Unexpected behavior of while loop in c++ - c++

Question

Made something like this:
int main()
{
while (true)
{
std::cout << "Enter a number between one and nine. \n";
int oneandnine;
std::cin >> oneandnine;
if (std::cin.fail())
{
std::cin.clear();
std::cin.ignore(100, '\n');
std::cout << "INVALID UNPUT!\n";
}else if (oneandnine <= 9 && oneandnine >= 1)
{
break;
}else
{
std::cout << "INVALID UNPUT!\n";
}
}
return 0;
}
and when input is provided something like this 456aihdb, getting something like this:
INVALID UNPUT!
Enter a number between one and nine.
INVALID UNPUT!
Enter a number between one and nine.
Why does it loop twice like this? is it because when the 456 is discarded and the rest aihdb isn't which causes it to loop again and skip a cin input?

Answer 1

It is exactly as you think it is.
The fail flag isn't set immediately, instead the formatted input operator reads the integer 456 into oneandnine, but doesn't set the fail flag since it's a valid integer value. That leads to the else case executing since std::cin.fail() is false and oneandnine is not between 1 and 9.
The next iteration you read the invalid input and the fail flag will be set leading to the second error output.
One common way to handle validation is to read the whole line into a string, put that string into an std::istringstream and use that to attempt to parse the input:
if (!std::getline(std::cin, line))
{
// Failure of some kind, could be EOF or something else
// Probably best not to continue in this case
}
std::istringstream iss(line);
if (!(iss >> oneandnine))
{
// Invalid input, report it as such
}
if (oneandnine < 1 || oneandnine > 9)
{
// Invalid number, report it as such
}
// Correct input, continue with program
Note that input such as 6abc will be considered valid by the above code. The 6 will be extracted into oneandnine and the abc part will silently be discarded. If that's not wanted there are other ways for the parsing (e.g. std::stoi or std::strtol if exceptions are not wanted). Do that instead of the >> extraction, but the rest of the code above should be fine.

Answer 2

std::istream's operator >> doesn't read in whole lines. It reads until it finds an invalid character or whitespace, if it has found a valid character before the invalid character the read operation succeeds and the invalid character is left in the stream.
In your example the first iteration successfully reads 456 and leaves aihdb in the stream. This fails your range check and the second iteration then tries to read the remaining characters which fails as the first character isn't a number.
If you want to read whole lines use std::getline then parse the whole line into a number. For example:
#include <iostream>
#include <string>
using std::cout;
int main()
{
while (true)
{
std::cout << "Enter a number between one and nine. \n";
std::string line;
std::getline(std::cin, line);
int oneandnine;
size_t pos;
try
{
oneandnine = std::stoi(line, &pos);
}
catch ( std::exception& )
{
oneandnine = -1;
}
if (pos != line.size() || oneandnine > 9 || oneandnine < 1)
{
std::cout << "INVALID INPUT!\n";
}
else
{
break;
}
}
return 0;
}