C++ Beginner Infinite Loop When Input Wrong Data Type and Help Evaluate My Code

Question

I have an assignment to create a menu-based program to calculate area of several shapes using user-defined function. My code creates an infinite loop when a user input a cha

Answer 1

With float r1; cin >> r1; and non-numeric input, there is nothing consumed from input. Repeating cin >> r1 results in an endless loop.

What to do?

1. Check whether cin >> r1 was successful: if (cin >> r1) { /* process r1 */ }
2. If not successful, do something to consume wrong input: else { cin.clear(); cin.ignore(); }

std::ios::clear()

Sets the stream error state flags by assigning them the value of state. By default, assigns std::ios_base::goodbit which has the effect of clearing all error state flags.

std::istream::ignore()

Extracts and discards characters from the input stream until and including delim.

Example:

#include <iostream>
#include <limits>

int main()
{
  double value = 0.0;
  for (;;) {
    std::cout << "Value: ";
    if (std::cin >> value) {
      break; // success -> bail out of loop
    } else {
      std::cerr << "Wrong input!\n";
      std::cin.clear(); // reset state
      std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // consume wrong input
    }
  }
  std::cout << "Value: " << value << '\n';
  return 0;
}

Output:

Value: Hello↵
Wrong input!
Value: 1.23↵
Value: 1.23

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