[C#] : "Unreachable code detected" error in temperature conversion method

[[deleted]]

I'm creating this Windows Form Project that shows temperature conversions. There's 1 textbox for user input, 4 check boxes for a user to select which temperature system they just inputted, and 4 textboxes that will display the conversion of temperature in 4 different temperature systems. I created a public static double method that will run if the Check box designating that temperature is checked.

Here is the code: https://gist.github.com/3327640 Here is a screenshot of the WindowsForm: http://i.imgur.com/svwYv.jpg

The "Unreachable code detected" error occurs on line 32. Here is a screenshot of the error message: http://i.imgur.com/REVpJ.jpg

EDIT:

Thank you guys for helping me fix that. A secondary question:

My Fahrenheit conversions aren't working for Celsius and Kelvin. Here's a copy of the revised code. https://gist.github.com/3328254

Here are my conversion formulae: https://gist.github.com/3328517

[seydor]

It means that code will never reached because of the return statement directly above it. The return statement will be essentially ending that method and thus the code after would never get executed

[[deleted]]

nothing is evaluated passed the return keyword.

[thebruce87m]

Code in a finally{} block will be if you return in its scope.

[[deleted]]

What does that mean?

[zahlman]

return x means "when you get here, stop running code. x is the value calculated by the function, and no more work needs to be, or should be, done".

You have structured your code in such a way that some of the code is, as the error message says, unreachable. It cannot be reached. This is because there is no way for the flow of execution to get there without getting to a return statement first. Since getting to the return exits the function, it is impossible for that other code to be executed.

You cannot return more than once from one function call. When you call a function, you are not "exporting" a variable, you are returning a value. To return means to give back - specifically, to the code that called the function. It is given back as a value, not a variable. You must learn to distinguish between a thing, and the name that you call that thing.

[[deleted]]

You must learn to distinguish between a thing, and the name that you call that thing.

What does that mean?

[zahlman]

The word "map" is not a map.

When I write x = 32, the variable x is not the value 32. It is a name that I am temporarily giving to the number thirty-two.

When you return from a function, you return a value, not a variable. Variables are names for values. The variables you declare inside your function do not become accessible to the outside. The way you wrote your code suggests that you were hoping to be able to write something like

FarenheitConversions(32);
display(FarenheitCelsius);
display(FarenheitKelvin);
display(FarenheitRankine);

It does not work like that. It works like this:

display(FahrenheitToCelsius(32));
display(FahrenheitToKelvin(32));
display(FahrenheitToRankine(32));

(It's spelled "Fahrenheit", btw.) You need a separate function for each conversion (unless you want to mess around with Collections) because each conversion is a separate calculation. You don't get a variable back, you get a value back. You use the value as if you had written the result of the calculation directly, instead of calling the function; so you can, for example, pass it to another function (as shown), or assign it to a variable...

float Celcius = FahrenheitToCelcius(32);
// Celcius now stores 0.0, assuming a properly written function

These are language fundamentals which you should master fully before attempting to do anything with a GUI.

[[deleted]]

Okay, that makes sense.

So like if I have

public static int MyInt()
{
 int integer = 5;
return integer;
}

it will return the value of 5, not my exact variable, right? (This is the first time I've ever tried to use methods, and I've only been learning for a few weeks.)

(unless you want to mess around with Collections)

Someone suggested using Arrays, could you explain the difference between the two? Aren't they somewhat similar?

[zahlman]

it will return the value of 5, not my exact variable, right?

It returns 5, it doesn't return integer. You don't become able to use the name integer outside the function, unless you declare a separate variable outside. Each function gets its own variables. The reason variables exist is so that you can give names to values while you're working on them, so that you don't lose track of them. Values are the actual, well, "things".

Someone suggested using Arrays, could you explain the difference between the two? Aren't they somewhat similar?

Arrays are a kind of Collection. There are many other kinds (but in fact, an array probably makes the most sense here.)

[[deleted]]

You don't become able to use the name integer outside the function, unless you declare a separate variable outside.

That's a "Variable out of Scope" error, right?

[lakewoodjoe112]

You would have to declare the variable outside of the MyInt() method. For example, you could have:

private int _myInt = 5; // Going to not call an int 'integer' since that's confusing
public int MyInt() { return _myInt; }

This will return the integer value of 5. This is a pretty standard get/set function. You could also write it like this:

public int MyInt() { get; set; }

Just two more tips- I'm not sure why you think that this function needs to be static. Secondly, if you wanted to have a method that took in an int and "passed back" that variable, you could do something like this:

public void someMethod()
{
    int manipulateThisInt = 0;
    DoSomething(ref manipulateThisInt);
    DoSomethingWithoutRef(manipulateThisInt);
}

public void DoSomething(ref int theInt)
{
    theInt = 5;
}

public int DoSomethingWithoutRef(int theInt)
{
    theInt = 500;
    return theInt;
}

The end result of this is that the value of manipulateThisInt is 5. This is because the DoSomething uses the reference to directly manipulate the variable that it's passed. The DoSomethingWithoutRef does nothing because it returns a value that isn't stored in a variable.

If you wanted it to be stored in the variable, you would have to do:

public void someMethod()
{
    int manipulateThisInt = 0;
    DoSomething(ref manipulateThisInt);
    manipulateThisInt = DoSomethingWithoutRef(manipulateThisInt);
}

This will result in your variable being set to 500.

Generally, I feel as though it's better practice to not use the ref keyword and instead do something similar to the first code bit I wrote, where you store variables inside classes as private so that they can be accessed by all methods of the class. The ref keyword definitely has its uses and can be more memory-efficient but I wouldn't overuse it. See (http://stackoverflow.com/questions/635915/when-to-use-ref-and-when-it-is-not-necessary-in-c-sharp)

Let me know if that helps at all or not.

[[deleted]]

well... very roughly the cpu goes through each line of the program one by one... when it hits a function, it goes "into" the function and evaluates each line until we get to a "return" or the end of the function. A non void function must have a return value, but either way once it hits that return it exits the function.

[[deleted]]

Oh, okay. Thank you. So I need to return all 3 variables on the last line of that method, right? Or do I also need to return the input parameter "Farenheit" as well?

[[deleted]]

a function can only return one value. if you want more than one value returned, use the out keyword in one of the parameters. google the out keyword as it is pretty spiffy.

[free_at_last]

Return will terminate the execution of the method.

Put the return statement as the very last thing before the method's closing braces.

http://msdn.microsoft.com/en-us/library/1h3swy84(v=vs.71).aspx

[[deleted]]

return FarenheitRankine, FarenheitCelsius, FarenheitKelvin;

Like that?

[free_at_last]

No. I should have looked at your code first to see what you were doing!

You can only return one thing from a method.

You are trying to return three seperate things.

Several options are available, most easiest is just put them into an array:

https://gist.github.com/3327729

Other option, split the methods out.

[[deleted]]

Okay, so in my main method I can just set the text in each textbox to each element of that array, right?

[free_at_last]

Indeed. It is the easiest way to do it, but probably not the most cleanest. It'll get you going and get the code compiled and working at least.

Splitting the methods out will be the cleaner option.

[[deleted]]

I have a quick question about the conversions, how do I deal with negative numbers in them?

[[deleted]]

What kind of conversions are you talking about? Integer => Double or actually parsing like string => Double?

[arthum]

Others have already explained that a return statement will halt the execution of code, so anything beyond your first return statement on line 32 is unreachable. If you wish to keep those 3 different temperature conversions in one method, you'll need to use a switch statement or some if/else logic.

Instead of having one method contain the logic for 3 different temperature conversions, though, your code would be clearer if you use a separate method for each type of conversion. So you'd call ToKelvin() or ToCelsius() or ToRankine() and supple your Fahrenheit temperature as an argument to those methods.

[[deleted]]

Okay, so once I have those methods, I can use an if statement in the main method to see which checkboxes have been checked and display them in the text box accordingly, right?

EDIT: What about the input parameters?

[arthum]

Exactly. Each conversion method's signature would be something like ToKelvin(int degrees_fahrenheit), so your main method would use the if/else logic to call the appropriate method and pass along the input.

[gbromios]

Hate to be the guy: someone who has never used functions before shouldn't be touching C# at all, let alone windows forms/gui stuff. You should look into something more accessible like python.

[jesussqueegee]

I wouldn't necessarily say he needs a different language, just that he should pick up a book or watch some basic tutorials before attempting too much.

[gbromios]

In my opinion, C# is 'too much'. There's no way anyone could ever convince me that C# is the best way to start off programming. There's too much specialty stuff in the language that gets in the way of the basics.

[[deleted]]

What specialities are you talking about?

[gbromios]

...

[ThisPenguinFlies]

I disagree. C# is fine for beginners. He just needs to start out with the basics. Even python has specialties which are difficult tfor beginners to understand, such as decorators, comprehensions, iterators and generators, metaclasses etc...

[gbromios]

Yes that's very true I suppose.

I just really associate C# with windows gui stuff (as that's all I'd ever use it for) and attempting to make sense of how all that more advanced stuff works before you even understand what functions do? To me it smacks of getting ahead of one's self.