I am having issues with my simple calculator program breaking in C# when I enter a letter rather than a variable

Question

For example when my program asks for a number and I enter a letter the program exits. So how can I make it do nothing when a letter Is entered? i have seen this with 'int' or 'strings' but not with 'doubles' it is really stumping me and i don't quite understand how even those were fixed

 static void Main(string[] args)
    {
        Console.WriteLine("type 'Exit' to leave this program.");
        Console.ReadLine();
        Console.Clear(); 
    Valuechoice: Console.WriteLine("Please choose a number.");
        double value1 = Convert.ToDouble(Console.ReadLine());     
        Console.WriteLine("Please choose a second Number.");
        double value2 = Convert.ToDouble(Console.ReadLine());
        Console.WriteLine("What would you like to do to these numbeers?");
        Console.WriteLine("Multiply");
        Console.WriteLine("Divide");
        Console.WriteLine("Add");
        Console.WriteLine("subtract");
        string operation = Console.ReadLine();
        Console.WriteLine();
        Console.Clear();
        if (operation == "Multiply")
        {
            double total1;
            total1 = (value1 * value2);
            Console.WriteLine(total1);
            Console.ReadLine();
            Console.Clear();
            goto Valuechoice;
        }
        if (operation == "Divide")
        {
            double total2;
            total2 = (value1 / value2);
            Console.WriteLine(total2);
            Console.ReadLine();
            Console.Clear();
            goto Valuechoice;
        }
        if (operation == "Add")
        {
            double total3;
            total3 = (value1 + value2);
            Console.WriteLine(total3);
            Console.ReadLine();
            Console.Clear();
            goto Valuechoice;

        }
        if (operation == "Subtract")
        {
            double total4;
            total4 = (value1 - value2);
            Console.WriteLine(total4);
            Console.ReadLine();
            Console.Clear();
            goto Valuechoice;

        }
        if (Console.ReadLine() == "Exit")
        {
            Environment.Exit(0);
        }

Just read input from the user as a `String` then determine whether or not it is a letter before trying to convert it to a `Double` — Hunter McMillen, Mar 02 '13 at 22:38
This problem is really too localized. You'd better formulate your problem in a bit more general form. — chtenb, Mar 02 '13 at 22:40
You could also catch the exception raised by Convert.ToDouble. — chtenb, Mar 02 '13 at 22:42

Andrew Whitaker · Answer 1 · 2013-03-02T22:44:07.613

4

Convert.ToDouble will throw a FormatException if the input is not a valid double. Use double.TryParse instead.

For example, you could do something like this to keep asking the user for a valid number:

double value1;

while(!double.TryParse(Console.ReadLine(), out value1)) 
{
    Console.WriteLine("Please choose a valid number");
}

Also there's really no reason to use goto here. I'd recommending restructuring your program to use a loop instead.

edited Mar 02 '13 at 22:44

answered Mar 02 '13 at 22:38

Andrew Whitaker

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thanks it works like a charm. could you maybe explain how to use a loop? I am just learning how to program. – TheMightyQuin Mar 03 '13 at 01:31
@TheMightyQuin: [MSDN's documentation on the while loop](http://msdn.microsoft.com/en-us/library/2aeyhxcd(v=vs.71).aspx) is pretty good--Basically it just repeats the code inside of the block until the condition inside of the parenthesis is false. – Andrew Whitaker Mar 03 '13 at 13:24

score 1 · Answer 2 · answered Mar 02 '13 at 22:39

1

Use TryParse - something like this:

double value1;
while (!double.TryParse(Console.ReadLine(), out value1) {
  Console.WriteLine("Please enter a valid number");
}

answered Mar 02 '13 at 22:39

MiMo

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score 0 · Answer 3 · answered Mar 02 '13 at 23:16

You can't convert a letter to a double, which is a number. It's like saying turn "e" into the decimal 34.72. It's impossible.

Try something like this:

bool valid = true; // locally declared variable, this is important!

string value = Console.ReadLine();
valid = validateInput(value);

if (valid == true) {
double value1 = Convert.ToDouble(value);
// Proceed with calculations, etc
}

private bool validateInput(string value) {
bool valid = true;
foreach (char c in value)
    {
        if (char.IsLetter(c))
            valid = false;
        Console.WriteLine("Input is invalid!");
    }
return valid;
}

I am having issues with my simple calculator program breaking in C# when I enter a letter rather than a variable

3 Answers3