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A1.2 Operations, strings and random numbers

Programming techniques and object-oriented programming · A level · AQA 7517 4.1.1.3, Eduqas A500QS 1.4 · about 20 min

BugBotLab

What this lesson is about

Integer division and MOD with negatives, rounding and truncation, XOR, string and date conversions, and pseudo-random numbers.

Questions 6 marks in all

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

    print(-17 // 5, -17 % 5)
    Answer:
    -4 3

    Python's integer division rounds down to -4, and the remainder takes the divisor's sign: 5 * -4 + 3 is -17.

  2. [1 mark]What does this program print?

    import math
x = -2.6
print(round(x), int(x), math.floor(x))
    Answer:
    -3 -2 -3

    Rounding gives the nearest whole number, -3; truncation removes the fraction, -2; floor rounds down, -3.

  3. [1 mark]What is True XOR True?

    1. AFalse
    2. BTrue
    Answer: A. XOR is true only when exactly one input is true.

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

    s = "R135,F20"
comma = s.find(",")
print(comma, int(s[1:comma]) + 1, s[comma + 1:] + "!")
    Answer:
    4 136 F20!

    The comma is at index 4, the substring from index 1 to before 4 is "135", and the rest of the string is "F20".

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

    heading = 30
for turn in [-90, -45, 200]:
    heading = (heading + turn) % 360
    print(heading)
    Answer:
    300
255
95

    30 - 90 is -60, which MOD 360 gives 300; then 255; then 455 MOD 360 is 95.

  6. [1 mark]Why does setting the same seed before generating numbers give the same sequence each time?

    1. AThe generator is pseudo-random: each number is calculated from the previous state, starting from the seed
    2. BThe computer stores every random number it has ever made
    3. CThe seed stops the numbers being random at all times
    4. DRandom numbers always repeat after one use
    Answer: A. A pseudo-random generator is deterministic: the same starting state gives the same sequence.

The task: drive a command string

A route arrives as one string of commands separated by commas: COMMANDS = "F20,L90,F15,R135,B5". Each command is a letter and a whole number: F drives forward that many cm, B drives backward that many cm, R turns right that many degrees and L turns left that many degrees. Go through the string and carry out each command on the robot. Keep two totals as you go: the distance driven in cm (forward and backward both add) and the heading in degrees, starting at 0, turning right adding and turning left subtracting, always kept from 0 to 359 with MOD 360. Convert each amount with int. At the end print driven <cm> cm, heading <degrees>, for this string driven 40 cm, heading 45. Work both totals out from the string; do not type them.

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

COMMANDS = "F20,L90,F15,R135,B5"

for command in COMMANDS.split(","):
    print(command)

The hint students can ask for: Each command is one letter followed by a number. Separate the commands, then take the first character as the letter and everything after it as the amount. Distances add up whichever way you drive; turns change the heading, and MOD keeps it in range.

A solution

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

COMMANDS = "F20,L90,F15,R135,B5"

driven = 0
bearing = 0
for command in COMMANDS.split(","):
    letter = command[0]
    amount = int(command[1:])
    if letter == "F":
        forward(50, distance=amount)
        driven = driven + amount
    elif letter == "B":
        backward(50, distance=amount)
        driven = driven + amount
    elif letter == "R":
        turn_right(30, angle=amount)
        bearing = (bearing + amount) % 360
    elif letter == "L":
        turn_left(30, angle=amount)
        bearing = (bearing - amount) % 360
print(f"driven {driven} cm, heading {bearing}")

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