AP CSP Day 67: Robot Grid with Complex Conditionals | Cycle 3
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AP CSP robot questions place a robot on a grid using MOVE_FORWARD, ROTATE_LEFT, ROTATE_RIGHT, and CAN_MOVE. Complex robot problems combine conditionals with loops, requiring students to track both position and direction through multiple iterations.
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Tracing Robot Movement Systematically
Track Direction Separately
After ROTATE_LEFT: up becomes left, left becomes down, down becomes right, right becomes up. After ROTATE_RIGHT the reverse applies. The most common error is forgetting the current direction.
Paired Rotations Cancel Out
When ROTATE_LEFT and ROTATE_RIGHT appear together in a loop body, they cancel each other out. The robot ends each iteration facing the same direction it started, making the movement pattern consistent across iterations.
Practice Question
A robot is in a 6×6 grid at row 5, column 2, facing right. The following code is executed. Row 1 is the top, column 1 is the left.
REPEAT 3 TIMES
{
MOVE_FORWARD()
ROTATE_LEFT()
MOVE_FORWARD()
ROTATE_RIGHT()
}What is the robot’s final position and direction?
Start: (5,2) facing right. Iteration 1: MOVE_FORWARD → (5,3). ROTATE_LEFT → up. MOVE_FORWARD → (4,3). ROTATE_RIGHT → right. Iteration 2: MOVE_FORWARD → (4,4). ROTATE_LEFT → up. MOVE_FORWARD → (3,4). ROTATE_RIGHT → right. Iteration 3: MOVE_FORWARD → (3,5). ROTATE_LEFT → up. MOVE_FORWARD → (2,5). ROTATE_RIGHT → right. Final: row 2, column 5, facing right.
A) Wrong column and wrong direction. The ROTATE_RIGHT at the end of each iteration restores the original direction. C) Represents the state mid-iteration 3 (after ROTATE_LEFT and MOVE, before ROTATE_RIGHT). D) Correct direction but wrong column, likely from miscounting rightward moves.
Students forget that ROTATE_RIGHT undoes the ROTATE_LEFT within each iteration. They track the first rotation but miss the correction, believing the robot spirals rather than moving diagonally.
Draw the grid on scratch paper. Mark the robot’s position with an arrow showing its direction. Update both after every command.
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