Lists in Python

What is a list?

• Lists are used to store multiple items in a single variable.
• List items are:
• - Ordered: items take order in which they are written in code
• - Changeable: you can use the assignment operator to change the items
• - And allow duplicate values
• List items are indexed, the first item has index [0], the second item has index [1], the third item has index [2], so and so forth.

Indexing (Part 1)

• # Accessing list items by using their index
• # Note : index start from 0
• # Note : Negative index start from -1
• thislist = ["apple", "banana", "cherry"]
• print(thislist[0])
• print(thislist[1])
• print(thislist[2])
• print(thislist[-1]) # Prints the last element

Indexing (Part 2)

• Using negative numbers as indices to write a slice.
• x = ["apple", "banana", "orange", "watermelon", "cherry"]

print(thislist[-1]) # Prints the last element

print(thislist[len(thislist) - 1])

# Index “-1” is equivalent to index “len(thislist) – 1”.

print(thislist[-2]) # Prints the second last element

print(thislist[len(thislist) - 2])

print(thislist[-3]) # Prints the third last element

print(thislist[len(thislist) - 3])

Slicing (Part 1)

• Slice syntax = [start index : end index : step (optional)]
• Q: You are given that a list has 10 items.
• Which all indices are covered by the slice [2:5]?
• Ans: Three parts of the slice [2:5] are:
• start index = 2
• end index = 5
• step = 1 (takes the default value as it is not mentioned)
• Slicing is “exclusive” of end index, that means index 5 will not be a part of the slice [2:5].
• That means indices covered are: 2, 3 and 4.

Slicing (Part 2)

• What does this slice expand to - [:4]
• Start index is not mentioned. That means start index takes the default value: 0
• End index is given to be 4.
• Step is not mentioned. Step takes the default value of ‘1’

Slicing (Part 3)

• What does this slice expand to... [4:]
• Start index is mentioned.
• It has the value: 4
• End index is not mentioned.
• It has the default value of: len()
• Step is not mentioned. Step takes the default value of ‘1’

Slicing (Part 4)

• You are given a list of 10 elements.
• Ques: Which all indices are present in slice... [2:10:2]
• Ans:
• Start index is 2. Why? Because it is the number mentioned before the first colon.
• End index is 10.
• ‘Step’ is mentioned after the second colon. Step has value 2.
• So, 2 will come. After that, 4 will come. After that, 6 will come. After that, 8 will come. That is it, because it is exclusive of end index.

Slicing (Part 5)

• A slice has three parts: start index, end index and step.
• Default value of start index: 0
• Default value of end index: len()
• Default value of step: 1
• Q: What does this [2:5] expand to?
• [2:5:1]
• Q: What does this [:4] expand to?
• Here, we can see that we have a single colon. This means that we have assume default value for step = 1.
• Because the number before colon is missing that has to be start index. And number after colon is: end index.
• [0:4:1]
• Q: What does this [2:] expand to?
• Answer: [2:len():1]

Slicing (Part 6)

• # Accessing multiple items by using a range of indexes
• thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
• print(thislist[2:5]) # Inclusive of 2 and exclusive of 5
• print(thislist[:4]) # From Begining to specified index
• # Note "kiwi" not included in the result
• # This slice [:4] -> [0:4:1]
• print(thislist[2:])
• # This slice [2:] -> [2:7:1]
• # 7 because it is the length of the list.
• # Using Negative indexes
• print(thislist[-4:-1]) # Note - last item index value is -1 and this not included in result

Assignment of another list on a slice of list

• With assignment operator, you can assign to an index and assign to a slice.
• When you assign a longer list to a shorter slice, it increases the length of the list.
• When you assign a shorter list to a longer slice, it decreases the length of the list.

Adding elements to a list

• Insert(): Used to insert element at a specified index.
• Takes two arguments. First argument is the index.
• Append(): takes one argument. That argument is an element to be appended in the list.
• Extend(): takes one argument but that argument is a list for us.
• Here, usage is: mainlist.extend(listtoappend)
• It is equivalent to: mainlist = mainlist + listtoappend
• Note: They lead to increase in the length of the original list.

Removing items from a list

• Remove(): takes one argument. And argument is the element itself.
• For ex: if it is list of strings, the argument would be a string. If it is a list of integers, the argument would be an int.
• Pop(): also takes one argument (defaults to ‘len()-1’ (which means the last element) if no argument is passed) but that argument is the index.
• Del: using the keyword ‘del’ delete from a specified index.
• Clear(): empties a list

>>> l = ['alpha', 'beta', 'gamma']
>>> l.remove('alpha')
>>> l
['beta', 'gamma']
>>> 
>>> 
>>> l.pop(0)
'beta'
>>> 
>>> l
['gamma']
>>> 
  

Problem

• Q: Let the input list be: x = ["apple", "banana", "orange", "watermelon", "cherry"]
• Show how you can reverse this list in three ways.

Solution

• 1) Through slicing:
• x[::-1]
• 2) Using the built-in function:
• list(reversed(x))
• 3) Using method invocation:
• x.reverse()

List Comprehension (Part 1)

• Creating a new list from an existing one:

# Creating a new list from an existing one

l = ["apple", "banana", "cherry", "guava"]

n = []

for i in l:

n.append(i)

print(n)

['apple', 'banana', 'cherry', 'guava']

# NOW USING LIST COMPREHENSION:

l = ["apple", "banana", "cherry", "guava"]

n = [ str(i) for i in l ]

print(n)

List Comprehension (Part 2)

# Syntax of list comprehension: [ some_function(i) for i in some_list ]

my_list = ["apple", "banana", "cherry", "guava"]

n = [ str(i) for i in my_list ]

print(n)

Output:

['apple', 'banana', 'cherry', 'guava']

Here:

some_function() is your: str()

some_list is your: my_list = ["apple", "banana", "cherry", "guava"]

Problem on List Comprehension

• You have a list of 10 numbers.
• [5, 2, 9, 1, 8, 3, 6, 0, 4, 7]
• Create a new list by adding 5 to each of these numbers.

Solution

# Create a new list by adding 5 to each of these numbers.

l = [5, 2, 9, 1, 8, 3, 6, 0, 4, 7]

k = [i + 5 for i in l]

print(k)

[10, 7, 14, 6, 13, 8, 11, 5, 9, 12]

Problem 1 on: Assignment to a slice

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]

thislist[1:3] = ['potato', 'tomato', 'brinjal']

What will be the output?

Solution 1 on: Assignment to a slice

thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]

len(thislist)

Out: 7

# What is being replaced?

thislist[1:3]

Out: ['banana', 'cherry']

thislist[1:3] = ['potato', 'tomato', 'brinjal']

thislist

Out: ['apple', 'potato', 'tomato', 'brinjal', 'orange', 'kiwi', 'melon', 'mango']

len(thislist)

8

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