Answers

2.3 The depth grid

Sensing · Robot club · about 20 min

What this lesson is about

64 readings, 8 columns: seeing left and right, not just ahead.

• tof_grid()
• Seeing it as a picture
• Columns are directions
• Left and right

Questions 7 marks in all

1. [1 mark]The grid is 8 rows of 8, and a reading is at grid[row * 8 + col]. At what position in the list is row 2, column 5?

   Answer: 21. Rows and columns both count from 0. Row 2 starts at 2 times 8, which is 16, and column 5 is 5 along from there: 21.

2. [1 mark]Which rows of the grid look level, straight ahead?

   1. ARows 2 and 3
   2. BRows 0 and 1
   3. CRows 6 and 7
   4. DAll eight rows
   Answer: A. Row 0 is the top of the view and looks over most things. The bottom rows see the mat. Rows 2 and 3 are the ones to read.

3. [1 mark]Why are the bottom rows of the grid always close readings?

   1. AThe sensor sits 3 cm above the mat, so the bottom rows see the mat a few centimetres ahead
   2. BThere is always a wall in front
   3. CThe bottom rows are broken
   4. DThe robot is always driving uphill
   Answer: A. The view tilts down at the bottom, and the mat is right there, so those readings are always short.

4. [1 mark]Which direction does column 0 look?

   1. AAbout 20 degrees to the left
   2. BAbout 20 degrees to the right
   3. CStraight ahead
   4. DStraight down at the mat
   Answer: A. Columns go left to right: 0 looks about 20 degrees left, 7 about 20 degrees right, and 3 and 4 straight ahead.

5. [1 mark]What does this program print?

   row = [30, 90, 60, 20]
   line = ""
   for d in row:
       line += "#" if d < 40 else ("+" if d < 80 else ".")
   print(line)

   Answer:

   #.+#

   Under 40 is #, under 80 is +, and anything further is .. 30 and 20 are close, 60 is medium and 90 is far.

6. [1 mark]What does this program print?

   left_side = 45
   right_side = 80
   if left_side < right_side:
       print("more room on the right")
   else:
       print("more room on the left")

   Answer:

   more room on the right

   The left reading is smaller, so things are closer on the left, and there is more room on the right.

7. [1 mark]The level-row readings for columns 0 to 7 are 35, 38, 90, 120, 110, 40, 33, 30. Which way is the way through?

   1. AColumn 3, about straight ahead
   2. BColumn 0, the left edge
   3. CColumn 7, the right edge
   4. DColumn 5
   Answer: A. The largest reading, 120, is in column 3. The biggest distance is where there is most room to drive.

The task: widest gap

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# 64 distances, 8 rows of 8
grid = tof_grid()
best_col = 0
best = 0
# find the column whose reading in the level rows is the largest
print("clear: column", best_col)

from bugbot import *
connect()
grid = tof_grid()
best_col = 0
best = 0
for col in range(8):
    d = min(grid[row * 8 + col] for row in (2, 3))   # the level rows
    if d > best:
        best = d
        best_col = col
print('clear: column', best_col)