C# Check input for Calculator

Discussion in 'Mixed Languages' started by Windows4live, Mar 10, 2013.

  1. Windows4live

    Windows4live MDL Addicted

    Jan 31, 2011
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    #1 Windows4live, Mar 10, 2013
    Last edited: Mar 10, 2013
    Hi,
    I'm struggling a little bit.

    I wrote a simple calculor for the operation + and want to check the input.

    int = num1
    and so on

    //Type first number and check if input is a letter

    Console.WriteLine("Type first number:");
    num1 = Convert.ToInt32(Console.ReadLine());
    if (num1 == string)
    {
    Console.WriteLine(num1 + " Only numbers allowed");
    }

    This doesn't work

    if (num1 = string)

    I get the error invalid expression.
    What's wrong?

    Thx for your help.
     
  2. stevemk14ebr

    stevemk14ebr MDL Senior Member

    Jun 23, 2010
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    i don't program in c# but from the looks of it you convert your input to int immediately so it will never be string and also you may want to try num1.gettype() == typeof(String) or something similar for your data type check (you'll probably need to do a different check anyways because logically it is impossible for it to be true)
     
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  3. BobSheep

    BobSheep MDL Guru

    Apr 19, 2010
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    #3 BobSheep, Mar 10, 2013
    Last edited by a moderator: Apr 20, 2017
    First string is a type and num1 is a variable of type int. You cannot compare these two.
    You should capture the input as a string and test/convert it to numeric.

    Here is an example for you to rewrite into your own code. You can use int instead of decimal.

    Code:
    string input = "2.0";            
    decimal number = decimal.MinValue;
    decimal.TryParse(input, out number);
    if(number != decimal.MinValue) // if the value changed then the input is valid.
    {
                    // input is ok.
    }
     
  4. Pr3acher

    Pr3acher MDL Member

    Aug 24, 2012
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    Hi, also dont forget that to test equality you must use == whereas to give a variable a value, just use = .
     
  5. BobSheep

    BobSheep MDL Guru

    Apr 19, 2010
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    I think he knew that

     
  6. Windows4live

    Windows4live MDL Addicted

    Jan 31, 2011
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    Yes, just typo.
    and thx for the help.
     
  7. Marvelous

    Marvelous MDL Novice

    Apr 10, 2013
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    Hello @Windows4live , as an independent C# developer I suggest you to study very well the programming language (no offense though)
    Foregoing code has few lines of code that doesn't make sense as stevemk14ebr mentioned earlier:
    -First of all if you want to define a variable in C# it ought to be like this int num; , NOT int = num; . In your case 'int' becomes a variable that has num as a value.
    -You converted the variable thus there will be a problem once a user input letters.
    -there is another option for casting in C# instead of using Covert object you can use int.Parse(Console.ReadLine());
    Any question feel free to ask me either on private message or here!
    thank you and have a wonderful day!
     
  8. Stannieman

    Stannieman MDL Guru

    Sep 4, 2009
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    The Convert.ToInt32() function will throw an exception when the string given can't be converted to an int. You can do 2 things:
    1) use a try catch block with the convert and everything that should happen if ok in the try block. It will go to catch of it fails.
    2) What I usually do is write an bool IsInt32(String str) method that also uses try catch but returns a boolean (to avoid mixing the exception stuff with your actual logic). This method usually goed into a Validator class or something like that.
     
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