Building programs with Python

Arrays, Lists etc

Arrays in Python

So we can use variables to hold values which we can then manipulate - useful! But what about when we need to hold many different values, such as a set of phone numbers?

One of the most fundamental data structures in any language is the array, used to hold many values at once. Python doesn’t have a native array data structure, but it has the list which is much more general and can be used as a multidimensional array quite easily.

List basics

A list in python is just an ordered collection of items which can be of any type. By comparison an array is an ordered collection of items of a single type - so a list is more flexible than an array.

We can also add and delete elements from a Python list at any time - Python lists are what is known as a dynamic and mutable type.

Lists are built into the language (so we don’t have to load a library to use them).

To define a list we simply write a comma separated list of items in square brackets:

odds = [1, 3, 5, 7, 9, 11, 15]
print('Odds are:', odds)

Odds are: [1, 3, 5, 7, 9, 11, 15]

This looks like an array because we can use indexing to pick out an individual element - indexes start from 0.

Programming languages like Fortran and MATLAB start counting at 1, because that’s what human beings have done for thousands of years. Languages in the C family (including C++, Java, Perl, and Python) count from 0 because that’s simpler for computers to do.

It takes a bit of getting used to, but one way to remember the rule is that the index is how many steps we have to take from the start to get the item we want.

We select individual elements from lists by indexing them:

print('first and last:', odds[0], odds[-1])

Which will print first and last elements, i.e. value 1 and 15 in this case.

first and last: 1 15

See slide Indexing a List Example I.

Similarly to change the seventh element we can assign directly to it:

odds[6] = 13

See slide Indexing a List Example II.

Slicing

The Slicing notation looks like array indexing but it is a lot more flexible. For example:

odds[2:5]

[5, 7, 9]

See slide Slicing a List Example I.

is a sublist from the third element to the fifth i.e. from odds[2] to odds[4]. Notice that the final element specified i.e. [5] is not included in the slice.

Also notice that you can leave out either of the start and end indexes and they will be assumed to have their maximum possible value. For example:

odds[5:]

[11, 13]

is the list from odds[5] to the end of the list and

odds[:5]

[1, 3, 5, 7, 9]

is the list up to and not including odds[5] and

odds[:]

[1, 3, 5, 7, 9, 11, 13]

is the entire list.

Slicing strings

A section of an array is called a slice. We can take slices of character strings as well:

element = 'oxygen'
print('first three characters:', element[0:3])
print('last three characters:', element[3:6])

See slide Slicing a List Example II.

first three characters: oxy
last three characters: gen

List slicing is more or less the same as string slicing except that we can modify a slice. For example:

odds[0:2]=[17,19]

has the same effect as

odds[0]=17
odds[1]=19

NOTE:

Finally it is worth knowing that the list we assign to a slice doesn’t have to be the same size as the slice - it simply replaces it even if it is a different size.

Thin slices

The expression element[3:3] produces an empty string, i.e., a string that contains no characters.

Lists and Strings

There is one important difference between lists and strings: we can change the values in a list, but we cannot change the characters in a string. For example:

names = ['Newton', 'Darwing', 'Turing'] # typo in Darwin's name
print('names is originally:', names)
names[1] = 'Darwin' # correct the name
print('final value of names:', names)

names is originally: ['Newton', 'Darwing', 'Turing']
final value of names: ['Newton', 'Darwin', 'Turing']

works, but:

name = 'Bell'
name[0] = 'b'

>>> name[0]='b'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

does not.

There are many ways to change the contents of lists besides assigning new values to individual elements:

odds.append(21)
print('odds after adding a value:', odds)

odds after adding a value: [17, 19, 5, 7, 9, 11, 13, 15, 21]

del odds[0]
print('odds after removing the first element:', odds)

odds after removing the first element: [19, 5, 7, 9, 11, 13, 15, 21]

odds.reverse()
print('odds after reversing:', odds)

odds after reversing: [21, 15, 13, 11, 9, 7, 5, 19]

While modifying in place, it is useful to remember that python treats lists in a slightly counterintuitive way.

If we make a list and (attempt to) copy it then modify in place, we can cause all sorts of trouble:

odds = [1, 3, 5, 7]
primes = odds
primes += [2]
print('primes:', primes)
print('odds:', odds)

primes: [1, 3, 5, 7, 2]
odds: [1, 3, 5, 7, 2]

This is because python stores a list in memory, and then can use multiple names to refer to the same list. If all we want to do is copy a (simple) list, we can use the list() command, so we do not modify a list we did not mean to:

odds = [1, 3, 5, 7]
primes = list(odds)
primes += [2]
print('primes:', primes)
print('odds:', odds)

primes: [1, 3, 5, 7, 2]
odds: [1, 3, 5, 7]

This is different from how variables worked in lesson 1, and more similar to how a spreadsheet works.

Basic array operations

So far so good, and it looks as if using a list is as easy as using an array.

Where things start to go wrong just a little is when we attempt to push the similarities between lists and arrays one step too far. For example, suppose we want to create an array initialised to a particular value. Following the general array idiom in most languages we might initialise the elements to a value, say, 1. e.g.:

myList=[]
myList[1]=1
myList[2]=1
...

only to discover that this doesn’t work because we can’t assign to a list element that doesn’t already exist.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list assignment index out of range

One solution is to use the append method to add elements one by one:

myList=[]
myList.append(1)
myList.append(1)
...

This works but it only works if we need to build up the list in this particular order - which most of the time you want to do anyway.

string_for_slicing = "Observation date: 02-Feb-2013"
list_for_slicing = [["fluorine", "F"], ["chlorine", "Cl"], ["bromine", "Br"], ["iodine", "I"], ["astatine", "At"]]

"2013"
[["chlorine", "Cl"], ["bromine", "Br"], ["iodine", "I"], ["astatine", "At"]]

counts = [2, 4, 6, 8, 10]
repeats = counts * 2
print(repeats)