Lists

Overview

Teaching: 15 min
Exercises: 10 min

Questions

• How can I store multiple values?

Objectives

• Explain why programs need collections of values.

• Write programs that create flat lists, index them, slice them, and modify them through assignment and method calls.

A list stores many values in a single structure.

• When we wish to work with hundreds of variables defining them all explicitly would be at least as slow as doing them by hand. For example if we had many regions in the brain that we wished to analyse we might have many numbers. Each number would correspond to the volume of a particular ROI like cingulate, limbic, etc.
• Use a list to store many values together.
  • Contained within square brackets: [...].
  • Values separated by commas: ,.
• Use len to find out how many values are in a list.

roi_volumes = [2.73,145.3,12.7,16.2, 27.6]
# Volume of ROIs :  
roi_volumes

[2.73,145.3,12.7,16.2, 27.6]

# length: 
len(roi_volumes)

5

Use an item’s index to fetch it from a list.

• Just like strings.

# The item with index 0 in the list is : 
roi_volumes[0]

2.73

# The item with index 4 in the list is : 
roi_volumes[4]

27.6

Lists’ values can be replaced by assigning to them.

• Use an index expression on the left of assignment to replace a value.

roi_volumes[0] = 26.5
# Our list is now: 
roi_volumes

Our list is now: [26.5,145.3,12.7,16.2, 27.6]

Appending items to a list lengthens it.

• Use list_name.append to add items to the end of a list.

# Our list is initially: 
roi_volumes 

[26.5,145.3,12.7,16.2, 27.6]

roi_volumes.append(14.2)
roi_volumes.append(140)
# Our list has now become: 
roi_volumes

[26.5,145.3,12.7,16.2, 27.6,14.2,140]

• append is a method of lists.
  • Like a function, but tied to a particular object.
• Use object_name.method_name to call methods.
  • Deliberately resembles the way we refer to things in a package. We’ll learn about packages later.
• We will meet other methods of lists as we go along.
  • In IPython, after typing a period after an object we can preview the methods available to us by hitting the tab-key. Alternatively we can see the methods by using the dir function or by accessing the help for an object.

Use del to remove items from a list entirely.

• del list_name[index] removes an item from a list and shortens the list.
• Not a function or a method, but a statement in the language.

# roi_volumes before removing last item: 
roi_volumes

[26.5,145.3,12.7,16.2, 27.6,14.2,140]

del roi_volumes[6]
# roi_volumes after removing last item: 
roi_volumes

[26.5,145.3,12.7,16.2, 27.6,14.2]

The empty list contains no values.

• Use [] on its own to represent a list that doesn’t contain any values.
  • “The zero of lists.”
• Helpful as a starting point for collecting values which we will see when we learn about iteration.

Lists may contain values of different types.

• A single list may contain numbers, strings, and anything else.

goals = [1, 'Create lists.', 2, 'Extract items from lists.', 3, 'Modify lists.']

While this list can be created there are better ways of representing this type of information.

Character strings can be indexed like lists.

• As we saw before, indexing into strings is the same as lists.

label_for_roi = 'cortex_left'
# The character indexed by 0: 
label_for_roi[0]
# The character indexed by 3: 

c

label_for_roi[3]

t

Character strings are immutable.

• Cannot change the characters in a string after it has been created.
  • Immutable: cannot be changed after creation.
  • In contrast, lists are mutable: they can be modified in place.
• Python considers the string to be a single value with parts, not a collection of values.

label_for_roi[0] = 'C'

TypeError: 'str' object does not support item assignment

• Lists and character strings are both collections.

Indexing beyond the end of the collection is an error.

• Python reports an IndexError if we attempt to access a value that doesn’t exist.
  • This is a kind of runtime error.
  • Cannot be detected as the code is parsed because the index might be calculated based on data.

# The character indexed by 99 is: 
label_for_roi[99]

IndexError: string index out of range

Fill in the Blanks

Fill in the blanks so that the program below produces the output shown.

values = []
values.____(1)
values.____(3)
values.____(5)
print('first time:', values)
values = values[____]
print('second time:', values)

first time: [1, 3, 5]
second time: [3, 5]

Solution

values = []
values.append(1)
values.append(3)
values.append(5)
print('first time:', values)
values = values[1:3]
print('second time:', values)

How Large is a Slice?

If ‘low’ and ‘high’ are both non-negative integers, how long is the list values[low:high]?

Solution

high-low

From Strings to Lists and Back

Given this:

print('string to list:', list('ADHD'))
print('list to string:', ''.join(['c', 'n', 's']))

string to list: ['A', 'D', 'H', 'D']
list to string: 'cns'

1. Explain in simple terms what list('some string') does.
2. What does '-'.join(['x', 'y']) generate?

Working With the End

What does the following program print?

roi_label = 'hippocampus'
print(roi_label[-1])

1. How does Python interpret a negative index?
2. If a list or string has N elements, what is the most negative index that can safely be used with it, and what location does that index represent?
3. If values is a list, what does del values[-1] do?
4. How can you display all elements but the last one without changing values? (Hint: you will need to combine slicing and negative indexing.)

Solution

1: It retrieves elements by stepping backwards through the string.
2: -N. This location is the first element in the list.
3: It deletes the last element in the list.
4: values[0:-1]

Stepping Through a List

What does the following program print?

roi_label = 'hippocampus'
print(roi_label[::2])
print(roi_label[::-1])

1. If we write a slice as low:high:stride, what does stride do?
2. What expression would select every other element, starting from the second?

Solution

hpoaps
supmacoppih

1: stride is the step to go through the list

The answer to question 2 is:

listobject[1::2]

Slice Bounds

What does the following program print?

roi_label = 'hippocampus'
print(roi_label[0:20])
print(roi_label[-1:3])

Solution

hippocampus
       

Sort and Sorted

roi_label_as_list = list(roi_label)

In simple terms, explain the difference between sorted(roi_label_as_list) and roi_label_as_list.sort().

Copying (or Not)

What do these two programs print? In simple terms, explain the difference between new = old and new = old[:]. For more information read about deep copy vs shallow copy

# Program A
old = list('Brain')
new = old      # simple assignment
new[0] = 'D'
print('new is', new, 'and old is', old)

# Program B
old = list('Brain')
new = old[:]   # assigning a slice
new[0] = 'D'
print('new is', new, 'and old is', old)

Key Points

• A list stores many values in a single structure.

• Use an item’s index to fetch it from a list.

• Lists’ values can be replaced by assigning to them.

• Appending items to a list lengthens it.

• Use del to remove items from a list entirely.

• The empty list contains no values.

• Lists may contain values of different types.

• Character strings can be indexed like lists.

• Character strings are immutable.

• Indexing beyond the end of the collection is an error.