Lists: one-dimensional arrays

Making, indexing and changing a list; append, remove, in, split and join.

F3.3Strings, lists and recordsGCSE15 min

A variable holds one value. A list holds many, in order, under one name. Exams call this an array. The robot's depth sensor gives 64 readings at once, as a list, and by the end of this module you will store whole surveys of a room in lists.

Making and reading a list

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

legs = [20, 15, 30]
print(legs)
print("first:", legs[0])
print("last:", legs[2])
print("also last:", legs[-1])
print("how many:", len(legs))

Run this in the simulator

Square brackets make a list, with commas between the items (also called elements). Items are numbered from 0, exactly like the characters of a string, so legs[0] is the first and legs[2] the third. len() counts them. Asking for legs[3] would be an IndexError: there is no fourth item.

Using items

An item is used like any variable:

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

legs = [20, 15, 30]
forward(60, distance=legs[0])
right(60, distance=legs[1])
forward(60, distance=legs[2])
print("total:", legs[0] + legs[1] + legs[2])

Run this in the simulator

Changing a list

Unlike a string, a list can be changed after it is made:

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

notes = [262, 294, 330]
notes[0] = 523          # replace the first item
notes.append(349)       # add one to the end
print(notes, "has", len(notes), "notes")
notes.remove(294)       # take out the first 294
print(notes)
last = notes.pop()      # take the last item off, and keep it
print("popped", last, "leaving", notes)

Run this in the simulator

list[index] = value replaces an item. append adds to the end, so the list grows. remove takes out the first item equal to a value; pop takes the last item off and gives it back.

An empty list that fills up

Many lists start empty and grow as a program runs:

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

readings = []
for i in range(4):
    readings.append(distance())
    forward(50, distance=10)
print("readings:", readings)
print("the second reading was", readings[1])

Run this in the simulator

Collecting readings into a list and looking at them afterwards is how you find out what a sensor actually does.

Is it in the list?

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

colours = ["red", "green", "blue"]
answer = input("Pick a colour: ")
if answer in colours:
    led(answer)
    print("item", colours.index(answer), "of the list")
else:
    print("not one of", colours)

Run this in the simulator

in works on lists the way it works on strings. index gives the position of an item.

Lists and strings

A string is a sequence of characters and a list is a sequence of anything, so many things work on both: indexing, slicing, len, in. split and join convert between them:

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

line = "30,20,30,20"
sides = line.split(",")
print(sides)
print(int(sides[0]) + int(sides[1]))
print(" then ".join(sides))

Run this in the simulator

split breaks a string into a list of strings at every comma. The items are still text, so int is needed before adding them.

In the exam

An array stores many values of the same type under one identifier, each reached by its index. Python's list is more relaxed: it can mix types and change size, so exams say "array (or list)". Questions ask for the value at an index, for code that changes an item, and why an array is better than many separate variables.

OCR 2.2.3

The Exam Reference Language declares an array with its size, and indexes from 0:

array notes[3]
notes[0] = 262
notes[1] = 294
notes[2] = 330
print(notes[1])

AQA 3.2.6

AQA pseudo-code makes an array with square brackets and indexes from 0:

notes ← [262, 294, 330]
OUTPUT notes[1]
OUTPUT LEN(notes)

Edexcel 6.3.1

Edexcel lists one-dimensional arrays among the structured data types, and uses Python lists for them in Paper 2, with append, len and indexing as here.

Task: a changing tune

Start with notes = [262, 294, 330]. Change the first note to 523, add 349 to the end, print how many notes there are as 4 notes, then play every note in the list for 0.3 seconds. Change the list with list operations, not by typing a new list.

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

notes = [262, 294, 330]

Challenges

Make a list of four colours and show each on the LED with a loop over the indexes.
Ask for side lengths as 30,20,30,20 and drive them.
What happens with notes[10] = 440? And with notes.append(440) ten times?