Special moves (castling, en passant) in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if castling is allowed by verifying the king's and rook's {{BLANK_1}} status.

LLD

if not king.[1] and not rook.[1]:
    allow_castling = True

Drag options to blanks, or click blank then click option'

Ais_attacked

Bcan_move

Cis_checked

Dhas_moved

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'has_moved' with 'can_move' or 'is_checked'.

Castling is only allowed if neither the king nor the rook has moved before.

2fill in blank

medium

Complete the code to detect if en passant capture is possible by checking the opponent pawn's {{BLANK_1}} move.

LLD

if opponent_pawn.last_move == [1]:
    allow_en_passant = True

Drag options to blanks, or click blank then click option'

Asingle_step

Bdouble_step

Ccapture_move

Dcastle_move

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'single_step' or other move types instead of 'double_step'.

En passant is only possible if the opponent pawn just moved two squares forward in one move.

3fill in blank

hard

Fix the error in the castling validation by correctly checking if the squares between king and rook are {{BLANK_1}}.

LLD

for square in squares_between:
    if square.[1]:
        return False

Drag options to blanks, or click blank then click option'

Ais_occupied

Bis_empty

Cis_attacked

Dis_safe

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if squares are empty instead of occupied.

Castling is not allowed if any square between the king and rook is occupied.

4fill in blank

hard

Fill both blanks to correctly update the board after an en passant move.

LLD

board.move_piece(pawn, [1])
board.remove_piece([2])

Drag options to blanks, or click blank then click option'

Atarget_square

Bcaptured_pawn_square

Cpawn.position

Den_passant_capture_square

Attempts:

3 left

💡 Hint

Common Mistakes

Removing the wrong piece or moving the pawn incorrectly.

The pawn moves to the target square, and the captured pawn is removed from its square.

5fill in blank

hard

Fill all three blanks to implement castling: move the king, move the rook, and update their {{BLANK_1}}, {{BLANK_2}}, and {{BLANK_3}} status.

LLD

board.move_piece(king, [1])
board.move_piece(rook, [2])
king.[3] = True
rook.[3] = True

Drag options to blanks, or click blank then click option'

Aking_target_square

Brook_target_square

Chas_moved

Dcan_castle

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong target squares or wrong status properties.

Castling moves the king and rook to their target squares and marks both as having moved.

Practice

1. Which of the following conditions must be true for castling to be allowed in a chess game?

easy

A. The king is currently in check and moves two squares towards the rook.

B. Neither the king nor the rook involved has moved before, and no pieces are between them.

C. The rook has moved once, but the king has not moved.

D. The king moves diagonally two squares towards the rook.

Solution

1. Step 1: Understand castling rules

   Castling requires that neither the king nor the rook involved has moved before, and the squares between them are empty.

2. Step 2: Check king safety conditions

   The king cannot be in check, nor can it pass through or land on a square under attack during castling.

3. Final Answer:

   Neither the king nor the rook involved has moved before, and no pieces are between them. -> Option B

4. Quick Check:

   Castling conditions = Neither the king nor the rook involved has moved before, and no pieces are between them. [OK]

Hint: Castling needs unmoved king and rook with clear path [OK]

Common Mistakes:

• Allowing castling when king is in check
• Ignoring if rook has moved
• Allowing king to move diagonally during castling

2. Which code snippet correctly checks if an en passant move is possible in a chess game for an opponent pawn landing to the right?

easy

A. if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (1, 0): allow_en_passant()

B. if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (0, -1): allow_en_passant()

C. if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (0, 1): allow_en_passant()

D. if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (-1, 0): allow_en_passant()

Solution

1. Step 1: Understand en passant position logic

   En passant capture happens when an opponent's pawn moves two squares forward and lands beside your pawn horizontally.

2. Step 2: Identify correct position offset

   The opponent pawn must be exactly one square horizontally adjacent (x+1 or x-1), so position + (1, 0) is correct for right side.

3. Final Answer:

   if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (1, 0): allow_en_passant() -> Option A

4. Quick Check:

   En passant horizontal check = if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (1, 0): allow_en_passant() [OK]

Hint: En passant checks horizontal adjacency after two-step pawn move [OK]

Common Mistakes:

• Checking vertical instead of horizontal adjacency
• Using wrong coordinate offsets
• Ignoring the two-square move condition

3. Given this simplified code snippet for castling validation, what will be the output if the king has moved before?

def can_castle(king_moved, rook_moved, path_clear):
    if king_moved or rook_moved:
        return False
    if not path_clear:
        return False
    return True

print(can_castle(True, False, True))

medium

A. None

B. True

C. False

D. Error

Solution

1. Step 1: Analyze input parameters

   king_moved is True, rook_moved is False, path_clear is True.

2. Step 2: Follow function logic

   Since king_moved is True, the first if condition triggers and returns False immediately.

3. Final Answer:

   False -> Option C

4. Quick Check:

   King moved disables castling = False [OK]

Hint: If king moved, castling returns False immediately [OK]

Common Mistakes:

• Ignoring king_moved condition
• Assuming path_clear overrides king_moved
• Expecting True despite king having moved

4. Identify the bug in this en passant validation code snippet:

def can_en_passant(pawn_pos, opponent_pawn_pos, last_move):
    if last_move == 'two_squares_forward' and abs(pawn_pos[0] - opponent_pawn_pos[0]) == 1:
        return True
    return False

# Example call
print(can_en_passant((4,4), (5,4), 'two_squares_forward'))

medium

A. The function should return False when pawns are adjacent.

B. The function incorrectly compares x-coordinates instead of y-coordinates.

C. The last_move parameter should be a boolean, not a string.

D. The function does not check if pawns are on the same rank (y-coordinate).

Solution

1. Step 1: Understand en passant position requirements

   En passant requires pawns to be on the same rank (same y-coordinate) and adjacent files (x-coordinates differ by 1).

2. Step 2: Analyze code logic

   The code checks x-coordinate difference but does not verify if y-coordinates are equal, missing a key condition.

3. Final Answer:

   The function does not check if pawns are on the same rank (y-coordinate). -> Option D

4. Quick Check:

   Missing same rank check = The function does not check if pawns are on the same rank (y-coordinate). [OK]

Hint: En passant needs same rank check besides adjacency [OK]

Common Mistakes:

• Ignoring y-coordinate equality
• Assuming x difference alone suffices
• Misusing last_move parameter type

5. You are designing a chess game system that supports castling and en passant. Which design approach best ensures correct validation of these special moves while keeping the system scalable?

hard

A. Track each piece's move history and board state; validate special moves by checking move history and current board conditions.

B. Only check the current board state without tracking move history, assuming special moves are rare.

C. Allow special moves without validation to simplify the system and fix errors later.

D. Hardcode special move rules without tracking piece movement or board state.

Solution

1. Step 1: Understand requirements for special moves

   Castling and en passant depend on move history (e.g., whether king or rook moved, or if pawn moved two squares last turn) and current board state.

2. Step 2: Evaluate design options

   Tracking move history and board state allows accurate validation and supports scalability as game complexity grows.

3. Final Answer:

   Track each piece's move history and board state; validate special moves by checking move history and current board conditions. -> Option A

4. Quick Check:

   Move history + board state = correct scalable validation [OK]

Hint: Track moves and board state for reliable special move validation [OK]

Common Mistakes:

• Ignoring move history for special moves
• Hardcoding rules without flexibility
• Skipping validation to simplify design