How to break loop by Enter (c++)? - c++

Question

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;
}

Answer 1

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 +

Answer 2

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;
}

Answer 3

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;
}