Very Interesting Question

[Rakanoth]

hey i am begginer and exploring all documentation. i encounter a very interesting thing.. in this piece of code if i trying to enter a character
or very big number the program just told me that i enter number (different number that i give)
and keep running it over and over... why its happens?

 // number echoer

#include <iostream>
using namespace std;

int main ()
{
  unsigned long n;
  do {
    cout << "Enter number (0 to end): ";
    cin >> n;
    cout << "You entered: " << n << "\n";
  } while (n != 0);
  return 0;
}

[LaboPie]

beacuse you are doing a cycle, which means that the programm will continue running until the condition is validated.
In your code you (or the from this random guy code) you are saying:

do this thing

 cout << "Enter number (0 to end): ";
    cin >> n;
    cout << "You entered: " << n << "\n";

and stop it when n will be 0.

[Cubbi]

If I understand this correctly, you entered a number bigger than what can fit in an unsigned long, and the program got stuck in an endless loop. This happened because you didn't test if the input operation (cin >>n in this case) succeeded. Always check if, for example, replace that line with

if (cin >> n)
   cout << ...
else
    break;

or rearrange the loop to make only one decision to continue

while( cout << "Enter..."
      && cin >> n
      && n != 0 )
    cout << ...

[Rakanoth]

But why the program keeps running i if i put value 21 all is good but if i put character
or very big number the program says that i put the number 3435973836 and keeps running it without asking me for another input whats the logic?

[Olysold]

This may not be 100% accurate.

When you input a number that is bigger than what the specific data type can hold (in this case, unsigned long), the stream buffer cin goes into a fail state. Since it's in a fail state it cannot accept anymore input until it's state has been corrected.

http://www.cplusplus.com/reference/ios/ios/fail/