Lists Python Lecture Notes PDF

Lists Week 4: Lecture 2 DATE : 19/11/2024 1 Learning Outcomes 2 Learning Outcomes By the end of this section, you will be able to: Create lists in python and identify elements using indices. Sort lists of numbers and text....

Lists Week 4: Lecture 2 DATE : 19/11/2024 1 Learning Outcomes 2 Learning Outcomes By the end of this section, you will be able to: Create lists in python and identify elements using indices. Sort lists of numbers and text. 3 Introduction 4 Outline Lists Insertion, deletion, etc Sorting 5 What are lists? A list is a sequence of data values called items or elements. An item can be of any type. Can you think of some real-world examples of lists? A shopping list A to-do list A Class list An attendance list …. 6 Lists in Python Lists in Python are like arrays in other languages but much more powerful. To define a list, we use the the square brackets []: # This code assigns a three-element list to x. This is a list of integers x = [1, 2, 3] fruits = ["apples", "oranges", "cherries"] # A list of strings y = [] # an empty list What do you think the following code will output: print(type(x)) >>> list 7 Other examples Unlike other languages, in Python a list can contain elements of different types: # First element is a number, second is a string, third is another list. x = [2, "two", [1, 2, 3]] 8 The len function The most basic built-in list function is the len function, which returns a number of elements in a list: x = [2, "two", [1, 2, 3]] print(len(x)) >>> 3 9 List indices Each item in a list has a unique index that specifies its position. The index of the first item is 0, and the index of the last item is the length of the list minus 1. x = ["first", "second", "third", "fourth"] x >>> 'first' x >>> 'third’ x[len(x)-1] >>> 'fourth’ x[len(x)] IndexError: list index out of range 10 Negative indexes Python indices can be negative. A negative index means start counting from the end, so a -1 = first element from the end, -2 = 2nd element from the end, etc. x = ["first", "second", "third", "fourth"] x[-1] >>> x[len(x)-1] >>> 'fourth' x[-4] >>> x >>> 'first' x[-5] IndexError: list index out of range 11  List Operations: Slicing Sometimes we need to access a portion of a list, this portion is often referred to as a slice. Slicing is the process by which we obtain part of a list. Slicing takes the form Including start and excluding end list[slice_start : slice_end : step] Examples: >>> grades = [60, 70, 50, 80, 100] >>> print(grades [1:3]) [ 70, 50] >>> print(grades[2: ]) -5 -4 -3 -2 -1 [50, 80, 100] >>> print(grades[ :4]) 60 70 50 80 100 [60, 70, 50, 80] 0 1 2 3 4 >>> print(grades[-10:10]) [60, 70, 50, 80,100] >>> print(grades[ :4:2]) #we included a step factor [60,50] #we start at the beginning, and then add 2, move to right direction 12  Slicing: more examples >>> print(grades[1:-1]) [70,50,80] we start at 1, always moving to the right.. -1 marks the end of the last item, so we stop at the element just before it. It makes no difference whether we start at 1 or -4, both will yield the same result because both refer to the same element >>> print(grades[-4:-1]) [70,50,80] -5 -4 -3 -2 -1 >>> print(grades[-5:3]) 60 70 50 80 100 [60,70,50] >>> print(grades[-1:-5:-1]) 0 1 2 3 4 the –ve step at end, means that we reverse the direction from moving from left to right to moving from right to left. [100, 80, 50, 70] >>> print(grades[-2:-5:-2]) #same as print(grades[3:0:-2]) [80, 70] >>> print(grades[2::-1]) #same as print(grades[-3::-1]) [50, 70, 60] >>> print(grades[::-2]) #bonus question! [100,50,60] 13 Indices for Slices x = ["first", "second", "third", "fourth"] print(x[0:3]) >>> ['first', 'second', 'third’] print(x[1:2]) >>> ['second’] print( x[-2:-1]) >>> ['third’] print( x[1:-1]) >>> ['second', 'third'] 14 Modifying lists: Replacing an element x = [1, 2, 3, 4] x = "two" print(x) >>> [1, 'two', 3, 4] 15 Excersise Write a Python function modify_lists(list1, list2) that takes two lists of numbers as arguments. The function should check if both lists are equal in length: If the lists are equal in length, ask the user for a new value to change the first element of list1. If the lists are not equal in length, ask the user for a new value to change the last element of list2. 16  Answer def modify_lists(list1, list2): if len(list1) == len(list2): new_value = int(input("Enter a new value to change the first element of list1: ")) list1 = new_value else: new_value = int(input("Enter a new value to change the last element of list2: ")) list2[-1] = new_value print(f"Modified list1: {list1}") An f-string allows you to embed expressions print(f"Modified list2: {list2}") inside string literals, using curly braces {} # Example usage list1 = [1, 2, 3] list2 = [4, 5, 6] modify_lists(list1, list2) 17