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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
Related
I have a question. I am testing a lib from me, which is generation some text in xml-style. Up to now, I am testing with the function
Assert.AreEqual(string1, string2);
But the strings, which are in xml-style, are more than 300 characters long. And when I make a little mistake in one character, the test is failing and the output is, that the strings are not equal. But the test does not say, at which position they are not equal.
So my question is: Is there already an implemented function, which compares two strings and tell me also, at which position they differ + output of the strings ... ?
try this way
var indexBroke = 0;
var maxLength = Math.Min(string1.Length, string2.Length);
while (indexBroke < maxLength && string1[indexBroke] == string2[indexBroke]) {
indexBroke++;
}
return ++indexBroke;
the logic is that you compare each character step by step and when you get the first difference the function exit returninng the last index with equal characters
For that reason (and many others more), I can recommend using FluentAssertions.
With FluentAssertions you would formulate your assertion like this:
string1.Should().Be(string2);
In the case the strings do not match, you get a nice informative message helping you to tackle down the problem:
Expected string to be
"<p>Line one<br/>Line two</p>" with a length of 28, but
"<p>Line one<br>Line two</p>" has a length of 27.
Additionally, you can give a reason to make the error message even more clear:
string1.Should().Be(string2, "a multiline-input should have been successfully parsed");
That would give you the following message:
Expected string to be
"<p>Line one<br/>Line two</p>" with a length of 28 because a multiline-input should have been successfully parsed, but
"<p>Line one<br>Line two</p>" has a length of 27.
These reason arguments are especially valuable when comparing values that provide no meaning by themselves, such as booleans and numbers.
BTW, FluentAssertions also helps greatly in comparing object graphs.
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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.
I want to evaluate a math expression which the user enters in a textbox. I have done this so far
string equation, finalString;
equation = textBox1.Text;
StringBuilder stringEvaluate = new StringBuilder(equation);
stringEvaluate.Replace("sin", "math.sin");
stringEvaluate.Replace("cos", "math.cos");
stringEvaluate.Replace("tan", "math.tan");
stringEvaluate.Replace("log", "math.log10");
stringEvaluate.Replace("e^", "math.exp");
finalString = stringEvaluate.ToString();
StringBuilder replaceI = new StringBuilder(finalString);
replaceI.Replace("x", "i");
double a;
for (int i = 0; i<5 ; i++)
{
a = double.Parse(finalStringI);
if(a<0)
break;
}
when I run this program it gives an error "Input string was not in a correct format." and highlights a=double.Parse(finalStringI);
I used a pre defined expression a=i*math.log10(i)-1.2 and it works, but when I enter the same thing in the textbox it doesn't.
I did some search and it came up with something to do with compiling the code at runtime.
any ideas how to do this?
i'm an absolute beginner.
thanks :)
The issue is within your stringEvaluate StringBuilder. When you're replacing "sin" with "math.sin", the content within stringEvaluate is still a string. You've got the right idea, but the error you're getting is because of that fact.
Math.sin is a method inside the Math class, thus it cannot be operated on as you are in your a = double.Parse(finalStringI); call.
It would be a pretty big undertaking to accomplish your goal, but I would go about it this way:
Create a class (perhaps call it Expression).
Members of the Expression class could include Lists of operators and operands, and perhaps a double called solution.
Pass this class the string at instantiation, and tear it apart using the StringBuilder class. For example, if you encounter a "sin", add Math.sin to the operator collection (of which I'd use type object).
Each operator and operand within said string should be placed within the two collections.
Create a method that evaluates the elements within the operator and operand collection accordingly. This could get sticky for complex calculations with more than 2 operators, as you would have to implement a PEMDAS-esque algorithm to re-order the collections to obey the order of operations (and thus achieve correct solutions).
Hope this helps :)
The .Parse methods (Int.Parse, double.Parse, etc) will only take a string such as "25" or "3.141" and convert it to the matching value type (int 25, or double 3.141). They will not evaluate math expressions!
You'll pretty much have to write your own text-parser and parse-tree evaluator, or explore run-time code-generation, or MSIL code-emission.
Neither topic can really be covered in the Q&A format of StackOverflow answers.
Take a look at this blog post:
http://www.c-sharpcorner.com/UploadFile/mgold/CodeDomCalculator08082005003253AM/CodeDomCalculator.aspx
It sounds like it does pretty much what you're trying to do. Evaluating math expressions is not as simple as just parsing a double (which is really only going to work for strings like "1.234", not "1 + 2.34"), but apparently it is possible.
You can use the eval function that the framework includes for JScript.NET code.
More details: http://odetocode.com/code/80.aspx
Or, if you're not scared to use classes marked "deprecated", it's really easy:
static string EvalExpression(string s)
{
return Microsoft.JScript.Eval.JScriptEvaluate(s, null, Microsoft.JScript.Vsa.VsaEngine.CreateEngine()).ToString();
}
For example, input "Math.cos(Math.PI / 3)" and the result is "0.5" (which is the correct cosine of 60 degrees)
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
Using a string.CompareTo(string) i can get around this slightly but is not easy to read and i have read on that locallity settings might influence the result.
Is there a way to just simply use < or > on 2 Strings in a more straightforward way?
You can overload operators but you seldom should. To me "stringA" > "stringB" wouldn't mean a damn thing, it's not helping readability IMO. That's why operator overloading guidelines advise not to overload operators if the meaning is not obvious.
EDIT: Operator Overloading Usage Guidelines
Also, in case of String I'm afraid you can't do it seeing as you can put operator-overloading methods only in the class in which the methods are defined.
If the syntax of CompareTo bothers you, maybe wrapping it in extension method will solve your problem?
Like that:
public static bool IsLessThan(this string str, string str2) {
return str.Compare(str2) < 0;
}
I still find it confusing for reader though.
The bottom line is, you can't overload operators for String. Usually you can do something like declaring a partial and stuffing your overloads there, but String is a sealed class, so not this time. I think that the extension method with reasonable name is your best bet. You can put CompareTo or some custom logic inside it.
CompareTo is the proper way in my opinion, you can use the overloads to specify culture specific parameters...
You mention in a comment that you're comparing two strings with values of the form "A100" and "B001". This works in your legacy VB 6 code with the < and > operators because of the way that VB 6 implements string comparison.
The algorithm is quite simple. It walks through the string, one character at a time, and compares the ASCII values of each character. As soon as a character from one string is found to have a lower ASCII code than the corresponding character in the other string, the comparison stops and the first string is declared to be "less than" the second. (VB 6 can be forced to perform a case-insensitive comparison based on the system's current locale by placing the Option Compare Text statement at the top of
the relevant code module, but this is not the default setting.)
Simple, of course, but not entirely logical. Comparing ASCII values skips over all sorts of interesting things you might find in strings nowadays; namely non-ASCII characters. Since you appear to be dealing with strings whose contents have pre-defined limits, this may not be a problem in your particular case. But more generally, writing code like strA < strB is going to look like complete nonsense to anyone else who has to maintain your code (it seems like you're already having this experience), and I encourage you to do the "right thing" even when you're dealing with a fixed set of possible inputs.
There is nothing "straightforward" about using < or > on string values. If you need to implement this functionality, you're going to have to do it yourself. Following the algorithm that I described VB 6 as using above, you could write your own comparison function and call that in your code, instead. Walk through each character in the string, determine if it is a character or a number, and convert it to the appropriate data type. From there, you can compare the two parsed values, and either move on to the next index in the string or return an "equality" value.
There is another problem with that, I think:
Assert.IsFalse(10 < 2);
Assert.IsTrue("10" < "2");
(The second Assert assumes you did an overload for the < operator on the string class.)
But the operator suggests otherwise!!
I agree with Dyppl: you shouldn't do it!