Numbers and names

Variables, arithmetic, and driving an exact distance instead of counting seconds.

0.2Python quick startRobot club10 min

Do this lesson in the simulator

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

Typing the same number in five places is how mistakes happen. Python lets you name a number once and use the name.

A name for a number

from bugbot import *
connect()

# a name, and what it stands for
speed = 60
seconds = 1.5

forward(speed)
wait(seconds)
stop()
print("drove at", speed, "for", seconds, "seconds")

Run this in the simulator

speed = 60 makes a variable. After that, speed means 60 everywhere in the program. Change the number once at the top and the whole program changes with it.

Names can hold the answer to a sum, too.

from bugbot import *
connect()

cm_per_second = 18
seconds = 2
print("that should be about", cm_per_second * seconds, "cm")

Run this in the simulator

Python does the usual arithmetic: +, -, * for times, / for divide.

Driving an exact distance

Counting seconds is guesswork. Every driving command takes a distance instead, in centimetres, and the robot works out the timing itself.

from bugbot import *
connect()

# 20 cm forward, then stop by itself
forward(50, distance=20)
print("position now", position())
# 15 cm to the right, still facing the same way
right(50, distance=15)
print("position now", position())
stop()

Run this in the simulator

distance=20 is a named argument: it says which 20 you mean. With a distance, the command waits until the robot has gone that far, so you do not need wait() after it.

It is still not perfect. The robot slides a little when it stops, and no two BugBots drift the same way. Getting from "about right" to "exact" is what Module 3 is for.

Task: two legs

Drive 40 cm forward, then 45 cm to the right, and stop in the green zone. Use distance= rather than counting seconds, and give the two distances names at the top.

from bugbot import *
connect()

first = 40
# your second distance here

forward(50, distance=first)
stop()

Challenges

  1. Do the same journey with the two legs the other way round. Does it end up in the same place?
  2. Make a variable slow = 25 and run the journey at that speed. Is it closer to the middle of the zone?
  3. Print the distance travelled in metres, not centimetres, using / 100.