Introduction to Python Lists: A Comprehensive Guide with Examples, Study notes of Advanced Computer Programming

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Introduction to Python

Lists

Topics

1. Lists
2. List indexing
3. Traversing and modifying a list
4. Summing a list
5. Maximum/Minimum of a list
6. List Methods

Lists Lists are the basic ordered and mutable data collection type in Python. They can be defined with comma-separated values between square brackets. L = [ 2 , 3 , 5 , 7 ] print(len(L)) # 4, len() also works with strings L.append( 11 ) # append to the end of the list print(L) # [2, 3, 5, 7, 11]

Indexing Indexing is a means the fetching of a single value from the list. This is a 0-based indexing scheme. L = [ 2 , 3 , 5 , 7 , 11 ] print(L[ 0 ]) # 2 print(L[ 1 ]) # 3 print(L[ 5 ]) # index out of bounds error.

Lists can contain different types of objects List can contain different types and even other lists. L = [ 1 , 'two', 3.14, [- 2 , 3 , 5 ]] print(L[ 0 ]) # 1 print(L[ 1 ]) # two print(L[ 3 ]) # [-2, 3, 5] print(L[ 3 ][0]) # - 2 print(L[ 3 ][1]) # 3 print(L[ 3 ][2]) # 5

Modifying a List Indexing can be used to set elements as well as access them. L = [ 2 , 3 , 5 , 7 , 11 ] L[ 0 ] = 100 print(L) # [100, 3, 5, 7, 11] L[ 2 ] = - 4 print(L) # [100, 3, - 4, 7, 11]

Modifying a list Consider the following code that is intended to change all even numbers in a list to 0. nums = [ 24 , 3 , 34 , 6 , - 5 , 4 ] for x in nums: if x % 2 == 0: x = 0 print(nums) Output: [24, 3, 34, 6, - 5, 4]

Note: The list is unchanged? Why? How can we fix it? 10

Modifying a list Here's the correct code to change all even numbers in a list to 0. Compare the following code to the previous slide. nums = [ 24 , 3 , 34 , 6 , - 5 , 4 ] for i in range(len(nums)): if nums[i] % 2 == 0: nums[i] = 0 print(nums) Output: [0, 3, 0, 0, - 5, 0]

Creating a list with list comprehensions List comprehensions is a way to create a list in Python that is concise and elegant. Its main use is to create a new list from a given list. Instead of: squares = [] # create empty list for i in range(5): squares.append(i ** 2) # add each square to list Do this: squares = [i ** 2 for i in range(5)] # one line! List comprehensions allow you to use a conditional. even_squares = [i ** 2 for i in range(5) if i % 2 == 0] print(even_squares) # [0, 4, 16]

Algorithms to know The following algorithms are useful. Know how to implement these algorithms!

1. Find sum of a list of numbers.
2. Find the average of a list of numbers.
3. Find the maximum/minimum of a list of numbers.

Sum Function Write a function that accepts a list of numbers as a parameter and returns its sum. def sum(nums): s = 0 for x in nums: s += x return s lst = [ 2 , - 1 , 3 , 4 , - 3 ] print(sum(lst)) # 5 lst2 = [ 1 , 5, 4, 2] a = sum(lst2) print(a) # 12

Average Function Write a function that accepts a list of numbers as a parameter and returns its average. def average(nums): s = 0 for x in nums: s += x return s/len(nums) lst = [ 2 , 5, 4, 3] a = average(lst) print(a) # 3.

Find Maximum Function Write a function that accepts a nonempty list of numbers as a parameter and returns its maximum value. Does the code below work? def maximum(nums): current_max = 0 for x in nums: if x > current_max: current_max = x return current_max lst = [- 2 , - 5, - 12, - 3 ] a = maximum(lst) print(a) # 0 INCORRECT! No! What if the list contains only negative numbers?This function returns 0 which is not even in the list! 19

Find Maximum Function Here's the correct implementation of maximum. The minimum function is similar. def maximum(nums): current_max = nums[0] # the first value is maximum for x in nums: # until a bigger value shows up if x > current_max: current_max = x return current_max lst = [ 2 , 5, 12, 3, 4, 11] a = maximum(lst) print(a) # 12