Related
I wonder how to print an error message if user's input is not a number.
Console.WriteLine("Water amount in ml:");
int waterAmount = Convert.ToInt32(Console.ReadLine());
Most answers from other posts don't work, because waterAmount is a Int32, not a string.
Also, sorry if my English is weak, it's not my native language
I see you did not accept the other answers. Maybe you want to make the user try again after he did not input a number. You can do that with a while loop, or easier, using the goto keyword. You simply put a tag before everything is happening (in my case, waterAmountInput), and if the input was not a number, you write your error message and go to the beginning again. Here is the code:
int waterAmount;
waterAmountInput:
Console.Write("Water amount in ml: ");
try
{
waterAmount = Convert.ToInt32(Console.ReadLine());
}
catch
{
Console.WriteLine("The water amount needs to be a number!");
goto waterAmountInput;
}
You can try using C#'s TryParse() functions.
These attempt to convert values, but if they fail they return false rather than erroring.
I would suggest trying this code:
Console.WriteLine("Water amount in ml:");
string input = Console.ReadLine();
if (Int32.TryParse(input, out var value))
// Do something here. The converted int is stored in "value".
else
Console.WriteLine("Please enter a number");
You should use TryParse function:
Console.WriteLine("Water amount in ml:");
int waterAmount = 0;
if (int.TryParse(Console.ReadLine(), waterAmount)) {
} else {
Console.WriteLine("Error Message");
}
You will need to use TryParse before presuming it to be a number.
After Console.WriteLine you can capture and parse the input and if TryParse returns false, you can display an error message.
Note: if the conversion succeeds the numeric value is available in the waterAmt local variable.
Console.WriteLine("Water amount in ml:");
var input = Console.ReadLine();
if(!int.TryParse(input, out int waterAmt))
Console.WriteLine("Input is not a number");
I am trying to convert a string variable to a double.
So far I have tried Convert.ToDouble()
string userAge;
Console.WriteLine("What is your age?");
Console.ReadLine();
Convert.ToDouble(userAge);
and when I tried to do operations on userAge it shows this error:
Program.cs(23,27): error CS0019: Operator '/' cannot be applied to operands of type 'string' and 'double' [/home/ccuser/workspace/csharp-working-with-numbers-arithmetic-operators-csharp/e3-workspace.csproj]
Program.cs(29,28): error CS0029: Cannot implicitly convert type 'string' to 'double' [/home/ccuser/workspace/csharp-working-with-numbers-arithmetic-operators-csharp/e3-workspace.csproj]
Program.cs(17,24): error CS0165: Use of unassigned local variable 'userAge' [/home/ccuser/workspace/csharp-working-with-numbers-arithmetic-operators-csharp/e3-workspace.csproj]
The build failed. Fix the build errors and run again.
Any suggestions?
To begin with, you need to assign the result of those method calls (to get the user input and then convert the string to a doulbe) to some variables so you can use them later in the code:
Console.WriteLine("What is your age?");
string input = Console.ReadLine();
double age = Convert.ToDouble(input);
But now we see there is a problem - if the user enters a non-numeric input, we'll get a FormatException.
Luckily there's a better method we can use for parsing strings to doubles: double.TryParse. This method takes in a string (the input), and an out parameter that it will set to the converted value on success (or the default value of 0 on failure). And the best thing is that it returns a bool that indicates if it was successful or not, so we can use it as a condition for a loop:
Console.Write("What is your age? ");
string userAge = Console.ReadLine();
double age;
while (!double.TryParse(userAge, out age))
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine("Invalid input, please try again.");
Console.ResetColor();
Console.Write("What is your age? ");
userAge = Console.ReadLine();
}
// Now 'age' is the converted value entered by the user
Now we have a solution that will loop until the user enters a valid number. But that's a fair amount of code. What if we have to get another number from them? Probably it would be better to extract this into a method that takes in a string (to use as a prompt) and which returns the strongly-typed double result:
public static double GetDoubleFromUser(string prompt)
{
bool isValid = true;
double result;
do
{
if (!isValid)
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine("Invalid input, please try again.");
Console.ResetColor();
}
else isValid = false;
Console.Write(prompt);
} while (!double.TryParse(Console.ReadLine(), out result));
return result;
}
Now our main code is much more simple:
double userAge = GetDoubleFromUser("What is your age? ");
double userWeight = GetDoubleFromUser("What is your weight? ");
Now, if we want to get a little fancier, we can include an optional 'validator' argument, which is a function that takes in a double and returns a bool, which we can use to further validate the result and force the user to enter a valid number.
For example, what if we want them to choose a number from 1 to 10? We don't want to have to setup a loop again for this further validation, so let's pass a Func<double, bool> to the method so it can do the validation for us!
For example:
public static double GetDoubleFromUser(string prompt, Func<double, bool> validator = null)
{
bool isValid = true;
double result;
do
{
if (!isValid)
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine("Invalid input, please try again.");
Console.ResetColor();
}
else isValid = false;
Console.Write(prompt);
} while (!double.TryParse(Console.ReadLine(), out result) &&
(validator == null || !validator.Invoke(result)));
return result;
}
Now we can pass in whatever validation we want to do on the input to the method! This would look something like the line of code below, and the method will not return until the user enters a valid number that's greater than or equal to 1 and less than or equal to 10:
double number = GetDoubleFromUser("Choose a number from 1 to 10: ",
x => x >= 1 && x <= 10);
You aren't assigning the result of your Convert call to a variable, you're just throwing it away. Convert doesn't alter the type of the existing variable (because, apart from other considerations, you simply can't do that in a strongly-typed language), instead it produces a new variable for you to use in your maths.
The error is, I presume without seeing the relevant code, because you tried to use userAge in your calculations which, as I've just explained, is still a string.
Also, to go back a step, you've never actually assigned the result of the ReadLine operation to the userAge variable in the first place.
This:
Console.WriteLine("What is your age?");
string userAge = Console.ReadLine();
double age = Convert.ToDouble(userAge);
would make more sense. And then use age in your calculations afterwards.
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 currently working on a program and I am finalising it by going over with error handling. I have several cases which look like:
int stockbankInput = Convert.ToInt32(Console.ReadLine());
Here, the user must enter either 1, 2, 3. I have tried to use an if statement to catch the error if anybody inputs a blankspace/string/character or a number that is not 1,2 or 3 but it doesn't work in the same sense as a string input. Below is what I have tried:
if(stockbankInput == null)
{
Console.WriteLine("Error: Please enter either 1, 2 or 3");
stockbankInput = 0;
goto menuRestartLine;
}
However, you cannot link 'null' with an integer input, only a string. Can anybody help with this please?
Use the Int32 TryParse method:
int input;
var successful = Int32.TryParse(Console.ReadLine(), out input);
if (!successful)
// do something else
else
return input;
You're checking if an int is null, which will always return false because an int cannot be null.
You can use 'int?' (Nullable int) but Convert.ToInt32 will not return null. If the value of the int cannot be resolved it will resolve to the default value of zero. You can either check if the returned int is zero or do some further checking of the returned string:
int input = 0;
string errorMessage = "Error: Please enter either 1, 2 or 3";
while(true)
{
try
{
input = Convert.ToInt32(Console.ReadLine());
if (input == 0 || input > 3)
{
Console.WriteLine(errorMessage);
}
else
{
break;
}
}
catch(FormatException)
{
Console.WriteLine(errorMessage);
}
}
With this you your returned value "int input" will either be 0 or the number you entered and FormatExceptions caused by the string to convert containing symbols other than the digits 0-9 will be caught in the try/catch statement.
give this sample program a try:
static void Main(string[] args)
{
int stockbankInput = 0;
bool firstTry = true;
while(stockbankInput < 1 | stockbankInput > 3)
{
if(!firstTry)
Console.WriteLine("Error: Please enter either 1, 2 or 3");
firstTry = false;
Int32.TryParse(Console.ReadLine(), out stockbankInput);
}
}
First of all, don't use goto statements. They are considered bad practice, and it's like a blinding red light when reading your question - that's all I can focus on.
As per your question, an int or Int32 cannot be null. So you can't compare it to null. Give it a default value, and then check that.
This is a scenario where you don't need to check for an error, but just need to validate input. Use TryParse, which will set your out parameter if the parse is successful, or else set it to 0.
Next, you want to loop until you are given good input. An if statement is executed once, a loop will guarantee that when you leave it, your input will be valid.
Lastly, the firstTry is just a nice way to let the user know, after their first try, that they screwed up.
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.