# Lists

Last updated on 2023-05-08 | Edit this page

## Overview

### 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.

• Scenario: You have set up an Arduino to do temperature measurements in a storage room for rare books.
• Doing calculations with a hundred variables called temperature_001, temperature_002, etc., would be at least as slow as doing them by hand.
• 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.

### PYTHON

temperatures = [17.3, 17.5, 17.7, 17.5, 17.6]
print('temperatures:', temperatures)
print('length:', len(temperatures))

### OUTPUT

temperatures: [17.3, 17.5, 17.7, 17.5, 17.6]
length: 5

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

• Just like strings.

### PYTHON

print('zeroth item of temperatures:', temperatures[0])
print('fourth item of temperatures:', temperatures[4])

### OUTPUT

zeroth item of temperatures: 17.3
fourth item of temperatures: 17.6

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

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

### PYTHON

temperatures[0] = 16.5
print('temperatures is now:', temperatures)

### OUTPUT

temperatures is now: [16.5, 17.5, 17.7, 17.5, 17.6]

## Appending items to a list lengthens it.

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

### PYTHON

print('temperatures is initially:', temperatures)
temperatures.append(17.9)
temperatures.append(18.2)
print('temperatures has become:', temperatures)

### OUTPUT

temperatures is initially: [16.5, 17.5, 17.7, 17.5, 17.6]
temperatures has become: [16.5, 17.5, 17.7, 17.5, 17.6, 17.9, 18.2]
• 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 library.
• We will meet other methods of lists as we go along.
• Use help(list) for a preview.

## 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.

### PYTHON

primes = [2, 3, 5, 7, 11]
print('primes before removing last item:', primes)
del primes[4]
print('primes after removing last item:', primes)

### OUTPUT

primes before removing last item: [2, 3, 5, 7, 11]
primes after removing last item: [2, 3, 5, 7]

## 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 in the next episode).

## Lists may contain values of different types.

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

### PYTHON

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

## Character strings can be indexed like lists.

• Get single characters from a character string using indexes in square brackets.

### PYTHON

element = 'carbon'
print('zeroth character:', element[0])
print('third character:', element[3])

### OUTPUT

zeroth character: c
third character: b

## 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.

### PYTHON

element[0] = 'C'

### ERROR

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.

### PYTHON

print('99th element of element is:', element[99])

### OUTPUT

IndexError: string index out of range

### Fill in the Blanks

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

### PYTHON

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

### OUTPUT

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

### PYTHON

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

### OUTPUT

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

### How Large is a Slice?

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

The list’s length would be equal to high - low.

Given this:

### PYTHON

print('string to list:', list('tin'))
print('list to string:', ''.join(['g', 'o', 'l', 'd']))

### OUTPUT

['t', 'i', 'n']
'gold'
1. Explain in simple terms what list('some string') does.
2. What does '-'.join(['x', 'y']) generate?
1. It creates a list of the some strings characters as elements.
2. It creates a string composed of x and y, separated by a hyphen character(-).

### Working With the End

What does the following program print?

### PYTHON

element = 'helium'
print(element[-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.)

### OUTPUT

m
1. A negative index begins at the final element.
2. -(N) corresponds to the first index, which is the [0] index.
3. It removes the final element of the list.
4. You could do the following: print(values[0:-1])

### Stepping Through a List

What does the following program print?

### PYTHON

element = 'fluorine'
print(element[::2])
print(element[::-1])
1. If we write a slice as low:high:stride, what does stride do?
2. What expression would select all of the even-numbered items from a collection?

### OUTPUT

furn
eniroulf
1. stride indicates both the number of steps, and from which end: positive starts from first element, negative from the last element.
2. element[1::2]

### Slice Bounds

What does the following program print?

### PYTHON

element = 'lithium'
print(element[0:20])
print(element[-1:3])

### OUTPUT

lithium
''

There is no 20th index, so the entire string is captured.
There is no element after the -1 index.

### Sort and Sorted

What do these two programs print? In simple terms, explain the difference between sorted(letters) and letters.sort().

### PYTHON

# Program A
letters = list('gold')
result = sorted(letters)
print('letters is', letters, 'and result is', result)

### PYTHON

# Program B
letters = list('gold')
result = letters.sort()
print('letters is', letters, 'and result is', result)

Program A:

### OUTPUT

letters is ['g', 'o', 'l', 'd'] and result is ['d', 'g', 'l', 'o']

Program B:

### OUTPUT

letters is ['d', 'g', 'l', 'o'] and result is None

sorted(letters) returns a sorted copy of the list, while letters.sort() sorted the list in place. Thus, it was already sorted, and calling a further sort returns None.

### Copying (or Not)

What do these two programs print? In simple terms, explain the difference between new = old and new = old[:].

### PYTHON

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

### PYTHON

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

Program A:

### OUTPUT

new is ['D', 'o', 'l', 'd'] and old is ['D', 'o', 'l', 'd']

Program B:

### OUTPUT

new is ['D', 'o', 'l', 'd'] and old is ['g', 'o', 'l', 'd']

new = old is assigning old to new, whereas new = old[:] is a slice assignment, which will only return a copy of 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.