This question already has answers here:
Identify if a string is a number
(26 answers)
Closed 5 years ago.
Is there a way to write code so that when a user gives input, either a string or a number, that the program will choose the most appropriate of the available variables that have been declared?
Pseudocode example:
static void Main()
{
int A;
string B;
Console.Write("enter something: ");
if (user enters a number)
A = int.Parse(Console.ReadLine());
else
B = Console.ReadLine();
}
Simply said, since Console.ReadLine is the way to receive user input, and since it always returns a string, no matter what: No, you have to parse the string for being a number yourself, and in turn assign it to the most fitting variable yourself.
You can do that with int.TryParse, which returns true if the given string could be parsed into a (integral) number, assigned to the second out parameter in the same line:
static void Main()
{
Console.Write("enter something: ");
string B = Console.ReadLine();
if (int.TryParse(B, out int A))
Console.WriteLine($"Yay, user entered number {A}.");
else
Console.WriteLine($"Nay, user entered a boring string {B}.");
}
You can let int.TryParse() decide:
int A;
string B;
string userinput = Console.ReadLine();
// if parsing to int fails, assign to B
if (!int.TryParse(userinput, out A)
{
B = userinput;
}
Related
This question already has answers here:
How can I convert String to Int?
(31 answers)
Closed 3 months ago.
I want to add two numbers using Console.ReadLine(), but it gives me the error
Invalid expression term 'int'
Here is my code:
using System;
namespace MyApplication
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Enter a number:");
string number1 = Console.ReadLine();
Console.WriteLine("Enter another number:");
string number2 = Console.ReadLine();
Console.WriteLine("The sum of those numbers is:");
Console.WriteLine(int(number1) + int(number2));
}
}
}
Could you help?
Use the Convert.ToInt32() method to convert from string to int.
Console.WriteLine(Convert.ToInt32(number1) + Convert.ToInt32(number2));
See How can I convert String to Int? for more examples or alternatives.
Note: You write
string s = "42";
int i = int(s); // Compiler error Invalid Expression term
This syntax is used for type casting in languages like Pascal or Python. But in C based languages like C#, C++ or Java you use a different syntax:
string s = "42";
int i = (int)s; // Compiler error invalid cast
Round brackets around the type name, not around the value. This will still not work, though, because there is no direct cast from type string to type int in C#. It would work for different types:
double d = 42d;
int i = (int)d; // Works
Closed. This question needs details or clarity. It is not currently accepting answers.
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Closed 2 years ago.
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namespace comparision
{
class numbers
{
private static string comparision;
static void Main(string[] args)
{
Console.WriteLine("Enter number a:");
string a = Console.ReadLine();
Console.WriteLine("Enter number b:");
string b = Console.ReadLine();
if a > b;
Console.WriteLine("a is greater than b");
else;
Console.WriteLine("a is greater than b");
return;
}
}
}
What I have done wrong ?
If I assume I know what your end goal is, here's everything that is wrong with your program:
You can't compare strings with just a > b. At the very best this would be a lexicographical comparison, which would compare the string "2" to be "greater than" "10", because it compares it character by character.
I guess you actually meant the two things you input to be actual numbers, in which case you either want:
int a = int.Parse(Console.ReadLine()); // if integers (whole numbers)
or
double a = double.Parse(Console.ReadLine()); // if floating point
And same goes for the b variable
However, < and > are not defined for string comparison. If you actually want to do string comparison, and not numeric comparison, you could use one of the members of the StringComparer class to pick the type of string comparison, such as StringComparer.CurrentCultureIgnoreCase.Compare(a, b) which would return a number less than 0 (if a < b), greater than zero (if a > b) or equal to zero (if a == b).
The syntax for an if-statement is if (...), not if ...;
The syntax for else doesn't require a semicolon. What you've actually done is to say "else do nothing", the semicolon means "do nothing" when placed like this. You want to take that out.
As for other improvements, that aren't actual errors:
You don't need a return statement for a method that returns void, if it's the last thing you do in the method
You have no use for the static string field you've declared
The two messages you output are the same, which means the program will say the same regardless of which number is the greatest one
What happens if you input two equal numbers?
So here's one different version of your program:
namespace comparision
{
class numbers
{
static void Main(string[] args)
{
Console.WriteLine("Enter number a:");
int a = int.Parse(Console.ReadLine());
Console.WriteLine("Enter number b:");
int b = int.Parse(Console.ReadLine());
if (a > b)
Console.WriteLine("a is greater than b");
else if (a < b)
Console.WriteLine("a is less than b");
else
Console.WriteLine("a is equal to b");
}
}
}
Ok at the beggining I think you make some mistakes. From the start conditional for if should be inside () and after it you don't use semicolon same with else. You can't compare strings like that so you need change type for a and b for example on int and Convert string input in int. Here is corrected code:
namespace comparision
{
class numbers
{
static void Main(string[] args)
{
Console.WriteLine("Enter number a:");
int a = Convert.ToInt32(Console.ReadLine());
Console.WriteLine("Enter number b:");
int b = Convert.ToInt32(Console.ReadLine());
if (a > b)
Console.WriteLine("a is greater than b");
else
Console.WriteLine("a is not greater than b");
Console.ReadKey();
}
}
}
What I am looking for is how to read an integer that was given by the user from the command line (console project). I primarily know C++ and have started down the C# path. I know that Console.ReadLine(); only takes a char/string. So in short I am looking for the integer version of this.
Just to give you an idea of what I'm doing exactly:
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
Console.ReadLine(); // Needs to take in int rather than string or char.
I have been looking for quite a while for this. I have found a lot on C but not C#. I did find however a thread, on another site, that suggested to convert from char to int. I'm sure there has to be a more direct way than converting.
You can convert the string to integer using Convert.ToInt32() function
int intTemp = Convert.ToInt32(Console.ReadLine());
I would suggest you use TryParse:
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
string input = Console.ReadLine();
int number;
Int32.TryParse(input, out number);
This way, your application does not throw an exception, if you try to parse something like "1q" or "23e", because somebody made a faulty input.
Int32.TryParse returns a boolean value, so you can use it in an if statement, to see whether or not you need to branch of your code:
int number;
if(!Int32.TryParse(input, out number))
{
//no, not able to parse, repeat, throw exception, use fallback value?
}
To your question: You will not find a solution to read an integer because ReadLine() reads the whole command line, threfor returns a string. What you can do is, try to convert this input into and int16/32/64 variable.
There are several methods for this:
Int.Parse()
Convert.ToInt()
Int.TryParse()
If you are in doubt about the input, which is to be converted, always go for the TryParse methods, no matter if you try to parse strings, int variable or what not.
Update
In C# 7.0 out variables can be declared directly where they are passed in as an argument, so the above code could be condensed into this:
if(Int32.TryParse(input, out int number))
{
/* Yes input could be parsed and we can now use number in this code block
scope */
}
else
{
/* No, input could not be parsed to an integer */
}
A complete example would look like this:
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Hello World!");
var foo = Console.ReadLine();
if (int.TryParse(foo, out int number1)) {
Console.WriteLine($"{number1} is a number");
}
else
{
Console.WriteLine($"{foo} is not a number");
}
Console.WriteLine($"The value of the variable {nameof(number1)} is {number1}");
Console.ReadLine();
}
}
Here you can see, that the variable number1 does get initialized even if the input is not a number and has the value 0 regardless, so it is valid even outside the declaring if block
You need to typecast the input. try using the following
int input = Convert.ToInt32(Console.ReadLine());
It will throw exception if the value is non-numeric.
Edit
I understand that the above is a quick one. I would like to improve my answer:
String input = Console.ReadLine();
int selectedOption;
if(int.TryParse(input, out selectedOption))
{
switch(selectedOption)
{
case 1:
//your code here.
break;
case 2:
//another one.
break;
//. and so on, default..
}
}
else
{
//print error indicating non-numeric input is unsupported or something more meaningful.
}
int op = 0;
string in = string.Empty;
do
{
Console.WriteLine("enter choice");
in = Console.ReadLine();
} while (!int.TryParse(in, out op));
Use this simple line:
int x = int.Parse(Console.ReadLine());
I didn't see a good and complete answer to your question, so I will show a more complete example. There are some methods posted showing how to get integer input from the user, but whenever you do this you usually also need to
validate the input
display an error message if invalid input
is given, and
loop through until a valid input is given.
This example shows how to get an integer value from the user that is equal to or greater than 1. If invalid input is given, it will catch the error, display an error message, and request the user to try again for a correct input.
static void Main(string[] args)
{
int intUserInput = 0;
bool validUserInput = false;
while (validUserInput == false)
{
try
{
Console.Write("Please enter an integer value greater than or equal to 1: ");
intUserInput = int.Parse(Console.ReadLine()); //try to parse the user input to an int variable
}
catch (Exception e) //catch exception for invalid input, such as a letter
{
Console.WriteLine(e.Message);
}
if (intUserInput >= 1) { validUserInput = true; }
else { Console.WriteLine(intUserInput + " is not a valid input, please enter an integer greater than 0."); }
} //end while
Console.WriteLine("You entered " + intUserInput);
Console.WriteLine("Press any key to exit ");
Console.ReadKey();
} //end main
In your question it looks like you wanted to use this for menu options. So if you wanted to get int input for choosing a menu option you could change the if statement to
if ( (intUserInput >= 1) && (intUserInput <= 4) )
This would work if you needed the user to pick an option of 1, 2, 3, or 4.
I used int intTemp = Convert.ToInt32(Console.ReadLine()); and it worked well, here's my example:
int balance = 10000;
int retrieve = 0;
Console.Write("Hello, write the amount you want to retrieve: ");
retrieve = Convert.ToInt32(Console.ReadLine());
Better way is to use TryParse:
Int32 _userInput;
if(Int32.TryParse (Console.Readline(), out _userInput) {// do the stuff on userInput}
Try this it will not throw exception and user can try again:
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
int choice = 0;
while (!Int32.TryParse(Console.ReadLine(), out choice))
{
Console.WriteLine("Wrong input! Enter choice number again:");
}
static void Main(string[] args)
{
Console.WriteLine("Please enter a number from 1 to 10");
int counter = Convert.ToInt32(Console.ReadLine());
//Here is your variable
Console.WriteLine("The numbers start from");
do
{
counter++;
Console.Write(counter + ", ");
} while (counter < 100);
Console.ReadKey();
}
You could create your own ReadInt function, that only allows numbers
(this function is probably not the best way to go about this, but does the job)
public static int ReadInt()
{
string allowedChars = "0123456789";
ConsoleKeyInfo read = new ConsoleKeyInfo();
List<char> outInt = new List<char>();
while(!(read.Key == ConsoleKey.Enter && outInt.Count > 0))
{
read = Console.ReadKey(true);
if (allowedChars.Contains(read.KeyChar.ToString()))
{
outInt.Add(read.KeyChar);
Console.Write(read.KeyChar.ToString());
}
if(read.Key == ConsoleKey.Backspace)
{
if(outInt.Count > 0)
{
outInt.RemoveAt(outInt.Count - 1);
Console.CursorLeft--;
Console.Write(" ");
Console.CursorLeft--;
}
}
}
Console.SetCursorPosition(0, Console.CursorTop + 1);
return int.Parse(new string(outInt.ToArray()));
}
Declare a variable that will contain the value of the user input :
Ex :
int userInput = Convert.ToInt32(Console.ReadLine());
I know this question is old, but with some newer C# features like lambda expressions, here's what I actually implemented for my project today:
private static async Task Main()
{
// -- More of my code here
Console.WriteLine("1. Add account.");
Console.WriteLine("2. View accounts.");
int choice = ReadInt("Please enter your choice: ");
// -- Code that uses the choice variable
}
// I have this as a public function in a utility class,
// but you could use it directly in Program.cs
private static int ReadInt(string prompt)
{
string? text;
do
{
Console.Write(prompt);
text = Console.ReadLine();
} while (text == null || !text.Where(c => char.IsNumber(c)).Any());
return int.Parse(new string(text.Where(c => char.IsNumber(c)).ToArray()));
}
The difference here is that if you accidentally type a number and any other text along with that number, only the number is parsed.
You could just go ahead and try :
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
int choice=int.Parse(Console.ReadLine());
That should work for the case statement.
It works with the switch statement and doesn't throw an exception.
I am trying to create a static method that validates if input is actually a number or not and I want to do it with a regex. I get an error "cannot convert int to string" when I try to do it, but it's my understanding that integers also can be compared using regex.
This is my static method so far.
public static void validatenumber(int number)
{
Regex regex = new Regex("^[0-9]+$");
if (regex.IsMatch(number))
{
Console.WriteLine("The input is a number");
}
else
{
Console.WriteLine("The input is not a number");
}
}
And this is the input that I am trying to validate, which is in the Main method.
Console.WriteLine("Please enter a number");
int number= Convert.ToInt32(Console.ReadLine());
validatenumber(number);
My variable number in my Main method needs to be an int, I cannot change that.
Your problem is not the Regex error you are getting, but the fact that you are trying to validate an int as a number. There is no sense in this validation, since the compiler will never allow an int variable to hold something else except a number.
It would make sense if yot method was receiving a string as a parameter:
validatenumber(string number)
In which case, you would not get this error, since Regex works with strings.
An issue with you code is found here, which leads to your regex (as you have to perform regex on a string not an int).
Console.WriteLine("Please enter a number");
int number= Convert.ToInt32(Console.ReadLine()); // <--- Here!
validatenumber(number);
Console.ReadLine() will return a string. So your code will throw an error or give you an unexpected result when you try to do Convert.ToInt32() on it.
You will need to change your code to look like the following:
static void Main(string[] args)
{
...
Console.WriteLine("Please enter a number");
string numberString = Console.ReadLine();
int number = ConvertAndValidateNumber(number); // we are now going to return an int
...
}
and then change the the validatenumber method to return the int type using the int.TryParse() method, which is much easier to read.
public static int ConvertAndValidateNumber(string numberString)
{
int.TryParse(numberString, out int number);
return number;
}
If you REALLY want to use RegEx (which makes sense in some very special cases), then you can change the above method to:
public static int ConvertAndValidateNumberUsingRegEx(string numberString)
{
int number;
//
// put your RegEx here...
//
// and don't forget to assign the converted string value into the number
number = (your converted string);
// return it
return number;
}
You also may want to validate that your regex is valid by testing it here http://regexr.com/
What I am looking for is how to read an integer that was given by the user from the command line (console project). I primarily know C++ and have started down the C# path. I know that Console.ReadLine(); only takes a char/string. So in short I am looking for the integer version of this.
Just to give you an idea of what I'm doing exactly:
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
Console.ReadLine(); // Needs to take in int rather than string or char.
I have been looking for quite a while for this. I have found a lot on C but not C#. I did find however a thread, on another site, that suggested to convert from char to int. I'm sure there has to be a more direct way than converting.
You can convert the string to integer using Convert.ToInt32() function
int intTemp = Convert.ToInt32(Console.ReadLine());
I would suggest you use TryParse:
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
string input = Console.ReadLine();
int number;
Int32.TryParse(input, out number);
This way, your application does not throw an exception, if you try to parse something like "1q" or "23e", because somebody made a faulty input.
Int32.TryParse returns a boolean value, so you can use it in an if statement, to see whether or not you need to branch of your code:
int number;
if(!Int32.TryParse(input, out number))
{
//no, not able to parse, repeat, throw exception, use fallback value?
}
To your question: You will not find a solution to read an integer because ReadLine() reads the whole command line, threfor returns a string. What you can do is, try to convert this input into and int16/32/64 variable.
There are several methods for this:
Int.Parse()
Convert.ToInt()
Int.TryParse()
If you are in doubt about the input, which is to be converted, always go for the TryParse methods, no matter if you try to parse strings, int variable or what not.
Update
In C# 7.0 out variables can be declared directly where they are passed in as an argument, so the above code could be condensed into this:
if(Int32.TryParse(input, out int number))
{
/* Yes input could be parsed and we can now use number in this code block
scope */
}
else
{
/* No, input could not be parsed to an integer */
}
A complete example would look like this:
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Hello World!");
var foo = Console.ReadLine();
if (int.TryParse(foo, out int number1)) {
Console.WriteLine($"{number1} is a number");
}
else
{
Console.WriteLine($"{foo} is not a number");
}
Console.WriteLine($"The value of the variable {nameof(number1)} is {number1}");
Console.ReadLine();
}
}
Here you can see, that the variable number1 does get initialized even if the input is not a number and has the value 0 regardless, so it is valid even outside the declaring if block
You need to typecast the input. try using the following
int input = Convert.ToInt32(Console.ReadLine());
It will throw exception if the value is non-numeric.
Edit
I understand that the above is a quick one. I would like to improve my answer:
String input = Console.ReadLine();
int selectedOption;
if(int.TryParse(input, out selectedOption))
{
switch(selectedOption)
{
case 1:
//your code here.
break;
case 2:
//another one.
break;
//. and so on, default..
}
}
else
{
//print error indicating non-numeric input is unsupported or something more meaningful.
}
int op = 0;
string in = string.Empty;
do
{
Console.WriteLine("enter choice");
in = Console.ReadLine();
} while (!int.TryParse(in, out op));
Use this simple line:
int x = int.Parse(Console.ReadLine());
I didn't see a good and complete answer to your question, so I will show a more complete example. There are some methods posted showing how to get integer input from the user, but whenever you do this you usually also need to
validate the input
display an error message if invalid input
is given, and
loop through until a valid input is given.
This example shows how to get an integer value from the user that is equal to or greater than 1. If invalid input is given, it will catch the error, display an error message, and request the user to try again for a correct input.
static void Main(string[] args)
{
int intUserInput = 0;
bool validUserInput = false;
while (validUserInput == false)
{
try
{
Console.Write("Please enter an integer value greater than or equal to 1: ");
intUserInput = int.Parse(Console.ReadLine()); //try to parse the user input to an int variable
}
catch (Exception e) //catch exception for invalid input, such as a letter
{
Console.WriteLine(e.Message);
}
if (intUserInput >= 1) { validUserInput = true; }
else { Console.WriteLine(intUserInput + " is not a valid input, please enter an integer greater than 0."); }
} //end while
Console.WriteLine("You entered " + intUserInput);
Console.WriteLine("Press any key to exit ");
Console.ReadKey();
} //end main
In your question it looks like you wanted to use this for menu options. So if you wanted to get int input for choosing a menu option you could change the if statement to
if ( (intUserInput >= 1) && (intUserInput <= 4) )
This would work if you needed the user to pick an option of 1, 2, 3, or 4.
I used int intTemp = Convert.ToInt32(Console.ReadLine()); and it worked well, here's my example:
int balance = 10000;
int retrieve = 0;
Console.Write("Hello, write the amount you want to retrieve: ");
retrieve = Convert.ToInt32(Console.ReadLine());
Better way is to use TryParse:
Int32 _userInput;
if(Int32.TryParse (Console.Readline(), out _userInput) {// do the stuff on userInput}
Try this it will not throw exception and user can try again:
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
int choice = 0;
while (!Int32.TryParse(Console.ReadLine(), out choice))
{
Console.WriteLine("Wrong input! Enter choice number again:");
}
static void Main(string[] args)
{
Console.WriteLine("Please enter a number from 1 to 10");
int counter = Convert.ToInt32(Console.ReadLine());
//Here is your variable
Console.WriteLine("The numbers start from");
do
{
counter++;
Console.Write(counter + ", ");
} while (counter < 100);
Console.ReadKey();
}
You could create your own ReadInt function, that only allows numbers
(this function is probably not the best way to go about this, but does the job)
public static int ReadInt()
{
string allowedChars = "0123456789";
ConsoleKeyInfo read = new ConsoleKeyInfo();
List<char> outInt = new List<char>();
while(!(read.Key == ConsoleKey.Enter && outInt.Count > 0))
{
read = Console.ReadKey(true);
if (allowedChars.Contains(read.KeyChar.ToString()))
{
outInt.Add(read.KeyChar);
Console.Write(read.KeyChar.ToString());
}
if(read.Key == ConsoleKey.Backspace)
{
if(outInt.Count > 0)
{
outInt.RemoveAt(outInt.Count - 1);
Console.CursorLeft--;
Console.Write(" ");
Console.CursorLeft--;
}
}
}
Console.SetCursorPosition(0, Console.CursorTop + 1);
return int.Parse(new string(outInt.ToArray()));
}
Declare a variable that will contain the value of the user input :
Ex :
int userInput = Convert.ToInt32(Console.ReadLine());
I know this question is old, but with some newer C# features like lambda expressions, here's what I actually implemented for my project today:
private static async Task Main()
{
// -- More of my code here
Console.WriteLine("1. Add account.");
Console.WriteLine("2. View accounts.");
int choice = ReadInt("Please enter your choice: ");
// -- Code that uses the choice variable
}
// I have this as a public function in a utility class,
// but you could use it directly in Program.cs
private static int ReadInt(string prompt)
{
string? text;
do
{
Console.Write(prompt);
text = Console.ReadLine();
} while (text == null || !text.Where(c => char.IsNumber(c)).Any());
return int.Parse(new string(text.Where(c => char.IsNumber(c)).ToArray()));
}
The difference here is that if you accidentally type a number and any other text along with that number, only the number is parsed.
You could just go ahead and try :
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
int choice=int.Parse(Console.ReadLine());
That should work for the case statement.
It works with the switch statement and doesn't throw an exception.