$1.$ A function of two arguments that returns $1$ if the first argument is negative but the second argument otherwise.

$2.$ Write a function called calculate$\_$circle$\_$area that takes the radius of a circle as an argument and returns its area.

$3.$ Create a second function called calculate$\_$rectangle$\_$area that calculates the area of a rectangle given its length and width.

$4.$ Implement a function that converts temperature from Celsius to Fahrenheit and vice versa, and demonstrate its usage.