Added 'Functions'

Author: Saidev

09_Functions.md

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+In Python, functions are defined using the `def` keyword, followed by the function name, parameters (optional), and a block of code. Functions are called by using their name followed by parentheses.
+
+### 1. **Defining a Function**
+To define a function, you use the `def` keyword followed by the function name and any parameters inside parentheses. The code block that follows the function definition is indented.
+
+```python
+def greet():
+    print("Hello, welcome to Python!")
+```
+
+In this example, `greet` is a function that doesn't take any parameters and simply prints a greeting message.
+
+### 2. **Calling a Function**
+Once a function is defined, you can call it by writing its name followed by parentheses.
+
+```python
+greet()  # Calling the function
+```
+
+### Output:
+```
+Hello, welcome to Python!
+```
+
+### 3. **Function with Parameters**
+Functions can accept parameters, which allow you to pass values to the function when calling it.
+
+```python
+def greet(name):
+    print(f"Hello, {name}!")
+```
+
+In this case, the `greet` function takes one parameter `name`.
+
+### Calling the function with an argument:
+```python
+greet("Saidev")  # Passing the argument "Saidev"
+```
+
+### Output:
+```
+Hello, Saidev!
+```
+
+### 4. **Function with Return Value**
+You can use the `return` keyword in a function to return a value to the caller. The `return` statement exits the function and sends a result back.
+
+```python
+def add(a, b):
+    return a + b
+```
+
+### Calling the function and using the return value:
+```python
+result = add(3, 5)
+print(result)
+```
+
+### Output:
+```
+8
+```
+
+### 5. **Function with Default Parameters**
+You can define default values for function parameters. If a value is not passed when calling the function, the default value is used.
+
+```python
+def greet(name="Guest"):
+    print(f"Hello, {name}!")
+```
+
+### Calling the function:
+```python
+greet("Alice")  # Passing a value
+greet()         # Using the default value
+```
+
+### Output:
+```
+Hello, Alice!
+Hello, Guest!
+```
+
+### 6. **Variable Scope in Functions**
+Variables defined inside a function are local to that function, meaning they can't be accessed outside of it.
+
+```python
+x = 20
+
+def example():
+    x = 10  # Local variable
+    print(x) # 10
+
+print(x) # 20
+example()
+print(x) # 20
+```
+
+### Summary:
+- **Defining a function**: Use `def` followed by the function name and parameters.
+- **Calling a function**: Use the function name followed by parentheses, passing arguments if required.
+- **Return values**: Use `return` to send values back from the function.
+- **Default parameters**: You can set default values for parameters to use when no argument is passed.
+- **Local variables**: Variables inside a function cannot be accessed outside the function.
+
+Let me know if you need more examples or clarification!