Add 01_Variables section

Author: C6OI

Resources/Exercises/01_Variables/README.md

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+# Variables
+
+When you declare a variable or constant in a program, you must either specify its type or use the var keyword to let the compiler infer the type.  
+After you declare a variable, you can't redeclare it with a new type, and you can't assign a value not compatible with its declared type.  
+When you declare a constant, you must specify its type and value, and once you have defined it, the value cannot be mutated.

Resources/Exercises/01_Variables/Variables1.cs

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+using System;
+
+class Variables1 {
+    public static void Main() {
+        // TODO: Add the missing keyword.
+        x = 5;
+
+        Console.WriteLine($"x has the value {x}");
+    }
+}

Resources/Exercises/01_Variables/Variables2.cs

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+using System;
+
+class Variables2 {
+    public static void Main() {
+        // TODO: Change the line below to fix the compiler error.
+        var x;
+
+        if (x == 10) {
+            Console.WriteLine("x is ten!");
+        } else {
+            Console.WriteLine("x is not ten!");
+        }
+    }
+}

Resources/Exercises/01_Variables/Variables3.cs

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+using System;
+
+class Variables3 {
+    public static void Main() {
+        // TODO: Change the line below to fix the compiler error.
+        int x;
+
+        Console.WriteLine($"Number {x}");
+    }
+}

Resources/Exercises/01_Variables/Variables4.cs

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+using System;
+
+class Variables4 {
+    public static void Main() {
+        // TODO: Change the line below to fix the compiler error.
+        const x = 42;
+
+        Console.WriteLine($"Constant {x}");
+    }
+}

Resources/Exercises/01_Variables/Variables5.cs

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+using System;
+
+class Variables5 {
+    public static void Main() {
+        // TODO: Change the line below to fix the compiler error.
+        int x = 34;
+        const int y = x + 8;
+
+        Console.WriteLine($"Number {y}");
+    }
+}

Resources/Exercises/01_Variables/Variables6.cs

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+using System;
+
+class Variables6 {
+    public static void Main() {
+        // TODO: Fix the constant initialization.
+        const int x;
+        x = 42;
+
+        Console.WriteLine($"Number {x}");
+    }
+}

Resources/Solutions/01_Variables/Variables1.cs

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+using System;
+
+class Variables1 {
+    public static void Main() {
+        // Declaring variables requires specifying the type...
+        int x = 5;
+        // ...or using the `var` keyword.
+        // var x = 5;
+
+        Console.WriteLine($"x has the value {x}");
+    }
+}

Resources/Solutions/01_Variables/Variables2.cs

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+using System;
+
+class Variables2 {
+    public static void Main() {
+        // The easiest way to fix the compiler error is to initialize the
+        // variable `x`. By setting its value to an integer, C# infers its type
+        // as `int` which is the default type for integers.
+        var x = 42;
+
+        // But we can enforce a type different from the default `int` by
+        // specifying the type explicitly:
+        // ushort x = 42;
+
+        if (x == 10) {
+            Console.WriteLine($"x is ten!");
+        } else {
+            Console.WriteLine($"x is not ten!");
+        }
+    }
+}

Resources/Solutions/01_Variables/Variables3.cs

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+using System;
+
+class Variables3 {
+    public static void Main() {
+        // Reading uninitialized variables isn't allowed in C#!
+        // Therefore, we need to assign a value first.
+        int x = 42;
+
+        Console.WriteLine($"Number {x}");
+
+        // It is possible to declare a variable and initialize it later.
+        // But it can't be used before initialization.
+        int y;
+        y = 42;
+        Console.WriteLine($"Number {y}");
+    }
+}