Discarding letters except numbers with cin

Question

I have this loop right now that reads in numbers and outputs them in decimal, octal, and hexadecimal:

while(1) {
    if (cin >> n)
    cout << internal << setfill(' ') << setw(10) << dec << n << internal << setw(12) << oct << n << internal << setw(9) << hex << uppercase << n << endl;
    if (cin.fail()) {
        break;
    }
}

However if I try to discard inputs that aren't numbers with this it won't read in the input after the letters:

if (cin.fail()) {
    cin.ignore();
}

How do I discard input but be able to read other input later on?

Sample Input:

23
678  786  abc
7777

Expected Output: dec, oct, hex

---

    23          27       17
   678        1246      2A6
   786        1422      312
  7777       17141     1E61

Answer 1

You need to consume the offending content from cin and reset the error state. As long as failbit is set, all input operations will fail immediately.

while(1) {
if (cin >> n)
    cout << internal << setfill(' ') << setw(10) << dec << n << internal << setw(12) << oct << n << internal << setw(9) << hex << uppercase << n << endl;
else {
    if (cin.eof())
        break;
    cin.clear();               // Reset the error state
    std::string dummy;  
    if (!(cin >> dummy))       // Consume what ever non-integer you encountered
        break;
  } 
}

---

Alternatively, you can just always read a std::string and then try to parse that to a number with std::stoi:

for (std::string word; std::cin >> word;) {
    try {
        int n = std::stoi(word);
        // You output logic here
    }
    catch (std::exception&) {}
}

But this will probably overuse exceptions as you state that invalid input is not exceptional. On the upside, it means doing less logic "by hand".