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0.6 Lists of readings

Python quick start · Robot club · about 12 min

BugBotLab

What this lesson is about

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

Questions 7 marks in all

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

    speeds = [20, 40, 60]
print(speeds[0])
print(len(speeds))
    Answer:
    20
3

    Counting starts at 0, so speeds[0] is the first item, 20. len() counts how many items there are: 3.

  2. [1 mark]speeds = [20, 40, 60]. Which gives you 60?

    1. Aspeeds[2]
    2. Bspeeds[3]
    3. Cspeeds[1]
    4. Dspeeds(3)
    Answer: A. Positions count from 0, so the three items are at 0, 1 and 2. speeds[3] would be a fourth item, which does not exist.

  3. [1 mark]How many readings does tof_grid() give you?

    Answer: 64. It is an 8 by 8 square of distances, and 8 times 8 is 64.

  4. [1 mark]What does this program print?

    grid = [10, 11, 12, 13, 14, 15]
print(grid[2:4])
    Answer:
    [12, 13]

    A slice goes from the first number up to but not including the second, so [2:4] is the items at positions 2 and 3.

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

    readings = [55, 32, 18, 70, 39]
close = 0
for value in readings:
    if value < 40:
        close = close + 1
print(close, "readings are under 40 cm")
    Answer:
    3 readings are under 40 cm

    The loop looks at each reading in turn. 32, 18 and 39 are under 40, so close goes up three times.

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

    readings = [27.43, 80.0, 45.2]
print("nearest:", round(min(readings), 1))
    Answer:
    nearest: 27.4

    min() picks the smallest item, 27.43, and round(..., 1) keeps one decimal place.

  7. [1 mark]Which readings does grid[24:32] give you?

    1. AThe 8 readings at positions 24 to 31
    2. BThe 9 readings at positions 24 to 32
    3. COnly readings 24 and 32
    4. DThe 32 readings after position 24
    Answer: A. A slice stops just before its second number, so [24:32] is 24, 25 and so on up to 31: eight readings, one row of the grid.

The 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

The hint students can ask for: tof_grid() hands back 64 whole-centimetre numbers. The bottom rows see the mat, so take min() over the rows that look straight ahead (rows 2 and 3).

A solution

from bugbot import *
connect()
grid = tof_grid()
print('nearest:', min(grid[16:32]))

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.