I wanted to break the loop when the user doesn't want to add anymore:
#include<iostream> using namespace std; int main() { int i = 0, a = 0, h = 0; cout << "Enter numbers to be added:\n "; for(i=0; ??; i++) { cout << "\n" << h << " + "; cin >> a; h = h+a; } return 0; } Use std::getline to read an input line and exit the loop when the line is empty.
#include<iostream> #include <sstream> int main() { int a = 0, h = 0; std::cout << "Enter numbers to be added:\n "; std::string line; std::cout << "\n" << h << " + "; while (std::getline(std::cin, line) && // input is good line.length() > 0) // line not empty { std::stringstream linestr(line); while (linestr >> a)// recommend better checking here. Look up std::strtol { h = h+a; std::cout << "\n" << h << " + "; } } return 0; } And output:
Enter numbers to be added: 0 + 1 2 3 4 5 6 7 8 9 1 + 3 + 6 + 10 + 15 + 21 + 28 + 36 + 45 + Note that this allows multiple entries per line and looks pretty ugly, so OP is probably more interested in:
#include<iostream> int main() { long a = 0, h = 0; std::cout << "Enter numbers to be added:\n "; std::string line; std::cout << "\n" << h << " + "; while (std::getline(std::cin, line) && // input is good line.length() > 0) // line not empty { char * endp; // will be updated with the character in line that wasn't a digit a = std::strtol(line.c_str(), &endp, 10); if (*endp == '\0') // if last character inspected was the end of the string // warning: Does not catch ridiculously large numbers { h = h+a; } else { std::cout << "Very funny, wise guy. Try again." << std::endl; } std::cout << "\n" << h << " + "; } return 0; } Output
Enter numbers to be added: 0 + 1 1 + 1 2 3 4 Very funny, wise guy. Try again. 1 + 2 3 + 44444 44447 + jsdf;jasdklfjasdklf Very funny, wise guy. Try again. 44447 + 9999999999999999999999 -2147439202 + It is easier to use a sentinel value that you can check against, something like this:
#include<iostream> #include<string> using namespace std; int main() { int sum = 0; string userInput; while(true) { cout<<"Enter number to be added ('q' to quit): "; cin >> userInput; if( (userInput == "q") || (userInput == "Q") ) { break; } try { sum += stoi( userInput ); } catch( const std::invalid_argument& e ) { cerr << "Invalid input \"" << userInput << "\" received!" << endl; return EXIT_FAILURE; } } cout << "Sum: " << sum << endl; return EXIT_SUCCESS; } std::getline(std::istream&, std::string&) happily gives all lines including empty lines:
#include <iostream> #include <string> int main() { long long accumulator = 0; while (true) { // read a (possibly empty) line: std::string buf; if (!std::getline(std::cin, buf)) { std::cerr << "The input stream is broken." << std::endl; break; } // was the entered line empty? if (buf.empty()) { std::cerr << "You entered a blank line" << std::endl; break; } // convert string to integer std::size_t pos; long long summand; try { summand = std::stoll(buf, &pos, 10); } catch (std::invalid_argument &) { std::cerr << "Not an integer: " << buf << std::endl; continue; } catch (std::out_of_range &) { std::cerr << "Out of range: " << buf << std::endl; continue; } if (pos != buf.size()) { std::cerr << "Not an integer on its own: " << buf << std::endl; continue; } // do something with the data: accumulator += summand; } std::cout << "accumulator = " << accumulator << std::endl; return 0; } 
if (buf.empty()) { std::cerr << "You entered a blank line" << std::endl; break; } hence my incomprehension. (btw checking whether multiple numbers are entered in your answer is nice, I haven't thought of that case)
A0ina, which means that manually entering this integer would stop the loop, I can't see how it is desirable (besides,cin >> a;doesn't even return when you only press enter on my machine... how do you want to testafor a line end if the function blocks until you've entered something relevant?). I suggest you use Kay's answer (or RyanP's answer which is fine too) which has the intended behaviour (and even checks for invalid values). I don't get why they were downvoted.