Add and subtract

Add and test the addition and subtraction methods.

Reading input with Console

In a C# console app you read user input with Console.ReadLine(). This method returns a string; to do numerical work you must convert that string to a number.

The double.Parse method converts a string to a double. For example, double.Parse("25") returns 25.0. If conversion fails it throws FormatException; we'll cover that in the final lesson.

Type conversion and error risk

double.Parse expects a valid number format. If the user enters a value like "hello", the method throws FormatException and program flow stops.

To catch these errors you use a try/catch block. You write the risky operation inside try and handle the error inside catch. This lesson only introduces the concept; the full pattern comes in the final lesson.

Exercise Read input and convert it to a number

Inside Main, declare a string variable and assign "25" (in a real app this value comes from Console.ReadLine()). Then convert it to double with double.Parse and print it with Console.WriteLine.

Show example solution

Convert the string with double.Parse and print with WriteLine.
Example solution:
using System;

class Calculator
{
    public double Add(double a, double b) { return 0; }
    public double Subtract(double a, double b) { return 0; }
    public double Multiply(double a, double b) { return 0; }
    public double Divide(double a, double b) { return 0; }
}

class Program
{
    static void Main()
    {
        // In a real app this comes from Console.ReadLine
        string input = "25";
        double number = double.Parse(input);
        Console.WriteLine(number);
    }
}

Test yourself

What type does Console.ReadLine() return?
- int
- double
- string
- char
Show answer

string — Correct; Console.ReadLine returns a string.

Convert the string to a number

string input = "25";
double number = ____;
Console.WriteLine(number);

Show answer

string input = "25";
double number = double.Parse(input);
Console.WriteLine(number);

Match each method or concept to its meaning

Console.ReadLine

Reads a line from the user
double.Parse

Converts a string to a number
Console.WriteLine

Writes text to the screen
FormatException

Invalid number format

Next lesson: Multiply and divide

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