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Comparing strings with if in C# code error [duplicate]

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compare two string value [closed]
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Closed 3 years ago.
I'm a beginner in c# and I'm making a console guess the number game. You enter a number and it tells you to guess higher or lower or if you guessed the number. Anyways, I'm having trouble comparing the answer with the users guess.
I've tried comparing string guess with string answer using a <= in an if statement. I got an error that says "Operator '<=' cannot be applied to operands of 'string' and'string'.
The code:
string answer = "537";
string guess = Console.ReadLine();
*if (guess <= answer)*
The code with asterisks is the code I'm getting an error from. Does anyone know what I'm doing wrong and a solution?

Since you've said that you're a beginner,
<= isn't valid for strings.
Imagine if I did this:
string foo = "Hello world";
string bar = "Wassup?"
if(foo <= bar)
{
/// do something
}
What, exactly, would foo <= bar mean in that context? We could trying to compare the length of the strings (bar is shorter than foo), the sum of the ASCII values of the characters in each string, or just about anything. It's possible to implement methods that do those things, but none of them make sense in the general case so the language doesn't try, and it shouldn't.
The difference between a string and an int is that the former is intended to contain character data, like a name or a sentence. Mathematical comparisons like <= apply to numeric data, like integers and floating point values. So, to get the behavior you're looking for, you need to convert your text data into a numeric type.
The nature of data types and how they are stored, comparisons, etc. is a nontrivial discussion. But, suffice it to say that the string "123" is NOT the same as the number (integer, most likely) 123.
The easiest fix for your code would be something like:
string answer = "537";
string guess = Console.ReadLine();
var intAnswer = Int32.Parse(answer);
var intGuess = Int32.Parse(guess);
if (intGuess <= intAnswer)
{
/// do something...
}
Note that this will throw an exception if the user enters anything in the console that is not a valid digit. (Look up TryParse for a better solution, but that's beyond the scope of this answer and I think it'll just confuse the issue in this case.)
I'd spend some time reading about data types, int vs string, etc. This is a reasonable question about something that is not obvious to those just getting started.
Keep at it. We all started somewhere, and this is as good a place as any.

strings cannot be treated as number, it will only compare if they are equal. if numbers are the input. convert it to int first, both the guess and answer. if the guess will always be a number this would suffice.
if (Convert.ToInt32(guess) <= Convert.ToInt32(answer))
{
}
if not try to do a try catch or Int32.TryParse

Is a C# decimal cultural dependent

I'm going through some code from another developer and I found some code that seems... well ugly and unnecessary.
I understand that when parsing strings, we need to use cultural info properly when otherwise you can get into the "1.2" vs "1,2" type issues. In this case, the developer is using a NumericUpDown which is stored as a decimal.
float fValue = System.Convert.ToSingle(numericUpDown1.Value, System.Globalization.CultureInfo.InvariantCulture)
My questions are:
- is the following code equivalent?
- Will cultural info be handled properly?
- Can this cast throw an exception?
float fValue = (float)numericUpDown1.Value;
If they are not equivalent (or safe) can you please explain why and propose a "correct" solution?
Thanks in advance!

Convert.ToSingle(value, culture) calls ((IConvertible)value).ToSingle(culture). decimal's IConvertible.ToSingle then ignores the passed-in culture and returns Convert.ToSingle(value), which just returns (float)value.
Yes, your code is equivalent.

Yes, your code is equivalent to that only if the numericupdown1 value is small.
but if that value is 41.00027357629127 or something like this it will return 4.10002732E+15.
so,by using float.Parse("numericUpDown1.Value", CultureInfo.InvariantCulture.NumberFormat); you can get the exact value

Why do all TryParse overloads have an out parameter? [closed]

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I have discovered that many times I don't need the out parameter of the TryParse method, but the problem that it is necessarily. Here I will show one example when it's not needed.
I want to check if a string is an integer, if it is an integer then print "An integer"; otherwise, print "Not an integer". So here is the code:
string value = Console.ReadLine(); //Get a value from the user.
int num; //Why should I have it?? No need at all !
if (int.TryParse(value, out num))
{
Console.WriteLine("An integer");
}
else
{
Console.WriteLine("Not an integer");
}
I am just wondering why TryParse always returns an out parameter? Why it doesn't have the overload without an out parameter?

Updated Answer:
In more recent versions of C# you can declare the output parameter inline, which allows you to remove the line of code you don't want in your example:
string value = Console.ReadLine(); //Get a value from the user.
if (int.TryParse(value, out int num))
{
Console.WriteLine("An integer");
}
else
{
Console.WriteLine("Not an integer");
}
You can simply ignore the result in your code and no longer have that extra line. You still have the extra parameter, but so?
The underlying "why" is still the same and is unlikely to ever change. The method needed to return two things, a bool indicating success and an int indicating the resulting value if successful. (I can't think of another way to convey the result, can you?) Since a method can only return one thing, and a custom result type seems like overkill for this, the decision was made to return the bool and have the result be an out parameter. And once that decision was made, it has to remain for the duration of the language.
"They" certainly could add an overload that doesn't output in the int value. But why? Why expend the effort in designing, documenting, testing, and as we've seen perpetually supporting a method that serves no purpose but to save a few keystrokes for an extreme minority of developers? Again, very unlikely.
For such features you are certainly welcome to propose a change. It would be pretty cool to have a proposal accepted, I imagine. I doubt this one would be, but if you're passionate about it then by all means have at it.
Original Answer:
The short answer is, "Because that's how the method is defined." Perhaps by chance someone from the C# language team might find this question and provide reasoning into why decisions were made, but that doesn't really change much at this point. C# is a statically compiled language and the method signatures need to match, so that's just the way it is.
(Imagine if they changed this and broke .TryParse() on all existing codebases. That would be... bad.)
You might be able to work around this in your own code, though. Something as simple as an extension method could do the trick for you:
public static bool IsInt(this string s)
{
int x = 0;
return int.TryParse(s, out x);
}
Then in your code you'd just need to call that method from the string value:
string value = Console.ReadLine();
if (value.IsInt())
Console.WriteLine("An integer");
else
Console.WriteLine("Not an integer");

It has the out parameter because the vast majority of the time when people use it, they want the int (or double, or decimal, or datetime, or whatever) that was parsed.
If you don't want/need the parsed value, and you find yourself doing it all the time, you could write your own "wrapper" on .TryParse() that just takes the string.
In this example (and you could make it more generic, I'm sure) you could do something like
public static bool TryParseNoOutput(this string value)
{
int noNeed = 0;
return int.TryParse(value, out noNeed);
}
Then in your code (in this case) you'd call:
string value = Console.ReadLine();
if(value.TryParseNoOutput()) Console.WriteLine("An Integer");
else Console.WriteLine("Not an integer."):
Edit: As has been pointed out in the comments, I tried to call "int.TryParseNoOutput" when I had defined it as an extension on a string. Answer has been updated to reflect that.

TryParse is a relatively complex operation to determine the int representation of a string. If there would be an overload that just returns a bool, it would be very likely that many (unexperienced) developers would follow this inefficient pattern:
int myInt = -1;
if(int.TryParse("123"))
myInt = int.Parse("123");

Why does TryParse have an out parameter?
For the very simple reason of how TryParse is implemented.
The way you determine whether or not something is parsable, is by parsing it! If you are able to parse something, then it is parsable. If you cannot parse it, then it is not parsable.
Therefore, by way of determining if something is parsable or not, if it is parsable, then we have already parsed it! It would be silly to throw away this parsed value (anyone who's wondering whether or not something is parsable is likely interested in the parsed result), so the parsed value is returned.
It has an out parameter because the parsed value is a by-product of a true-returning TryParse call.

Is there an Int.isWholeNumber() function or something similar?

I need to check if an input is an int or not. Is there a similar function to the String.IsNullOrEmpty(), like an Int.isWholeNumber() function?
Is there a way to validate this inside the if() statement only, without having to declare an int before? (As you need to do with TryParse())
EDIT
I need to validate an area code (five numbers)

I don't believe there's any such method within the BCL, but it's easy to write one (I'm assuming you're really talking about whether a string can be parsed as an integer):
public static bool CanParse(string text)
{
int ignored;
return int.TryParse(text, out ignored);
}
Add overloads accepting IFormatProvider values etc as you require them. Assuming this is for validation, you might want to expand it to allow a range of valid values to be specified as well...
If you're doing a lot of custom validation, you may well want to look at the Fluent Validation project.

I gather from your comments that your actual problem is "how do I determine if this string contains a valid Swedish postal code?" A Swedish postal code is a five digit number not beginning with a zero. If that's the problem you actually have to solve, then solve that problem. Rather than trying to convert the string to an integer and then check the integer, I would simply write checks that say:
is the string five characters long? If not, reject it.
is the first character of the string 1, 2, 3, 4, 5, 6, 7, 8 or 9? If not, reject it.
are the second, third, fourth and fifth characters of the string 0, 1, 2, 3, 4, 5, 6, 7, 8 or 9? If not, reject it.
Simple as that. If you're never going to do math on it, don't convert it to an integer in the first place.
This approach will further generalize to more complex forms. Swedish postal codes, I gather, are often written in the form "SE-12 345", that is, with the prefix "SE-" and a space between digits two and three. It's going to be awfully hard to write an integer-validating routine that deals with that format, but writing a string-validating routine is straightforward.
More generally, this illustrates some good advice for writing questions. Ask a question about the problem you actually must solve. You assumed a solution -- parse the string as an integer -- and then started asking questions about your assumed solution. That automatically precludes anyone from giving advice that is specific to your real problem. Maybe someone reading this has already developed a library of postal-code validating software; if they have, they'd never know to tell you about it from your original question.

You can achieve this by extension methods:
You use
bool isNumber = "-1990".IsNumber();
Here is code:
public static class NumberStringExtension
{
public static bool IsNumber(this string value)
{
int i = 0;
return int.TryParse(value, out i));
}
}

See if int.TryParse gives you an answer, and make sure there's not a decimal point in the input string. (This will give you a false positive if someone enters "1." as the input.)
string input;
int ignored;
bool wholeNumber = int.TryParse(input, out ignored) && input.indexOf('.') == -1;

Use Int.TryParse Or make a custom function.
function bool IsNumber(object number)
{
try
{
Convert.ToInt32(number.ToString());
return true;
}
catch
{
return false;
}
}

int types have to be whole numbers by definition. It would have to be type double, float, or decimal to be a non-whole number. You can always try testing the type of the input.
Or maybe the input is a string? If the input is a string, you could try searching for a '.' or ',' character (depending on the culture). Or you could try parsing as an integer.

Since every int is a whole number, this is easy:
public static bool IsWholeNumber(int i)
{
return true;
}

If you just want to validate it in the if statement you could just do
If ( val % 1 > 0)
//not whole number

Double.TryParse or Convert.ToDouble - which is faster and safer?

My application reads an Excel file using VSTO and adds the read data to a StringDictionary. It adds only data that are numbers with a few digits (1000 1000,2 1000,34 - comma is a delimiter in Russian standards).
What is better to check if the current string is an appropriate number?
object data, string key; // data had read
try
{
Convert.ToDouble(regionData, CultureInfo.CurrentCulture);
dic.Add(key, regionData.ToString());
}
catch (InvalidCastException)
{
// is not a number
}
or
double d;
string str = data.ToString();
if (Double.TryParse(str, out d)) // if done, then is a number
{
dic.Add(key, str);
}
I have to use StringDictionary instead of Dictionary<string, double> because of the following parsing algorithm issues.
My questions: Which way is faster? Which is safer?
And is it better to call Convert.ToDouble(object) or Convert.ToDouble(string) ?

I did a quick non-scientific test in Release mode. I used two inputs: "2.34523" and "badinput" into both methods and iterated 1,000,000 times.
Valid input:
Double.TryParse = 646ms
Convert.ToDouble = 662 ms
Not much different, as expected. For all intents and purposes, for valid input, these are the same.
Invalid input:
Double.TryParse = 612ms
Convert.ToDouble = ..
Well.. it was running for a long time. I reran the entire thing using 1,000 iterations and Convert.ToDouble with bad input took 8.3 seconds. Averaging it out, it would take over 2 hours. I don't care how basic the test is, in the invalid input case, Convert.ToDouble's exception raising will ruin your performance.
So, here's another vote for TryParse with some numbers to back it up.

To start with, I'd use double.Parse rather than Convert.ToDouble in the first place.
As to whether you should use Parse or TryParse: can you proceed if there's bad input data, or is that a really exceptional condition? If it's exceptional, use Parse and let it blow up if the input is bad. If it's expected and can be cleanly handled, use TryParse.

The .NET Framework design guidelines recommend using the Try methods. Avoiding exceptions is usually a good idea.
Convert.ToDouble(object) will do ((IConvertible) object).ToDouble(null);
Which will call Convert.ToDouble(string, null)
So it's faster to call the string version.
However, the string version just does this:
if (value == null)
{
return 0.0;
}
return double.Parse(value, NumberStyles.Float | NumberStyles.AllowThousands, provider);
So it's faster to do the double.Parse directly.

If you aren't going to be handling the exception go with TryParse. TryParse is faster because it doesn't have to deal with the whole exception stack trace.

I generally try to avoid the Convert class (meaning: I don't use it) because I find it very confusing: the code gives too few hints on what exactly happens here since Convert allows a lot of semantically very different conversions to occur with the same code. This makes it hard to control for the programmer what exactly is happening.
My advice, therefore, is never to use this class. It's not really necessary either (except for binary formatting of a number, because the normal ToString method of number classes doesn't offer an appropriate method to do this).

Unless you are 100% certain of your inputs, which is rarely the case, you should use Double.TryParse.
Convert.ToDouble will throw an exception on non-numbers
Double.Parse will throw an exception on non-numbers or null
Double.TryParse will return false or 0 on any of the above without generating an exception.
The speed of the parse becomes secondary when you throw an exception because there is not much slower than an exception.

Lots of hate for the Convert class here... Just to balance a little bit, there is one advantage for Convert - if you are handed an object,
Convert.ToDouble(o);
can just return the value easily if o is already a Double (or an int or anything readily castable).
Using Double.Parse or Double.TryParse is great if you already have it in a string, but
Double.Parse(o.ToString());
has to go make the string to be parsed first and depending on your input that could be more expensive.

Double.TryParse IMO.
It is easier for you to handle, You'll know exactly where the error occurred.
Then you can deal with it how you see fit if it returns false (i.e could not convert).

I have always preferred using the TryParse() methods because it is going to spit back success or failure to convert without having to worry about exceptions.

This is an interesting old question. I'm adding an answer because nobody noticed a couple of things with the original question.
Which is faster: Convert.ToDouble or Double.TryParse?
Which is safer: Convert.ToDouble or Double.TryParse?
I'm going to answer both these questions (I'll update the answer later), in detail, but first:
For safety, the thing every programmer missed in this question is the line (emphasis mine):
It adds only data that are numbers with a few digits (1000 1000,2 1000,34 - comma is a delimiter in Russian standards).
Followed by this code example:
Convert.ToDouble(regionData, CultureInfo.CurrentCulture);
What's interesting here is that if the spreadsheets are in Russian number format but Excel has not correctly typed the cell fields, what is the correct interpretation of the values coming in from Excel?
Here is another interesting thing about the two examples, regarding speed:
catch (InvalidCastException)
{
// is not a number
}
This is likely going to generate MSIL that looks like this:
catch [mscorlib]System.InvalidCastException
{
IL_0023: stloc.0
IL_0024: nop
IL_0025: ldloc.0
IL_0026: nop
IL_002b: nop
IL_002c: nop
IL_002d: leave.s IL_002f
} // end handler
IL_002f: nop
IL_0030: return
In this sense, we can probably compare the total number of MSIL instructions carried out by each program - more on that later as I update this post.
I believe code should be Correct, Clear, and Fast... In that order!

Personally, I find the TryParse method easier to read, which one you'll actually want to use depends on your use-case: if errors can be handled locally you are expecting errors and a bool from TryParse is good, else you might want to just let the exceptions fly.
I would expect the TryParse to be faster too, since it avoids the overhead of exception handling. But use a benchmark tool, like Jon Skeet's MiniBench to compare the various possibilities.