Readline errors when user submits null or string

Question

I am writing a console application, that gives the user a numerical choice to pick from (1-3). I have my logic down correctly, so that if they select 1-3 it works, but will cycle back through the loop if they choose any number outside that range and write a string error message "Invalid Entry! Please select 1-3 only.".
The problem I am running into is that if you just hit enter, without entering anything at all, it errors out. Or if you submit an alpha character instead of a number, it also errors out. How do I write it so that null entries or string/char entries will not bug it out when it is looking for only an int entry?

centra · Answer

I updated my code last night, and it is working much better now. The TryParse was the answer I was looking for , and after I updated my do/while logic from (race > 3) to (race 3), it started functioning better when the user submits nothing as a response. I spent most of my time, however, messing around w/ ReadKey. Reading about it I learned about the KeyAvailable property, and eventually I'm going to make the app more dyanmic using it so the user can enter in multiple choices to do different things.

centra · Answer

Cool, thanks for the info.

Vulpes · Answer

Just to clarify what happens if you pass int.TryParse a null or empty string - it returns false and places 0 in the output parameter. So, it's handled just the same as any other error. You can use Console.Readkey() to validate characters as they're typed in, rather than wait until enter is pressed and then validate the whole string. However, you'll find that doing that in a bomb-proof way is much more complicated than what you're doing now and most developers are therefore content to stick with the latter.

centra · Answer

That still helps Javeed, thanks. i like the (race 3) logic much better than simply (race > 3). also, i just looked into the Console.ReadKey();, and i'm probably going to do something w/ that.

Javeed M Shaikh · Answer

too late, you already got that working..but still... class Program { public static int health = 100, action = 100, magic = 100, race, prof; public static int damage = 5, armor = 5, evade = 5; public static string name, lastName; static void Main() { char recreate; char goodName; string chosenRace = ""; string chosenProf = ""; int race; Console.BackgroundColor = ConsoleColor.DarkRed; Console.Title = "Game Version 0"; do { Console.WriteLine("Please choose a character Race: 
1)Human 
2)Elf 
3)Orc"); // race = int.Parse(Console.ReadLine()); int.TryParse(Console.ReadLine(), out race); if (race == 1) { chosenRace = "Human"; health += 25; Console.WriteLine("You have chosen Human. Humans have a +25 health bonus."); Console.WriteLine("Health = {0} 
Action = {1} 
Magic = {2}", health, action, magic); } else if (race == 2) { chosenRace = "Elf"; magic += 25; Console.WriteLine("You have chosen Elf. Elves have a +25 magic bonus."); Console.WriteLine("Health = {0} 
Action = {1} 
Magic = {2}", health, action, magic); } else if (race == 3) { chosenRace = "Orc"; action += 25; Console.WriteLine("You have chosen Orc. Orcs have a +25 action bonus."); Console.WriteLine("Health = {0} 
Action = {1} 
Magic = {2}", health, action, magic); } else { Console.WriteLine("Invalid Entry! Please choose either 1, 2, or 3."); } } while (race 3); Console.ReadKey(); } }

centra · Answer

Ha! The int.TryParse worked. here's the before code: Console.WriteLine("Please choose a character Race: 
1)Human 
2)Elf 
3)Orc"); race = int.Parse(Console.ReadLine()); after code: Console.WriteLine("Please choose a character Race: 
1)Human 
2)Elf 
3)Orc"); int.TryParse(Console.ReadLine(), out race); beautiful. I have a few things to change, but that's the answer I was looking for. @Sam: with just the int.TryParse, if the entry is null it states "Invalid Entry!..." as per my conditions, but it submits nothing as the answer and progresses on towards the next step. So potentially, you could create a character with out ever choosing a race it seems. I'll be looking into this next...

centra · Answer

i see where I failed in my logic. Yes it will cycle through the question so long as the person is entering numerics. but I never thought what if the person just hits enter, or inputs any nonnumerics. Thanks to my wife for blowing it up for the first time :P

centra · Answer

ok, here is an actual snippet (unadjusted from the conversation here): class Game { public static int health = 100, action = 100, magic = 100, race, prof; public static int damage = 5, armor = 5, evade = 5; public static string name, lastName; public static void Main() { char recreate; char goodName; string chosenRace = ""; string chosenProf = ""; Console.BackgroundColor = ConsoleColor.DarkRed; Console.Title = "Game Version 0"; #region description do { do { Console.WriteLine("Please choose a character Race: 
1)Human 
2)Elf 
3)Orc"); race = int.Parse(Console.ReadLine()); if (race == 1) { chosenRace = "Human"; health += 25; Console.WriteLine("You have chosen Human. Humans have a +25 health bonus."); Console.WriteLine("Health = {0} 
Action = {1} 
Magic = {2}", health, action, magic); } else if (race == 2) { chosenRace = "Elf"; magic += 25; Console.WriteLine("You have chosen Elf. Elves have a +25 magic bonus."); Console.WriteLine("Health = {0} 
Action = {1} 
Magic = {2}", health, action, magic); } else if (race == 3) { chosenRace = "Orc"; action += 25; Console.WriteLine("You have chosen Orc. Orcs have a +25 action bonus."); Console.WriteLine("Health = {0} 
Action = {1} 
Magic = {2}", health, action, magic); } else { Console.WriteLine("Invalid Entry! Please choose either 1, 2, or 3."); } } while (race > 3); #endregion..... ***edit: added the class level variables so it makes more sense looking at the snippet***

Javeed M Shaikh · Answer

Thanks Sam, appreciate your reply and I do see that Vulpes code is much simpler to understand. I was not trying to prove anything but just wanted to learn if there was anything bad in using regex. thanks again.

centra · Answer

Javeed, I think all that Sam is getting at is that your suggestion was simply brilliant, while Vulpes's may be easier for a novice, such as myself, to understand. The suggest of the regex class does give me something to research however. thanks all for your input.

Sam Hobbs · Answer

Javeed, just look at Vulpes's code. It is simple and complete, except perhaps for the improvements I suggest. I am just saying that it is entirely possible to write code that is as simple to write as using regex and that satisfies requirements. Other than that, I am not sure how to explain what I mean.

Javeed M Shaikh · Answer

@Sam, can you please explain why regex is an overkill and unnecessary, as it takes care of the validation that you are try to do with multiple if conditions and tryparse. the part i am referring here is only checking if the input is within range, numeric, hit enter, alphanumeric. I would appreciate if you can throw some light and open my eyes to explain this so i can refrain myself before i suggest anything like this in my future posts.

Sam Hobbs · Answer

In my opinion, a regular expression (regex) is overkill for this; it is unnecesary. I certainly recommend the code that Vulpes suggests that uses TryParse. There is one thing I would add to the code that Vulpes shows, and that is to use the return code from TryParse. If the return code is false, then you know the input is not valid. Something else you could do is to first check to ensure that there was data read; if the string that Console.ReadLine() returns is empty, then you know that the input is not good. I am nbot sure what TryParse does when the string is empty; I assume it will report an error but probably it will work better if you simply ignore the data when there is none but then issue an error message if TryParse cannot convert the data otherwise. It would also be good to check if Console.ReadLine() returns null, indicating EOF. This is not likely to happen for this program, but it is a good thing to do anyway. So then the standard input could be redirected to a file. When executing the program nomally, I think you can cause EOF by using Ctrl-Z and that would be a reasonable alternative for determining the end of input.

centra · Answer

Ok, that makes sense. Thanks a lot for your input, I'll likely be testing both out tonight. I'm leaning more towards your solution Vulpes, only because it makes more sense to me. Though I am going to try learn more about regex. I just took a quick glance at it, and it seems like a pretty diverse class.

Vulpes · Answer

It will error out if you use int.Parse. If you look back to my code, I used int.TryParse. The difference is that TryParse never errors out. If it's not presented with the string representation of an integer, it returns false and sets the output parameter to 0. If it presented with a valid integer, it returns true and sets the output parameter to that integer. I think if you use that instead you won't have much difficulty in getting your code to work.

centra · Answer

Hey guys, thanks a lot for the speedy responses. I'm just now having the time to check this out. @Javeed, I have not tried regex. I am actually not familiar with that at all. I'm going to have to do some research on that, but looks like it might be the key. @Vulpes, I tried something similar to that, but it still errors out when anything other than a number is entered. I'll try to recreate the code off the top of my head... Please be understanding if there are any syntax errors below. I am but a novice programmer trying to learn, and I typed this all off the top of my head, so it may be different or wrong from what I actually have. public static int race; public static char create=''; Console.Writeline("Welcome to game version 0.0") Console.Writeline("Please chose a race:
 1) Human
2) Elf
3) Orc") Console.Readline(Int.Parse()); do{ if (race == 1) { Console.Writeline("You have chosen Human"); } else if (race == 2) { Console.Writeline("You have chosen Elf"); } else if (race == 3) { Console.Writeline("You have chosen Orc") } else { Console.Writeline("Invalid Intry! Please entry 1, 2 or 3 only") } Console.Writeline("Do you wish to chose a different race? Y or N") create = Console.Readline(ToUpper()); }while (create == Y)

Javeed M Shaikh · Answer

just tested, looks like working: static void Main(string[] args) { string minput = Console.ReadLine(); Regex mregex = new Regex("^[1-3]+$"); MatchCollection mmatch = mregex.Matches(minput); if (mmatch.Count == 0) { Console.WriteLine("error"); } else { Console.WriteLine("ok"); } }

Vulpes · Answer

Try this: using System; class Program { static void Main() { int n; while(true) { while(true) { Console.WriteLine(); Console.Write("Enter a number (1 - 3) : "); int.TryParse(Console.ReadLine(), out n); if (n 3) { Console.WriteLine("Invalid Entry! Please select 1 -3 only"); } else { Console.WriteLine("The number you entered was {0}", n); Console.ReadKey(); return; } } } } }

Javeed M Shaikh · Answer

did you try regex like this: "^[1-3]+$" --> untested.