Lists

Python has a number of built-in composite data structures: tuples, lists, sets, dictionaries. Here we take a look at lists and dictionaries.

A list stores many values in a single structure.

events = [267, 1332, 1772, 1994, 493, 1373, 1044, 156, 1515, 1788]  # array of years
print('events:', events)
print('length:', len(events))

print('first item of events is', events[0])   # indexing starts witgh 0
events[2] = 1773                              # individual elements are mutable
print('events is now:', events)

events.append(1239)   # append at the end
events.append(606)    # append at the end
print('events is now:', events)
events.pop(4)         # remove element #4
print('events is now:', events)
events.remove(267)    # remove by value (first occurrence)

a = []   # start with an empty list
a.append('Vancouver')
a.append('Toronto')
a.append('Kelowna')
print(a)

a[3]   # will give an error message (past the end of the array)
a[-1]   # display the last element; what's the other way?
a[:]    # will display all elements
a[1:]   # all elements starting from #1
a[:1]   # ending with but not including #1

Lists can be heterogeneous and nested:

a = [11, 21., 3.5]
b = ['Mercury', 'Venus', 'Earth']
c = 'hello'
nestedList = [a, b, c]
print(nestedList)

Exercise 5.1: double subsetting

How would you extract element “Earth” from nestedList?

You can search inside a list:

'Venus' in b                   # returns True
'Pluto' in b                   # returns False
b.index('Venus')               # returns 1 (positional index)
nestedList[1].index('Venus')   # same

And you sort lists alphabetically:

b.sort()
b             # returns ['Earth', 'Mercury', 'Venus']

Exercise 5.2

Write a script to find the second largest number in the list [77,9,23,67,73,21].