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
Related
This question already has answers here:
compare two string value [closed]
(6 answers)
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
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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'm working with the Umbraco CMS which holds lots of data as strings.
Sometimes I need to compare a stored value string value (which is an int stored as a string) to an enum, but is it best to compare them as strings:
if ( stringValue == ( (int) Enum.Option ).ToString() ){
}
Or to parse and compare as ints:
if ( int.Parse(stringValue) == (int) Enum.Option ){
}
Or does it just not matter either way!
You should compare data in its native/canonical form. So use integers. Performance is usually a second-order concern in such cases. Correctness is first.
Maybe you want to try to use Enum.Parse?
enum MyEnum
{
Option,
Option1 = 1,
Option2 = 2
}
string stringValue = "0";
if((MyEnum)Enum.Parse(typeof(MyEnum), stringValue) == MyEnum.Option)
{
//Do what you need
}
Note:
The value parameter contains the string representation of an enumeration member's underlying value or named constant, or a list of named constants delimited by commas (,).
So stringValue can be "Option" or "0".
Its even better if you will compare enums.
For the sake of code readability, I'd choose the second approach: it makes clear beyond doubt that your string is expected to contain an integer in that particular context, and you're treating it as such.
Second approach would also allow you to handle error cases more deeply (what if your string isn't an integer ? Second block would throw, first one would silently act just like your data was different from the enum).
Also, as already stated, comparing integers is always better performance-wise than comparing strings, but I believe there wouldn't be much real-world difference in this case.
Casting from int to an enum is extremely cheap... it'll be faster than a dictionary lookup. Basically it's a no-op, just copying the bits into a location with a different notional type.
Parsing a string into an enum value will be somewhat slower.
from this SO answer.
If you want to check the validity, you can use
int value;
Option option;
if (int.TryParse(stringValue, out value) &&
Enum.IsDefined(typeof(Option), value)) {
option=(Option)value;
}
A friend of mine came to me with this strange behavior which i can't explain, any insight view would be appreciated.
Im running VS 2005 (C# 2.0), the following code show the behavior
int rr = "test".IndexOf("");
Console.WriteLine(rr.ToString());
the above code, print "0" which clearly show it should have return -1
This also happen in Java where the following Class show the behavior:
public class Test{
public static void main(String[] args){
System.out.println("Result->"+("test".indexOf("")));
}
}
Im running Java 1.6.0_17
Quote from the C# documentation:
If value is Empty, the return value
is 0.
The behavior that you describe is entirely as expected (at least in C#).
0 is correct. Start at position zero and you can (trivially) match a zero-length string. Likewise, "" contains "".
This is not an exception to the rule, but rather a natural consequence of how indexOf and startsWith are defined.
You’re claiming that "test".indexOf("") should return -1. This is essentially equivalent to the claim that "test".startsWith("") should return false. Why is this? Although this case is specifically addressed in the documentation as returning true, this is not just an arbitrary decision.
How would you decide "test".startsWith("te"), for example? The simplest way is to use recursion. Since both strings start with the character 't', you call "est".startsWith("e") and return the result. Similarly, you will call "st".startsWith("") and return the result. But you already know that the answer should be true, so that is why every string starts with "".
0 is correct. The Javadocs point out that indexOf works as follows:
The integer returned is the smallest
value k such that:
this.startsWith(str, k)
Any string starting with "" is equal to the original string (and every string starts with ""), so the smallest k for str = "" is always 0.
Think of it this way: IndexOf, when looking for a string, will start at position 0, try to match the string, if it doesn't fit, move on to position 1, 2, etc. When you call it with an empty string, it attempts to match the empty string with the string starting at position 0 with length 0. And hooray, nothing equals nothing.
Side note: There's no real reason to use ToString when you're using Console.Write/WriteLine. The function automatically calls the ToString method of the object in question. (Unless overloading ToString)
It should return 0. You are looking for the first occurrence of an empty string, right? :)
More fun php actually does a way better job!
php -r "print strpos('test','');"
PHP Warning: strpos(): Empty delimiter. in Command line code on line 1
Just for the fun of it. It also works like that in python
>>> "test".startswith("")
True
>>> "test".index("")
0
Python throws a ValueError instead of the -1 that is nice.
>>> "test".index('r')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: substring not found
This seems so trivial but I'm not finding an answer with Google.
I'm after a high value for a string for a semaphore at the end of a sorted list of strings.
It seems to me that char.highest.ToString() should do it--but this compares low, not high.
Obviously it's not truly possible to create a highest possible string because it would always be lower than the same thing + more data but the strings I'm sorting are all valid pathnames and thus the symbols used are constrained.
In response to the comments:
In the pre-unicode days in Delphi I would simply have used #255. I simply want a string that will compare higher than any possible pathname. This should be trivial--why isn't it??
Response #2:
It's not the sorting that requires the sentinel, it's the processing afterwards. I have multiple lists that I am sort-of merging (a simplistic merge won't do the job.) and either I duplicate code or I have dummy values that always compare high.
A string representation of the highest character will only be one character long.
Why don't you just append it as a semaphore after sorting, rather than trying to make it something that will sort afterwards?
Alternatively, you could specify your own comparator that sorts your token after any other string, and calls the default comparator otherwise.
I had the same problem when trying to put null values at the bottom of a list in a LINQ OrderBy() statement. I ended up using...
Char.ConvertFromUtf32(0x10ffff)
...which worked a treat.
Something like this?
public static String Highest(this String value)
{
Char highest = '\0';
foreach (Char c in value)
{
highest = Math.Max(c, highest);
}
return new String(new Char[] { highest });
}