How to flush cin in c++ properly? [duplicate] - c++

Question

#include <iostream>
#include <limits>
using namespace std;
int main()
{
int x;
cout << "5 + 4 = ";
while(!(cin >> x)){
cout << "Error, please try again." << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
if (x == (5 + 4)){
cout << "Correct!" << endl;
}
else{
cout << "Wrong!" << endl;
}
return 0;
}
How can I check if the user inputs a valid integer? In this program I wrote above, if the user inputs 9, it should be correct, however, if the user inputs 9a for example, it should return an error, but it doesn't for some reason. How can I correct it?
How I did it using cin.peek()
#include <iostream>
#include <limits>
#include <stdio.h>
using namespace std;
int main()
{
int x;
bool ok;
cout << "5 + 4 = ";
cin >> x;
while(!ok){
cin >> x;
if(!cin.fail() && (cin.peek() == EOF || cin.peek() == '\n')){
ok = true;
}
else{
cout << "Error, please try again." << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
}
if (x == (5 + 4)){
cout << "Correct!" << endl;
}
else{
cout << "Wrong!" << endl;
}
return 0;
}

Answer 1

You could read a string, extract an integer from it and then make sure there's nothing left:
std::string line;
std::cin >> line;
std::istringstream s(line);
int x;
if (!(s >> x)) {
// Error, not a number
}
char c;
if (s >> c) {
// Error, there was something past the number
}

Answer 2

bool isIntegerNumber(const std::string& string){
std::string::const_iterator it = string.begin();
int minSize = 0;
if(string.size()>0 && (string[0] == '-' || string[0] == '+')){
it++;
minSize++;
}
while (it != string.end() && std::isdigit(*it)) ++it;
return string.size()>minSize && it == string.end();
}

Answer 3

You have a line oriented input, so you should probably be using
getline. Something like:
bool
getIntFromLine( std::istream& source, int& results )
{
std::string line;
std::getline( source, line );
std::istringstream parse( source ? line : "" );
return parse >> results >> std::ws && parse.get() == EOF;
}
should do the trick.
Using this, your loop would be:
while ( !getIntFromLine( std::istream, x ) ) {
std::cout << "Error, please try again." << std::endl;
}
Note that this technique also means that you don't have to worry
about clearing the error or resynchronizing the input.

Answer 4

For the reason this happens, take a look at this link:
Extracts and parses characters sequentially from the stream to
interpret them as the representation of a value of the proper type,
which is stored as the value of val. Internally, the function accesses
the input sequence by first constructing a sentry object (with
noskipws set to false). Then (if good), it calls num_get::get (using
the stream's selected locale) to perform both the extraction and the
parsing operations, adjusting the stream's internal state flags
accordingly. Finally, it destroys the sentry object before returning.
Then observe the behavior if you attempt something like this:
int x = 0;
cin >> x;
std::cout << x << std::endl;
std::cout << cin.good() << std::endl;
g++-4.8 -std=c++11 -O3 -Wall -pedantic -pthread main.cpp && echo "900a100" | ./a.out
// Output:
// 900
// 1
If you input "a100" instead, it outputs:
0
0

Answer 5

try this:
std::string input;
std::cin >> input;
if ( std::all_of(input.begin(), input.end(), std::isdigit) )
{
//input is integer
}
Refer this :
C++ Fix for checking if input is an integer

Answer 6

One I have seen that works for some situations is:
Read the input as string. cin >> str
Decode to number: atoi, or sscanf, or stringstream, etc.
print the number into a string (using sprintf or stringstream)
check if its equal to read string. (using strings ==, not char*)
Quick and simple to do. Uses the Cin>>str word breaking rules, accept negative numbers, rejects overflowing numbers. But it does reject "+10", which in somesituations you are happy wiht, and in some you are not.

Answer 7

If you can use C++11 (and your compiler has full regex support), you can also use the <regex> library:
#include <iostream>
#include <limits>
#include <regex>
#include <string>
#include <utility>
int main()
{
std::string line;
std::pair<int, bool> value = std::make_pair(0, false);
std::cout << "5 + 4 = ";
while (!value.second)
{
while (!std::getline(std::cin, line))
{
std::cout << "Error, please try again." << std::endl;
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
if (!std::regex_match(line, std::regex("(\\+|-)?[[:digit:]]+")))
{
std::cout << "Error, please try again." << std::endl;
}
else
{
value = std::make_pair(std::stol(line), true);
}
}
if (value.first == (5 + 4))
{
std::cout << "Correct!" << std::endl;
}
else
{
std::cout << "Incorrect!" << std::endl;
}
return 0;
}