Programming with Python
Storing Multiple Values in Lists
Learning Objectives
- Explain what a list is.
- Create and index lists of simple values.
Just as a for
loop is a way to do operations many times, a list is a way to store many values. Unlike NumPy arrays, lists are built into the language (so we don’t have to load a library to use them). We create a list by putting values inside square brackets:
odds = [1, 3, 5, 7]
print 'odds are:', odds
odds are: [1, 3, 5, 7]
We select individual elements from lists by indexing them:
print 'first and last:', odds[0], odds[-1]
first and last: 1 7
and if we loop over a list, the loop variable is assigned elements one at a time:
for number in odds:
print number
1
3
5
7
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'
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-8-220df48aeb2e> in <module>()
1 name = 'Bell'
----> 2 name[0] = 'b'
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(11)
print 'odds after adding a value:', odds
odds after adding a value: [1, 3, 5, 7, 11]
del odds[0]
print 'odds after removing the first element:', odds
odds after removing the first element: [3, 5, 7, 11]
odds.reverse()
print 'odds after reversing:', odds
odds after reversing: [11, 7, 5, 3]
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.
Turn a string into a list
Use a for-loop to convert the string “hello” into a list of letters:
["h", "e", "l", "l", "o"]
Hint: You can create an empty list like this:
my_list = []