input: Write a function to prompt the user to enter the value of radius and calculate the area of a circle. The function should return the calculated area. If the radius is not provided, the function should prompt the user again until a valid radius is entered.
  assert calculate_circle_area() == 78.5
  assert calculate_circle_area() == 153.86
  assert calculate_circle_area() == 0.0
output: def calculate_circle_area():
      while True:
          radius = input("Enter the value of radius: ")
          if radius.isdigit():
              radius = float(radius)
              return 3.14 * radius * radius
          else:
              print("Please enter a valid number.")