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How to prevent input of null enter in C# [duplicate]

This question already has answers here:
check for valid number input - console application
(5 answers)
Allow To Only Input A Number - C#
(1 answer)
Closed 2 years ago.
I have just started a table programme. I am just a trainee learning C# from internet, so I am not so good in this.
I just wanted the programme to run according to the user. I want that if the user hits enter simply, the programme should not crash. That is I just wanted to know how to prevent null enter. This is the code is used:
The "______" which used if for writing a line
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace tables
{
class Program
{
static void Main(string[] args)
{
goto found;
found:
Console.WriteLine("");
string textToEnter = "MULTIPLATION TABLES";
Console.WriteLine(String.Format("{0," + ((Console.WindowWidth / 2) + (textToEnter.Length / 2)) + "}", textToEnter));
Console.WriteLine("");
Console.WriteLine("________________________________________________________________________________");
Console.WriteLine("");
int num, j, i;
Console.Write("enter the number of which table u need ? :- ");
num = Convert.ToInt32( Console.ReadLine());
while (num == 0)
{
Console.WriteLine("please enter a valid input");
Console.Write("enter the number of which table u need ? :- ");
num = Convert.ToInt32(Console.ReadLine());
}
Console.Write("enter the number till which the table need to be ? :- ");
j = Convert.ToInt32(Console.ReadLine());
while (j == 0)
{
Console.WriteLine("please enter a valid input");
Console.Write("enter the number till which the table need to be ? :- ");
j = Convert.ToInt32(Console.ReadLine());
}
i = Convert.ToInt32(j);
for (j=1; ; j++)
{
if (j > i)
{
break;
}
Console.WriteLine(num + " * " + j + " = " + num * j);
}
string str;
Console.Write("do you want to continue? (y/n) :- " );
str= Console.ReadLine();
foreach (char ch in str)
{
if (ch == 'y')
{
goto found;
}
else if (ch=='n' )
{
Console.WriteLine("");
Console.WriteLine("THANK YOU FOR USING MY PRODUCT");
}
else
{
Console.WriteLine("please enter a valid input");
}
}
Console.ReadKey();
}
}
}

As suggested in the comments, I'd use int.TryParse(), but inside a do...while() loop. Use a separate flag (boolean) to track whether the user should keep trying again:
bool invalid;
int num, j, i;
do
{
invalid = true;
Console.Write("enter the number of which table u need ? :- ");
String response = Console.ReadLine();
if (int.TryParse(response, out num))
{
invalid = false;
}
else
{
Console.WriteLine("Invalid input. Please try again.");
}
} while (invalid);
// ...repeat the above do...while() block for "j" and "i"...

When you're accepting user input, it's important to perform validation on it. You can't assume that the user will always enter correctly formatted data that your program will be able to work with. As you discovered, a user who hits enter will give you an empty string (""), which can't be parsed to anything.
C# has several ways of attempting parsing. The first, which you're using, is Convert.ToInt32(), which throws an exception if the input it receives is not, in fact, a number. You have to catch the exception with a try/catch block, like so:
try
{
num = Convert.ToInt32(Console.ReadLine());
}
catch(FormatException ex)
{
Console.WriteLine("You didn't enter a proper number!");
}
However, in general, exceptions should be, well, exceptional. They should only be relied upon when rare failures occur, because unwinding the call stack can be expensive.
I would argue that C# has a better method for you to use in this instance: Int32.TryParse()
You can see the documentation here.
TryParse takes two parameters, the thing you're trying to parse (convert), and then a number to store the value in. It returns true or false, indicating if it succeeded or failed in converting the number.
You might use it like this:
var success = Int32.TryParse(Console.ReadLine(), num);
if (success)
{
// do something with 'num' -- it has a valid value now.
}
else
{
// Warn the user, perhaps prompt them to try again
Console.WriteLine("That wasn't a valid number!");
}

You can use a methode to get a safe int value:
private static int ReadIntValue(string psMessage)
{
int lnInt;
string lsValue = string.Empty;
do
{
Console.Write(psMessage);
lsValue = Console.ReadLine();
} while (!int.TryParse(lsValue, out lnInt));
return lnInt;
}
And then use this:
num = ReadIntValue("enter the number of which table u need ? :- ");

Small coding project won't catch FormatException

So I recently started my first coding course for C#. I am currently trying to write a short program that prints out Yay! as many times as I indicate. Now to prevent any format exceptions, I've tried to add a try and catch to it. But, for some reason this isn't working and I can't seem to figure out why. The code is below:
Console.Write("Enter the number of times to print \"Yay!\": ");
string entry = Console.ReadLine();
var number = int.Parse (entry);
bool print = true;
while(print)
{
try
{
if(number <= 0)
{
print = false;
}
else
{
Console.WriteLine("Yay!");
number -= 1;
}
}
catch(FormatException)
{
Console.WriteLine("You must enter a whole number.");
}
}
Now to my knowledge, I have everything I need to make this work. Can anyone see where I went wrong with this?
Thanks a lot for reading!

It's
var number = int.Parse (entry);
that should throw the exception, and since it's beyond try {} scope, the
exception has not been caught. Move the fragment into the scope:
Console.Write("Enter the number of times to print \"Yay!\": ");
string entry = Console.ReadLine();
bool print = true;
try {
// int.Parse is within the try {} scope now
var number = int.Parse (entry);
while(print) {
...
}
}
catch(FormatException) {
Console.WriteLine("You must enter a whole number.");
}
Or convert int.Parse into int.TryParse and drop try {...} catch {...} at all (a better solution)
Console.Write("Enter the number of times to print \"Yay!\": ");
int number;
if (!int.TryParse(Console.ReadLine(), out number)) {
Console.WriteLine("You must enter a whole number.");
return;
}
bool print = true;
// There's no need in try {} catch {} now
while(print) {
...
}

You need to place your int.Parse inside your try-catch block.
Here's a revised version of your code.
static void Main(string[] args)
{
bool print = true;
while (print)
{
Console.Write("Enter the number of times to print \"Yay!\": ");
string entry = Console.ReadLine();
try
{
var number = int.Parse(entry);
for (int i = 0; i < number; i++)
{
Console.WriteLine("Yay!");
}
}
catch (FormatException)
{
Console.WriteLine("You must enter a whole number.");
}
}
}

Breaking out of a for loop upon invalid user entry

edit: for those who have answered, I apologize, I should have been more clear, the assignment REQUIRES use of the for loop per my professor.
I'm doing a small assignment for class and am having trouble breaking out of a for loop and prompting the user to enter a valid value. My code is set up thus far as:
const int MINRANGE = 1;
const int MAXRANGE = 20;
int input = 0;
Console.Write("Enter the desired maximum: ");
string strInput = Console.ReadLine();
Console.WriteLine("\n\n\n");
for (int.TryParse(strInput, out input); input >= MINRANGE && input <= MAXRANGE; input--)
{
Console.WriteLine("{0,2} {1,5}", input, Math.Pow(input, 3));
}
The program displays everything I need it to correctly. When the user enters a value out of the range I have specified, I need to give them a short message and then break out of the loop and return to the beginning prompt. I think I need to use something like the following if statement:
if (input >= MAXRANGE || input <= MINRANGE)
{
Console.WriteLine("That is not a valid value, please try again.");
}
Perhaps with a break; following it? But I'm not sure how to use it inside of the for loop. I've tried placing it outside, but that doesn't get me back to the user prompt, but neither does placing it inside the loop, so I'm obviously doing something wrong.

You can use a while loop
int input;
while(!int.TryParse(Console.ReadLine(), out input) || input >= MAXRANGE)
{
Console.WriteLine("Not valid!");
}
Note that this may get the user "stuck" and a helpful message as to why it isn't valid would be nice
If for some bizarre reason you must use a for loop you can use the following but it is horrible code that I would never condone
int input;
for(;;)
{
if(int.TryParse(Console.ReadLine(), out input) && input < MAXRANGE)
break;
}

Its better to do the validation before entering the loop. Try this out.....
const int MINRANGE = 1;
const int MAXRANGE = 20;
int input = 0;
Console.Write("Enter the desired maximum: ");
string strInput = Console.ReadLine();
Console.WriteLine("\n\n\n");
if (input > MAXRANGE || input < MINRANGE)
{
Console.WriteLine("That is not a valid value, please try again.");
}
else
{
for (int.TryParse(strInput, out input); input >= MINRANGE && input <=
MAXRANGE; input--)
{
Console.WriteLine("{0,2} {1,3}", input, Math.Pow(input, 3));
}
}

I would rewrite your code as follows. Creating a method for asking the input:
const int MINRANGE = 1;
const int MAXRANGE = 20;
int input = 0;
string strInput = AskInput();
while (!int.TryParse(strInput, out input)) /*while the number is invalid*/
{
Console.WriteLine("Your input is invalid. Try again.");
strInput = AskInput();
}
while (input >= MINRANGE && input <= MAXRANGE)
{
Console.WriteLine("{0,2} {1,3}", input, Math.Pow(input, 3));
input--;
}
Where AskInput is:
private static string AskInput()
{
Console.Write("Enter the desired maximum: ");
string strInput = Console.ReadLine();
Console.WriteLine("\n\n\n");
return strInput;
}

for ([(fist) Initialization];
[(second)Condition];
[(third or exit the loop) Progression])
{
[(fourth) Loop's body -if we reached third than we got here ]
}
The for loop is constructed of three parts:
The initialization part which happens only once when the loop starts.
The condition evaluation part which happens for every iteration.
The indexer progression part which also happens for every loop.
The order is as follows:
Start Loop => init => CheckCondition => if OK Progress the indexer and execute loop's body, else the loop is done.
The initialization part happens just once, if you want it to happen foreach interation it should be inside the loop or in the conditional section of the loop instead.
Best practice would be to use a while loop when the number of iterations is unknown ahead, like in this case, when it's determined by user's input.

C# input has to be 5 digits

I have a question that asks the user to enter a student number, how can I make it so it will only accept a 5 digit number. The input is being added to an object like
console.writeline("Enter the student number: ");
then
studentObject.StudentNumber = int.Parse(Console.Readline());
I've tried using
if (Console.ReadLine().Length != 5)
{
//Do this
}
else
{
//Do this
}
But it won't work, the .Length says can't convert type int to bool. I'm stuck, any help please?

You can use regular expressions:
String input;
do {
Console.WriteLine("Please enter student number:");
input = Console.ReadLine();
}
while (!Regex.IsMatch(input, #"^\d{5}$")); // <- five digits expected
// input contains 5 digit string
int number = int.Parse(number);
P.S. In case that the input should be "five digit number, not starting with zero" the regular expression has to be changed to something like that:
while (!Regex.IsMatch("12345", #"^[1-9]\d{4}$")); // five digits, not zero-starting

Probably not the answer to your question, however one thing I noticed:
if you are using Console.Readline() in your if-statement and then want to store it in the studentObject you will need to store it in a variable first. Calling Console.Readline(); again to store it in the studentObject will cause another input to be expected, which nullifies your attempt to validate the input.
Something like this:
static void Main(string[] args)
{
Console.WriteLine("Please enter student number:");
//get the user input
var number = Console.ReadLine();
if (number.Length != 5)
{
Console.WriteLine("Invalid format.");
}
else
{
Console.WriteLine("Yay it works");
}
Console.ReadLine();
}

var input = Console.ReadLine();
int i;
if (input.Length > 0 && input.Length < 6 && Int32.TryParse(input, out i))
// i has 5 digit;
else
// i has zero

char[] cc = Console.Read().ToString().ToCharArray();
if (char.IsDigit(cc[0])&&
char.IsDigit(cc[1])&&
char.IsDigit(cc[2])&&
char.IsDigit(cc[3])&&
char.IsDigit(cc[4]))
{
//Life's GOOD!
}else { //bad input}
Note: I'm self-taught & don't have much knowledge. Correct me if wrong.

Limit Number of Hints a Gamer can Use : Number Guessing Game in C#

I'm designing a Number Guessing Game, that reads a 4 Digit number from the Console.
The computer randomly generates the 4 digit number and the user tries to guess the digits and their correct order. The program returns answer in a format 0A0B or 1A3B or 4A0B e.t.c, where it returns A if the number you guessed in correct and it's position is correct too, and will return B if the number you guessed is correct but it's position is wrong.
I want to implement a hint system, which provides hints when the user types the word 'hint'. How do I limit the number of times a User can type the word 'hint' ?
This is the code I have so far
static void Main(string[] args)
{
while (true)
{
Console.WriteLine("********************************");
Console.WriteLine("What number do you think it is ?");
Console.WriteLine("********************************");
Console.WriteLine();
bool GameOver = false;
int[] targetNumber = GenerateRandomNumber(); // Generates the Random Number
while (!GameOver)
{
Console.Write("Answer: ");
string answer = Console.ReadLine(); // Gets an answer from User
if (answer.ToLower() == "hint") // Designed to Provide a hint.
{
int Rando = GenerateRandomNumberforHint(); // function I created
// Console.WriteLine("The Random Number generated was {0}",Rando);
if (Rando == 0)
{
Console.WriteLine("{0}XXX", targetNumber[Rando]);
}
else if (Rando == 1)
{
Console.WriteLine("X{0}XX", targetNumber[Rando]);
}
else if (Rando == 2)
{
Console.WriteLine("XX{0}X", targetNumber[Rando]);
}
else if (Rando == 3)
{
Console.WriteLine("XXX{0}", targetNumber[Rando]);
}
continue;
}
/* Code to check if the number is correct */
Console.ReadLine();
}
}
Do I need to create a Data Structure, and if I do, how do I implement that? I'm extremely new to C Sharp so any help would be appreciated

I think this is exactly what you want:)
int hitcounter = 0;
int lastHint = -1;
while (!GameOver)
{
Console.Write("Answer: ");
string answer = Console.ReadLine();
if (answer.ToLower() == "hint")
{
if (hitcounter < 10)
{
//preveintg double hint. something like that.
//you can use later array or list if you need more hint
int hint = generaterandomhints();
while (hint == lastHint)
{
hint = generaterandomhints();
}
hitcounter++;
//write the hints
}
else
{
//write you reached the maximum of the hints
}
}
}