Lists of readings

Lists, len, min and max, slices and looping over the 64 readings of the depth grid.

0.6Python quick startRobot club12 min

Do this lesson in the simulator

Print it: worksheet (PDF) · answers (PDF) · as a web page

distance() is one number: what is straight ahead. The robot's depth sensor sees far more than that, and it hands it over as a list.

A list

from bugbot import *
connect()

# a list is several values in one name, in order
speeds = [20, 40, 60]
print(speeds)
print("the first one is", speeds[0])
print("how many:", len(speeds))

Run this in the simulator

Counting starts at 0, so speeds[0] is the first and speeds[2] is the last of three. len() says how many there are.

The depth grid

from bugbot import *
connect()

grid = tof_grid()
print("readings:", len(grid))
print("nearest thing:", round(min(grid), 1), "cm")
print("furthest:", round(max(grid), 1), "cm")

Run this in the simulator

tof_grid() gives 64 readings: an 8 by 8 square of distances in whole centimetres, left to right, top to bottom. min() and max() pick out the smallest and largest of any list.

With nothing on the mat this says the nearest is 8 cm and the furthest 400 cm. The sensor's bottom rows look down at the mat just in front of the robot, so they always see something close. The top rows look up over everything, and 400 is the most it ever reports: it means nothing there.

Going through a list

from bugbot import *
connect()

grid = tof_grid()
# row 3 (the fourth row), which looks straight ahead
row = grid[24:32]
for value in row:
    print(round(value, 1))

Run this in the simulator

for value in row: runs the indented lines once for each reading, with value being that reading. grid[24:32] is a slice: the readings from 24 up to but not including 32. On the empty mat they run 85, 83, 81, 81, 81, 81, 83, 85: the edge of the mat ahead, a little further away at the sides because those readings look across at an angle.

Counting how many readings meet a test is the same pattern with an if inside.

from bugbot import *
connect()

close = 0
for value in tof_grid():
    if value < 40:
        close = close + 1
print(close, "of the 64 readings are under 40 cm")

Run this in the simulator

On the empty mat that prints 32: the four bottom rows, all looking at the mat.

Task: how near is the nearest

There is a box on the mat in front of the robot. Print one line saying how far away it is, like nearest: 27, worked out with min() from tof_grid() rather than from distance(). The smallest of all 64 readings is the mat under the robot's nose, 8 cm, so only look at the rows that see straight ahead.

from bugbot import *
connect()

grid = tof_grid()
# your line here

Challenges

  1. Print the furthest reading as well, on its own line.
  2. Print how many readings are under 30 cm.
  3. Turn the robot 45 degrees, take the readings again, and see which way is clearest.

Where next

That is the Python this club needs. Module 1 starts on the robot itself: driving well, driving exactly, and why it never goes quite straight.