Checking user input in C#

Question

I just wrote my first C# console application, I am still a beginner. Anyway, I tried the code below and it seems to work, its for solving quadratic equations. I'd like to add code for a situation whereby a user inputs a string instead of an integer and give an error message any ideas as to how to implement this?

namespace Quadratic_equation
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("welcome to seyi's quadratic calculator!!!");
            Console.Write("a:");
            double a = Convert.ToInt32(Console.ReadLine());


            Console.Write("b:");
            double b = Convert.ToInt32(Console.ReadLine());
            Console.Write("c:");
            double c = Convert.ToInt32(Console.ReadLine());
            if ((b * b - 4 * a * c) < 0) {
                Console.WriteLine("There are no real roots!");
            }
            else {
                double x1 = (-b + Math.Sqrt((b*b)-4*a*c)) /2*a;
                double x2 = (-b + Math.Sqrt((b*b)-4*a*c)) /2*a;
                Console.WriteLine("x:{0}",x1);
                Console.WriteLine("y:{0}",x2);
            }

            Console.ReadKey();
        }
    }
}

Just use TryParse and if it returns false show error and don't continue. — user447356
– user447356, Commented Mar 30, 2014 at 13:39

Soner Gönül · Accepted Answer · 2014-03-30 13:56:59Z

2

You can use Int32.TryParse method to check your string is a valid integer or not. This method returns a boolean value for your conversation is succeed or not.

Converts the string representation of a number to its 32-bit signed integer equivalent. A return value indicates whether the conversion succeeded.

And I don't understand why you want to keep as double the return value of Convert.ToInt32 method. These factors (a, b, c) should be integer, not double.

int a;
string s = Console.ReadLine();
if(Int32.TryParse(s, out a))
{
   // Your input string is a valid integer.
}
else
{
  // Your input string is not a valid integer.
}

This Int32.TryParse(string, out int) overload uses NumberStyle.Integer as default. That means your string can have one of these;

Trailing white spaces
Leading white spaces
Leading sign character

edited Mar 30, 2014 at 13:56

answered Mar 30, 2014 at 13:42

Soner Gönül

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2 Comments

Sumner Evans Over a year ago

strings is an invalid variable type. Was that a typo?

Soner Gönül Over a year ago

@jsve Yeah, that was a typo. Fixed.

Tim Sneed · Accepted Answer · 2014-03-30 13:41:05Z

1

Check out int.TryParse

   int number;
  bool result = Int32.TryParse(value, out number);
  if (result)
  {
     Console.WriteLine("Converted '{0}' to {1}.", value, number);         
  }
  else
  {
     if (value == null) value = ""; 
     Console.WriteLine("Attempted conversion of '{0}' failed.", value);
  }

answered Mar 30, 2014 at 13:41

Tim Sneed

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Comments

Sumner Evans · Accepted Answer · 2014-03-30 13:43:17Z

1

Use a try-catch block in a do-while loop:

bool goToNextNum = false;
do
{
    try
    {
        double a = Convert.ToInt32(Console.ReadLine());
        goToNextNum = true;
    }
    catch
    {
        Console.WriteLine("Invalid Number");
    }
} while (goToNextNum == false);

This will loop until a is a valid number.

answered Mar 30, 2014 at 13:43

Sumner Evans

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