Develop Reference

A problem with getting/printing answers in a C# calculator

I'm trying to make simple projects for learning C# and have tried to make a simple console calculator. I have only found this current error when getting to the getting/printing the answer bit when test-running my program, so I have no idea if there are any other errors/things that will or may not work or run properly/as intended. So if there are any of those, please let me know and if you want to you can fix them yourself. It only recognized the error when it reached that specific line of code, and otherwise will run the program until it reaches the error.
Code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Calculator
{
class Program
{
static void Main(string[] args)
{
string num1;
string num2;
string condition;
string answer;
Console.WriteLine("Calculator");
Console.WriteLine("For division, use /. For multiplication, use *.\n");
while (true)
{
Console.WriteLine("Enter a number: "); // gets first number to add in problem
num1 = Console.ReadLine();
Console.WriteLine("Enter a condition: "); // gets condition to add in problem
condition = Console.ReadLine();
Console.WriteLine("Enter your second number: "); // gets second number to add in problem
num2 = Console.ReadLine();
Console.WriteLine("Calculating..");
// converting strings to int and working out answer
Convert.ToInt32(num1);
Convert.ToInt32(num2);
// error is from here on (not sure if the Convert.ToInt32() code above causes errors)
answer = num1 + condition + num2;
Convert.ToInt32(answer);
Console.WriteLine(answer);
// sets values to null after getting & printing answer (probably unnessessary)
answer = null;
num1 = null;
num2 = null;
condition = null;
}
}
}
}

When facing problems like this one - the routine is too complex to be tested:
if there are any other errors/things that will or may not work"
split routine into smaller ones: start extracting methods.
// Get integer value from user
public static int ReadInteger(string title) {
while (true) {
if (!string.IsNullOrEmpty(title))
Console.WriteLine(title);
if (int.TryParse(Console.ReadLine(), out int result))
return result;
Console.WriteLine("Sorry, not a valid integer value, please, try again.");
}
}
// Get character operator ('+', '-' etc.) from user
public static char ReadOperator(string title, string operators) {
while (true) {
if (!string.IsNullOrEmpty(title))
Console.WriteLine(title);
string input = Console.ReadLine().Trim();
if (input.Length == 1 && operators.Contains(input[0]))
return input[0];
Console.WriteLine("Sorry, not a valid operator, please, try again.");
}
}
Now we are ready to implement Main method:
static void Main(string[] args) {
while (true) {
int num1 = ReadInteger("Enter a number: ");
char op = ReadOperator("Enter a condition: ", "+-*/");
int num2 = ReadInteger("Enter your second number: ");
//TODO: I've skipped error handling (zero division, overflow)
int answer =
op == '+' ? num1 + num2 :
op == '-' ? num1 - num2 :
op == '*' ? num1 * num2 :
op == '/' ? num1 / num2 : 0;
Console.WriteLine($"{num1} {op} {num2} = {answer}");
//TODO: it's a right place here to ask user if (s)he wants to continue
Console.WriteLine();
}
}

You receive input (condition) as a string
cant do this : answer = num1 + condition + num2 because those variables are
string
you have to check that with switch , for example :
int num1 = 0, num2 = 0, answer = 0;
string condition;
Console.WriteLine("Calculator");
Console.WriteLine("For division, use /. For multiplication, use *.\n");
while (true)
{
Console.WriteLine("Enter a number: "); // gets first number to add in problem
num1 = int.Parse(Console.ReadLine());
Console.WriteLine("Enter a condition: "); // gets condition to add in problem
condition = Console.ReadLine();
Console.WriteLine("Enter your second number: "); // gets second number to add in problem
num2 = int.Parse(Console.ReadLine());
Console.WriteLine("Calculating..");
// converting strings to int and working out answer
Convert.ToInt32(num1);
Convert.ToInt32(num2);
// error is from here on (not sure if the Convert.ToInt32() code above causes errors)
switch (condition)
{
case "/":
answer = num1 / num2;
break;
case "*":
answer = num1 * num2;
break;
case "+":
answer = num1 + num2;
break;
case "-":
answer = num1 - num2;
break;
default:
Console.WriteLine("error : unknown operator");
break;
}
Console.WriteLine(answer);
// sets values to null after getting & printing answer (probably unnessessary)
}

You can't do this line because all the variables are strings and not the real values (e.g: num1 & num2 should ints/doubles... and condition should be a real operator).
answer = num1 + condition + num2;
Also, you can't do it because let's assume the user inputs the multiplication sign, and then the string of "*" is not equal to the sign of *.
Instead, you can do some switch-case statements to check the value of the condition variable. Also, you need to make sure num1 & num2 are numbers (you can parse/try-parse them to a number (int/double...)). You will also have to parse them to another variables as they are strings.
switch (condition)
{
case '*':
answer = numX * numY;
break;
case "/":
// Validate that numY is not 0 to avoid [DivideByZeroException][1]
answer = numX / numY;
break;
...
}
Note, it is just one way to do it and I just gave you a small example that might help you to continue.
I would also offer you to make a default case (in case that the input of the user is not something you expect, in this case, you may ask him again to write an input because the current input is invalid).

Display Console.ReadLine() in switch statement

I'm building console applications to sharpen my c# skills. I'm having two issues that I can't seem to find online.
Issue 1: I want to display whatever the user inputs in the output (user enters a numeric value.. when the program runs and displays an answer, I want the user input to be displayed as well)..
Issue 2: Currently you have to press the enter key twice to continue after the first input. Is there a way to only have to press the enter key once?
Thanks in advance!
Here is the existing code (I'll include both classes just incase you need the second for some reason):
namespace StaticDemo {
class Program
{
static void Main(string[] args)
{
string selection = string.Empty;
while (selection != "q" && selection != "Q")
{
Console.Write("Enter C)elsius to Fahrentheit or F(ahrenheit to Celsius or Q(uit. \nSelect C, F, or Q then press the enter key twice to continue: ");
selection = Console.ReadLine();
double fahrenheit = 0, celsius = 0;
string line = Console.ReadLine();
switch (selection)
{
case "C":
case "c":
Console.Write("Please enter the Celsius temperature that you want converted to a Fahrenheit temperature: ");
fahrenheit = TemperatureConverter.CelsiusToFahrenheit(Console.ReadLine());
Console.WriteLine($"The temperature you provided in celsius: {line}, would be: {fahrenheit} in fahrentheit");
break;
case "F":
case "f":
Console.Write("Please enter the Farentheit temperature that you want converted to a Celsius temperature: ");
celsius = TemperatureConverter.CelsiusToFahrenheit(Console.ReadLine());
Console.WriteLine($"The temperature you provided in Fahrentheit: {line}, would be: {celsius} in celsius");
break;
case "Q":
case "q":
break;
default:
Console.WriteLine("Enter C F or a Q, Moron!");
break;
}
Console.WriteLine();
Console.WriteLine();
}
}
} }
The second class code is below if you need it for whatever reason:
namespace StaticDemo
{
class TemperatureConverter
{
public static double CelsiusToFahrenheit(string tempCelsius)
{
double celcius = double.Parse(tempCelsius);
double fahrenheit = (celcius * 9 / 5) + 32;
return fahrenheit;
}
public static double FahrenheitToCelsius(string tempFahrenheit)
{
double fahrenheit = double.Parse(tempFahrenheit);
double celsius = (fahrenheit - 32) * 5 / 9;
return celsius;
}
}
}

This line is useless and it is the reason for the two input required to process the choice
string line = Console.ReadLine();
You have already read the selection variable two lines up.
To get the value for the temperature provided by the user, you need to get the input without passing it directly to the conversion class
Console.Write("Please enter the Celsius temperature that you want converted to a Fahrenheit temperature: ");
string input = Console.ReadLine();
fahrenheit = TemperatureConverter.CelsiusToFahrenheit(input);
Console.WriteLine($"The temperature you provided in Celsius: {input}, would be: {celsius} in Fahrenheit");
Of course the same applies to the other block of code that converts a Fahrenheit to Celsius. Notice also that I think you have inverted the message.

You are asking for the user input twice basically.
Console.Write("Enter C)elsius to Fahrentheit or F(ahrenheit to Celsius or Q(uit. \nSelect C, F, or Q then press the enter key twice to continue: ");
selection = Console.ReadLine().ToUpper();
Console.WriteLine("Please enter the temperature that you want converted: ");
string tempToConvert = Console.ReadLine();
switch (selection)
{
case "C":
fahrenheit = TemperatureConverter.CelsiusToFahrenheit(Console.ReadLine());
Console.WriteLine($"The temperature you provided in celsius: {tempToConvert }, would be: {fahrenheit} in fahrentheit");
break;
Consider using ToUpper or ToLower on user inputs so you don't have to worry about the formatting of the input when you make your cases.

It is because of line
string line = Console.ReadLine();
Change it to
string line;
I think you previously wanted that line to read value to convert.

Change the line
selection = Console.ReadLine();
to
selection = Convert.ToString(Console.ReadKey());
also i think after making the above change you do not need this line.
string line = Console.ReadLine();

Float/Double type input validation in C#

This is literally my first program I've ever written (started learning this past Monday); I am a total newbie.
My question is, how can I prevent exceptions from being thrown when a user enters an invalid character when the program prompts the user for fahreinheit or celsius entry (expecting a number)??? So for example, when a user enters "asfasd", the program throws an exception.
I did a lot of searching on the site before posting this, and I was successfully able to find other input validation questions, however, they were all concerning C and C++ and since I am such a newbie, I have a hard time with understanding those languages and how they relate to C#. Thank you. Please see code:
using System;
namespace Converter
{
class Program
{
static void Main()
{
float? FahrenheitInput = null;
double? CelsiusInput = null;
float? KilogramInput = null;
float? PoundsInput = null;
int UserChoice = 0;
do
{
Console.WriteLine("What would you like to convert? Enter the corresponding number.\n1. Fahrenheit to Celsius");
Console.WriteLine("2. Celsius to Fahrenheit\n3. Pounds to Kilograms\n4. Kilograms to pounds\n5. Exit program");
UserChoice = int.Parse(Console.ReadLine());
switch (UserChoice)
{
case 1:
Console.WriteLine("Enter the temperature in Fahreinheit, number only:");
FahrenheitInput = float.Parse(Console.ReadLine());
Console.Clear();
Console.WriteLine(FahrenheitInput + " degrees fahrenheit in Celsius is " + Program.FahrenheitToCelsius(FahrenheitInput) + "\n\n");
break;
case 2:
Console.WriteLine("Enter the temperature in Celsius, number only:");
CelsiusInput = double.Parse(Console.ReadLine());
Console.Clear();
Console.WriteLine(CelsiusInput + " degrees Celius in fahrenheit is " + Program.CelsiusToFahrenheit(CelsiusInput) + "\n\n");
break;
case 5:
break;
default:
Console.WriteLine("This is not a valid entry. Please enter 1, 2, 3, 4, or 5.");
break;
}
} while (UserChoice != 5);
}
public static float? FahrenheitToCelsius(float? INPUT)
{
return (INPUT - 32) * 5 / 9;
}
public static double? CelsiusToFahrenheit(double? INPUT)
{
return INPUT * 1.8 + 32;
}
}
}

You can either put it in Try-Catch block or use a while loop to validate the user input.
below is your code with a while loop which validates users input.
class Program
{
static void Main(string[] args)
{
double FahrenheitInput = 0;
double CelsiusInput = 0;
double KilogramInput = 0;
double PoundsInput = 0;
int UserChoice = 0;
do
{
Console.WriteLine("What would you like to convert? Enter the corresponding number.\n1. Fahrenheit to Celsius");
Console.WriteLine("2. Celsius to Fahrenheit\n3. Pounds to Kilograms\n4. Kilograms to pounds\n5. Exit program");
UserChoice = int.Parse(Console.ReadLine());
switch (UserChoice)
{
case 1:
Console.WriteLine("Enter the temperature in Fahreinheit, number only:");
while (!double.TryParse(Console.ReadLine(), out FahrenheitInput))
{
Console.WriteLine("Invalid format, please input again!");
};
Console.Clear();
Console.WriteLine(FahrenheitInput + " degrees fahrenheit in Celsius is " + Program.FahrenheitToCelsius(FahrenheitInput) + "\n\n");
break;
case 2:
Console.WriteLine("Enter the temperature in Celsius, number only:");
while (!double.TryParse(Console.ReadLine(), out CelsiusInput))
{
Console.WriteLine("Invalid format, please input again!");
};
Console.Clear();
Console.WriteLine(CelsiusInput + " degrees Celius in fahrenheit is " + Program.CelsiusToFahrenheit(CelsiusInput) + "\n\n");
break;
case 5:
break;
default:
Console.WriteLine("This is not a valid entry. Please enter 1, 2, 3, 4, or 5.");
break;
}
} while (UserChoice != 5);
}
public static double FahrenheitToCelsius(double INPUT)
{
return (INPUT - 32) * 5 / 9;
}
public static double CelsiusToFahrenheit(double INPUT)
{
return INPUT * 1.8 + 32;
}
}

TryParse is your good friend here. In most scenarios, you should favor using TryParse than Parse. In your example, you can do something like:
int validInt;
int.TryParse(Console.ReadLine(), out validInt);
float validFloat;
float.TryParse(Console.ReadLine(), out validFloat);
Parse vs. TryParse

The easiest way, IMHO, to change the routine is to rewrite Parse into corresponding TryParse:
// UserChoice = int.Parse(Console.ReadLine());
UserChoice = int.TryParse(Console.ReadLine(), out UserChoice) ? UserChoice : -1;
...
A bit more complex (you have to convert float into float?)
// FahrenheitInput = float.Parse(Console.ReadLine());
float v;
FahrenheitInput = float.TryParse(Console.ReadLine(), out v) ? (float?) v : null;
The same scheme for CelsiusInput
// CelsiusInput = double.Parse(Console.ReadLine());
double d;
CelsiusInput = double.TryParse(Console.ReadLine(), out v) d (double?) d : null;
The underlying mechanic of the code is
We try to parse user input TryParse(Console.ReadLine()...
If parse succeeds (and thus TryParse returns true) we just return the out (parsed value).
If parse fails (and thus TryParse returns false) we return some special a value (-1 for UserChoice or null in case of FahrenheitInput or CelsiusInput)
P.S. in the first switch you have just case 1, case 2 and case 5; however, you've put "This is not a valid entry. Please enter 1, 2, 3, 4, or 5." in the error message. It seems, that you have to either implement case 3 and case 4 in the switch or edit the error message.

Use int.TryParse instead of int.Parse and float.tryParse instead of float.Parse

While all the answers provided seem to work, the question you asked was
how can I prevent exceptions from being thrown [..]
and I just want to point out that you do this by putting the part which throws the exception in an try-catch-block. What this does is it executes the code within try until an exception is beeing thrown and then passes this exceptions as a parameter to the catch-part where you can handle it:
EXAMPLE
do
{
try
{
// the code you already have
}
catch (Exception ex)
{
Console.WriteLine("This is no valid input (" + ex.Message + ")! Try again...");
}
} while (UserChoice != 5);
Of course preventing exceptions from beeing thrown at all in the first place is the better way (as all the other answers do suggest), but this approach works as well and ist more generic in case you run into a similar problem in the future. Using switch-case-statements for error-handling is quite common practice...

My Loop is not working - it exits before it should

I'm new to programming. I tried this code and the loop is not working. The console application just ends after one operation, and I would like to continue doing more until I choose to say "no." Where am I making a mistake here? I want the program to keep going until I say so. Thanks in advance.
static void Main(string[] args)
{
Console.WriteLine("Program to convert USA Dollars to five different currencies,");
Console.WriteLine();
double dollars, euros, turkisLira, yen, britishPounds, mexicanPeso;
char option;
euros = 0.72;
turkisLira = 2.09;
yen = 101.73;
britishPounds = 0.59;
mexicanPeso = 13.03;
string answer = "Y";
do
{
Console.Write("Enter the Amount of USA Dollars to Convert:");
dollars = Convert.ToInt32(Console.ReadLine());
Console.WriteLine("1.Convert to Euros");
Console.WriteLine("2.Convert to Turkis Lira");
Console.WriteLine("3.Convert to Japanes Yen");
Console.WriteLine("4.Convert to British Pounds");
Console.WriteLine("5.Convert to Mexican Pesos");
Console.Write("Enter the option of Currency you wish to convert : ");
option = Convert.ToChar(Console.ReadLine());
switch (option)
{
case '1':
euros = dollars * euros;
Console.WriteLine("The amount of Euros given is : {0}", euros);
break;
case '2':
turkisLira = dollars * turkisLira;
Console.WriteLine("The amount of Turkis Liras given is : {0}", turkisLira);
break;
case '3':
yen = dollars * yen;
Console.WriteLine("The amount of Japanes Yen given is : {0}", yen);
break;
case '4':
britishPounds = dollars * britishPounds;
Console.WriteLine("The amount of British Pounds given is : {0}", britishPounds);
break;
case '5':
mexicanPeso = dollars * mexicanPeso;
Console.WriteLine("The amount of Mexican Pesos given is : {0}", mexicanPeso);
break;
}
Console.WriteLine(" Do you wish to do more conversions? (yes/no)");
answer = Console.ReadLine();
if (answer.Equals("Y"))
{
Console.WriteLine("Yes");
}
else if (answer.Equals("N"))
{
Console.WriteLine("No");
}
} while (answer.ToLower() == "y'");

Your requirement is
I will like to continue doing more until I choose to say No
while your exist condition is
(answer.ToLower() == "y'");
So, if somebody clicks anything other than y, the loop would exit. You would want the exist condition to be
(answer.ToLower() != "n'");-

I think you have miss type ' in "y'" where you just want receive "y" to continue.

A method using Boolean expression

The user has to pick 2 number to 1) Add 2) Subtract 3) Multiply 4) Divide 5) Quit.
The 2 number that the user entry can be 0-9, a decimal point, or minus sign, and anything else is a error.I need to create a method to check the user entry , by using a true or false Boolean expression. what do I put in the Boolean expression
Console.Write("Enter Number 1: ", num1);
num1 = Convert.ToDouble(Console.ReadLine());
Console.Write("Enter Number 2: ", num2);
num2 = Convert.ToDouble(Console.ReadLine());
if (true)
{
}
switch (input)
{
case 1:
Console.WriteLine("\tResults: {0}", Add(num1, num2));
break;
case 2:
Console.WriteLine("\tResults: {0}", Subtract(num1, num2));
break;
case 3:
Console.WriteLine("\tResults: {0}", Multiply(num1, num2));
break;
case 4:
Console.WriteLine("\tResults: {0}", Divide(num1, num2));
break;
default:
//Console.WriteLine("Invalid Menu Selection.\t Try Again");
//Console.ReadLine();
return;
}
Console.WriteLine("Press any key...");
Console.ReadKey();
Console.Clear();

If I understand this correctly, you can use Double.TryParse() to check whether user input is a valid convertible to double string, as well as actually converting it (if the input is valid). For example :
double num1 = 0;
bool isNum1Valid = Double.TryParse(Console.ReadLine(), out num1);
Actually, this question need more context, for example, how you want to use that function and it's signature (parameter type and return type). and showing what have you tried so far.

Check that the input is only numbers:
Regex r = new Regex(#"^[-.]?[\d]{1,}[.]{0,1}[\d]{0,}$");
if(r.IsMatch(num1) && r.IsMatch(num2) && num1.Count(j => j == '.') < 2 && num2.Count(j => j == '.') < 2)
{
//Do something
}
And if both numbers are only nums, then use your switch statement.
If the input contains letters, then show some message to the user telling that only numbers are available