A programmer is writing a game using a 3x3 grid. A square on the grid is referred to by a letter (A, B, C) and a number (1, 2, 3).

Figure 15 shows part of a Python program that checks the grid reference entered by a player.
The grid reference is valid if:

• there are exactly two characters
• the first character entered is A, B or C
• the second character entered is 1, 2 or 3.

Figure 15
python
check = False
while check == False:
square = ""
while len(square) != 2:
square = input("Enter grid reference (eg C2): ")
square = square.upper()
Extend the program from Figure 15 so it completes the other checks needed to make sure a valid grid reference is entered.

Your extended program must:

• use the variable check
• repeat the following steps until a valid grid reference is entered:
  • get the user to enter a grid reference
  • output an appropriate message if the grid reference is not valid.

This question requires extending an existing piece of code to perform full input validation. The goal is to create a loop that only terminates when the user enters a string that meets three criteria: correct length, valid first character, and valid second character. A good structure is a single `while` loop controlled by the `check` flag. Inside the loop, the program gets input and then uses a series of `if/else` statements to test the conditions. python # check is the flag that controls the loop check = False while check == False: # Get input on each iteration square = input("Enter grid reference (eg C2): ") square = square.upper() # First, check the length if len(square) == 2: # If length is OK, check the characters col = square[0] row = square[1] # Check if column and row are in the allowed sets if col in ["A", "B", "C"] and row in ["1", "2", "3"]: # All checks passed, set flag to True to exit loop check = True else: # Characters are invalid print("Invalid grid reference. Please try again.") else: # Length is invalid print("Input must be two characters long. Please try again.") This structure ensures that an error message is printed for the specific failure and the loop continues until all conditions are met simultaneously.