C++ - isdigit not working properly and causing never ending loop - c++

Question

I'm creating a program which converts decimal values into binary values. The issue I'm having is that in my if statement I'm checking whether the user input for my int decimal variable contains digits before it moves on to converting the values but when it is digits it considers them as alpha characters, which then cause the program to infinitely loop.
When I change isdigit(decimal) to !isdigit(decimal) the conversion works but if I put in alpha characters it will then infinitely loop again. Am I doing something really silly?
#include <iostream>
#include <string>
#include <ctype.h>
#include <locale>
using namespace std;
string DecToBin(int decimal)
{
if (decimal == 0) {
return "0";
}
if (decimal == 1) {
return "1";
}
if (decimal % 2 == 0) {
return DecToBin(decimal/2) + "0";
}
else {
return DecToBin(decimal/2) + "1";
}
}
int main()
{
int decimal;
string binary;
cout << "Welcome to the Decimal to Binary converter!\n";
while (true) {
cout << "\n";
cout << "Type a Decimal number you wish to convert:\n";
cout << "\n";
cin >> decimal;
cin.ignore();
if (isdigit(decimal)) { //Is there an error with my code here?
binary = DecToBin(decimal);
cout << binary << "\n";
} else {
cout << "\n";
cout << "Please enter a number.\n";
}
}
cin.get();
}

Answer 1

First of all, to check for a number in a mixture of number and characters, do not take input into an int. Always go with std::string
int is_num(string s)
{
for (int i = 0; i < s.size(); i++)
if (!isdigit(s[i]))
return 0;
return 1;
}
int main()
{
int decimal;
string input;
string binary;
cout << "Welcome to the Decimal to Binary converter!\n";
while (true) {
cout << "\n";
cout << "Type a Decimal number you wish to convert:\n";
cout << "\n";
cin >> input;
cin.ignore();
if (is_num(input)) { //<-- user defined function
decimal = atoi(input.c_str()); //<--used C style here
binary = DecToBin(decimal);
cout << binary << "\n";
} else {
cout << "\n";
cout << "Please enter a number.\n";
}
}
cin.get();
}
You can always write a function to check for number in a string as shown above. Now your code does not run into an infinite loop. Moreover if you want to take only one valid input and exit the program, u can add a break
if (is_num(input)) {
decimal = atoi(input.c_str());
binary = DecToBin(decimal);
cout << binary << "\n";
break; //<--
}