку и выполняет следующую подставляя в user_num_to_sum значение 0, которое было задано при инициализации. cout << "The sum of first " << user_num_to_sum << " numbers -> "; //код еще не готов полностью, программа смысла не имеет _getch(); return 0; }

I stop the while loop (cin >> num) by entering the error character '|'. This technique is used in his book Stroustrup. But then for some reason, after stopping the cycle, the reading of the whole number user_num_to_sum does not work.

PS The variant with cin.clear () worked, but only in case of completion of data input into the vector by pressing Ctrl + Z. If you complete it by entering incorrect data (for example, '|' when reading data of type int), then cin.clear () does not work. Question: why?

Accepted Answer · 2016-07-31T22:45:01

 while(cin >> num) { // Пока мы можем прочитать int, читаем его ... // и что-то делаем } // Вышли из цикла, т. к. int прочитать нельзя ... // Тут мы ничего не читали cin >> user_num_to_sum; // Опа! Чиатем ещё int

But the state of the stream has not changed - int as it was impossible to read, and now it is impossible.

If the input of a sequence of numbers ends with some non-numeric input, you should read this something as a string and then read the next number. If the input was completed by pressing ctrl + z, then there seems to be eof in general.

Also, to reset the error marker, you must call cin.clear() before reading.

By the way, if the input is completed by pressing ctrl + Z, then cin.clear () perfectly solves the problem (as I did) and the last (after the cycle) number is remarkably read.
Do not think it is stupid, I'm just learning, but why cin.clear () does not clear the last erroneous input for example the symbol '|'?
In general, I somehow prefer scanf - it has a lot of tricks with the format.
@ TarasSus, please add your last comment to the question itself, or ask a new one, I would love to know the answer

Answer 2 · 2016-07-31T22:59:42

 while(cin >> num) { numbers.push_back(num);// заносим числа в вектор } std::string s; cin.clear(); // сбрасываем ошибку в cin cin >> s; // убираем из cin нечисловые символы cout << "How much elements of vector do you want to sum?\n"; cin >> user_num_to_sum; // ERROR: программа пропускает данную строчку и выполняет следующую подставляя в user_num_to_sum значение 0, которое было задано при инициализации.

This is how your code will work correctly when filling in a vector with any non-numeric character.

Actually, the answer to the last question: cin.clear () clears the flags (state) of the error in the stream, but the last character entered (group of characters) remains in it, so they need to be removed from there, for example, into some unnecessary string.

P.S. The question turned out unexpectedly interesting and deserves encouragement in the form of a plus.

P.P.S. Found a variant without an extra variable (checked, you need to #include <limits> )

 cout << "Write a few numbers (end with any non-number input): \n"; while(cin >> num) { numbers.push_back(num);// заносим числа в вектор } cin.clear(); // сбрасываем флаг ошибки cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // пропускаем все до символа новой строки cout << "How much elements of vector do you want to sum?\n"; cin >> user_num_to_sum;

ideone.com/UhPFH7 - and so does not work with input redirection.
@Qwertiy found another thing, there is an example, almost like a vehicle: en.cppreference.com/w/cpp/io/basic_istream/ignore

C ++. The program skips the line of code (read variable)

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