Why do I get an infinite loop if I enter a letter rather than a number? [duplicate]

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This is how basic_istream works. In your case when cin >> num1 gets wrong input – failbit is set and cin is not cleared. So next time it will be the same wrong input. To handle this correctly you can add check for correct input and clear&ignore cin in case of wrong input. For example:

    #include<limits>

    //user enters a number
    cout << "\nPlease enter a positive number and press Enter: \n";
    do {    
        while(!(cin >> num1)) {
            cout << "Incorrect input. Please try again.\n";
            cin.clear();
            cin.ignore(numeric_limits<streamsize>::max(), '\n');
        }
        if(num1 < 0) cout << "The number you entered is negative. Please enter a positive number to continue.\n";
    } while(num1 < 0);

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