Python Lists and Tuples - CSM2170 PDF

Starting out with Python Fourth Edition Chapter 7 Lists and Tuples Reham Hamdy Abou-Zaid, 2024  Topics Sequences Introduction to Lists List Slicing Finding Items in Lists with the in Operator List Methods and Useful Built-in Functions ...

Starting out with Python Fourth Edition Chapter 7 Lists and Tuples Reham Hamdy Abou-Zaid, 2024  Topics Sequences Introduction to Lists List Slicing Finding Items in Lists with the in Operator List Methods and Useful Built-in Functions  Topics Copying Lists Processing Lists List Comprehensions Two-Dimensional Lists Tuples Plotting List Data with the matplotlib Package Sequences Sequence: an object that contains multiple items of data The items are stored in sequence one after another Python provides different types of sequences, including lists and tuples The difference between these is that a list is mutable and a tuple is immutable Lists Introduction to Lists (1 of 2) List: an object that contains multiple data items Element: An item in a list Format: list = [item1, item2, etc.] Can hold items of different types print function can be used to display an entire list numbers = [5, 10, 15, 20] print(numbers) list() function can convert certain types of objects to lists numbers = list(range(5)) Output: [0, 1, 2, 3, 4] Introduction to Lists (2 of 2) A list of integers A list of strings A list holding different types The Repetition Operator and Iterating over a List Repetition operator: makes multiple copies of a list and joins them together The * symbol is a repetition operator when applied to a sequence and an integer Sequence is left operand, number is right General format: list * n You can iterate over a list using a for loop Format: for x in list: Indexing Index: a number specifying the position of an element in a list Enables access to individual element in list Index of first element in the list is 0, second element is 1, and n’th element is n-1 Negative indexes identify positions relative to the end of the list The index -1 identifies the last element, -2 identifies the next to last element, etc. Indexing Indexing Indexing The len function An IndexError exception is raised if an invalid index is used len function: returns the length of a sequence such as a list Example: size = len(my_list) Returns the number of elements in the list, so the index of last element is len(list)-1 Can be used to prevent an IndexError exception when iterating over a list with a loop The len function Can be used to prevent an IndexError exception when iterating over a list with a loop Lists Are Mutable Mutable sequence: the items in the sequence can be changed Lists are mutable, and so their elements can be changed An expression such as list = new_value can be used to assign a new value to a list element Lists Are Mutable Must use a valid index to prevent raising of an IndexError exception Concatenating Lists Concatenate: join two things together The + operator can be used to concatenate two lists – Cannot concatenate a list with another data type, such as a number The += augmented assignment operator can also be used to concatenate lists Concatenating Lists Concatenating Lists List Slicing (1 0f 8) Slice: a span of items that are taken from a sequence List slicing format: list[start : end] Span is a list containing copies of elements from start up to, but not including, end If start not specified, 0 is used for start index If end not specified, len(list) is used for end index Slicing expressions can include a step value and negative indexes relative to end of list List Slicing (2 0f 8) List Slicing (3 0f 8) List Slicing (4 0f 8) List Slicing (5 0f 8) List Slicing (6 0f 8) List Slicing (7 0f 8) List Slicing (8 0f 8) Finding Items in Lists with the in Operator You can use the in operator to determine whether an item is contained in a list General format: item in list Returns True if the item is in the list, or False if it is not in the list Similarly, you can use the not in operator to determine whether an item is not in a list Finding Items in Lists with the in Operator List Methods and Useful Built-in Functions append(item): used to add items to a list – item is appended to the end of the existing list List Methods and Useful Built-in Functions List Methods and Useful Built-in Functions index(item): used to determine where an item is located in a list Returns the index of the first element in the list containing item Raises ValueError exception if item not in the list List Methods and Useful Built-in Functions List Methods and Useful Built-in Functions insert(index, item): used to insert item at position index in the list sort(): used to sort the elements of the list in ascending order remove(item): removes the first occurrence of item in the list reverse(): reverses the order of the elements in the list List Methods and Useful Built-in Functions insert(index, item): used to insert item at position index in the list List Methods and Useful Built-in Functions sort(): used to sort the elements of the list in ascending order Output: List Methods and Useful Built-in Functions remove(item): removes the first occurrence of item in the list List Methods and Useful Built-in Functions reverse(): reverses the order of the elements in the list List Methods and Useful Built-in Functions Method Description append(item) Adds item to the end of the list. index(item) Returns the index of the first element whose value is equal to item. A ValueError exception is raised if item is not found in the list. insert(index, item) Inserts item into the list at the specified index. When an item is inserted into a list, the list is expanded in size to accommodate the new item. The item that was previously at the specified index, and all the items after it, are shifted by one position toward the end of the list. No exceptions will occur if you specify an invalid index. If you specify an index beyond the end of the list, the item will be added to the end of the list. If you use a negative index that specifies an invalid position, the item will be inserted at the beginning of the list. sort() Sorts the items in the list so they appear in ascending order (from the lowest value to the highest value). remove(item) Removes the first occurrence of item from the list. A ValueError exception is raised if item is not found in the list. reverse() Reverses the order of the items in the list. List Methods and Useful Built-in Functions del statement: removes an element from a specific index in a list General format: del list[i] List Methods and Useful Built-in Functions min and max functions: built-in functions that returns the item that has the lowest or highest value in a sequence The sequence is passed as an argument Copying Lists (1 of 3) To make a copy of a list you must copy each element of the list Two methods to do this: Creating a new empty list and using a for loop to add a copy of each element from the original list to the new list Creating a new empty list and concatenating the old list to the new empty list Copying Lists (2 of 3) You can copy a list to another one by assigning the first list to a second list. Note that after this code executes, both variables list1 and list2 will reference the same list in memory. Copying Lists (3 of 3) Copying Lists (2 of 2) Processing Lists (1 of 5) List elements can be used in calculations To calculate total of numeric values in a list use loop with accumulator variable To average numeric values in a list: Calculate total of the values Divide total of the values by len(list) List can be passed as an argument to a function Processing Lists (2 of 5) A function can return a reference to a list “We will get to this later after we discuss chapters 5 & 6” To save the contents of a list to a file: Use the file object’s writelines method Does not automatically write \n at then end of each item Use a for loop to write each element and \n To read data from a file use the file object’s readlines method Processing Lists (3 of 5) Processing Lists (4 of 5) Processing Lists (5 of 5) List Comprehensions (1 of 7) List comprehension: a concise expression that creates a new list by iterating over the elements of an existing list. List Comprehensions (2 of 7) The following code uses a for loop to make a copy of a list: list1 = [1, 2, 3, 4] list2 = [] for item in list1: list2.append(item) The following code uses a list comprehension to make a copy of a list: list1 = [1, 2, 3, 4] list2 = [item for item in list1] List Comprehensions (3 of 7) The iteration expression works like a for loop In this example, it iterates over the elements of list1 Each time it iterates, the target variable item is assigned the value of an element. At the end of each iteration, the value of the result expression is appended to the new list. List Comprehensions (4 of 7) list1 = [1, 2, 3, 4] list2 = [item**2 for item in list1] After this code executes, list2 will contain the values [1, 4, 9, 16] List Comprehensions (5 of 7) str_list = ['Winken', 'Blinken', 'Nod'] len_list = [len(s) for s in str_list] After this code executes, len_list will contain the values [6, 7, 3] List Comprehensions (6 of 7) You can use an if clause in a list comprehension to select only certain elements when processing a list list1 = [1, 12, 2, 20, 3, 15, 4] list2 = [] for n in list1: if n < 10: list2.append(n) Works the same as… list1 = [1, 12, 2, 20, 3, 15, 4] list2 = [item for item in list1 if item < 10] List Comprehensions (7 of 7) list1 = [1, 12, 2, 20, 3, 15, 4] list2 = [item for item in list1 if item < 10] After this code executes, list2 will contain [1, 2, 3, 4] Two-Dimensional Lists Two-dimensional list: a list that contains other lists as its elements Also known as nested list Common to think of two-dimensional lists as having rows and columns Useful for working with multiple sets of data To process data in a two-dimensional list need to use two indexes Typically use nested loops to process Two-Dimensional Lists Two-dimensional list with three rows and three columns Two-Dimensional Lists (3 of 3) Tuples Tuples Tuple: an immutable sequence Very similar to a list Once it is created it cannot be changed Format: tuple_name = (item1, item2) Tuples Tuples support operations as lists Subscript indexing for retrieving elements Methods such as index Built in functions such as len, min, max Slicing expressions The in, +, and * operators Tuples Tuples do not support the methods: append remove insert reverse sort Tuples Tuples Advantages for using tuples over lists: Processing tuples is faster than processing lists Tuples are safe Some operations in Python require use of tuples list() function: converts tuple to list tuple() function: converts list to tuple Converting Tuple to List, And List to Tuple matplotlib Plotting Data with matplotlib (1 of 4) The matplotlib package is a library for creating two-dimensional charts and graphs. It is not part of the standard Python library, so you will have to install it separately, after you have installed Python on your system. Plotting Data with matplotlib (2 of 4) Watch this YouTube video for detailed instructions for Windows: https://youtu.be/4dCKMnXpcj0?si=jAddhqVexvjSmo4b Watch this YouTube video for detailed instructions for Mac: https://youtu.be/aKsSylN38Ok?si=BUE-t1hryVH9C8YQ Plotting Data with matplotlib (3 of 4) To verify the package was installed, start IDLE and enter this command: >>> import matplotlib If you don't see any error messages, you can assume the package was properly installed. Plotting Data with matplotlib (4 of 4) The matplotlib package contains a module named pyplot that you will need to import. Use the following import statement to import the module and create an alias named plt: import matplotlib.pyplot as plt For more information about the import statement, see Appendix E in your textbook. Plotting a Line Graph with the plot Function (1 of 4) Use the plot function to create a line graph that connects a series of points with straight lines. The line graph has a horizontal X axis, and a vertical Y axis. Each point in the graph is located at a (X,Y) coordinate. Plotting a Line Graph with the plot Function (2 of 4) Program 7-20 (line_graph1.py) 1 # This program displays a simple line graph. 2 import matplotlib.pyplot as plt 3 4 def main(): 5 # Create lists with the X and Y coordinates of each data point. 6 x_coords = [0, 1, 2, 3, 4] 7 y_coords = [0, 3, 1, 5, 2] 8 9 # Build the line graph. 10 plt.plot(x_coords, y_coords) 11 12 # Display the line graph. 13 plt.show() 14 15 # Call the main function. 16 if _ _name_ _ == '_ _main_ _': 17 main() Plotting a Line Graph with the plot Function (3 of 4) You can change the lower and upper limits of the X and Y axes by calling the xlim and ylim functions. Example: plt.xlim(xmin=1, xmax=100) plt.ylim(ymin=10, ymax=50) This code does the following: Causes the X axis to begin at 1 and end at 100 Causes the Y axis to begin at 10 and end at 50 Plotting a Line Graph with the plot Function (3 of 4) You can customize each tick mark’s label with the xticks and yticks functions. These functions each take two lists as arguments. The first argument is a list of tick mark locations The second argument is a list of labels to display at the specified locations. plt.xticks([0, 1, 2, 3, 4], ['2016', '2017', '2018', '2019', '2020']) plt.yticks([0, 1, 2, 3, 4, 5], ['$0m', '$1m', '$2m', '$3m', '$4m', '$5m']) Program 7-22 Output Program 7-22 Displayed by the Displayed by the title() function. yticks() function. Displayed by the ylabel() function. Displayed by the xticks() function. Displayed by the xlabel() function. Plotting a Bar Chart (1 of 6) Use the bar function in the matplotlib.pyplot module to create a bar chart. The function needs two lists: one with the X coordinates of each bar’s left edge, and another with the heights of each bar, along the Y axis. Plotting a Bar Chart (2 of 6) left_edges = [0, 10, 20, 30, 40] heights = [100, 200, 300, 400, 500] plt.bar(left_edges, heights) plt.show() Plotting a Bar Chart (3 of 6) The default width of each bar in a bar graph is 0.8 along the X axis. You can change the bar width by passing a third argument to the bar function. left_edges = [0, 10, 20, 30, 40] heights = [100, 200, 300, 400, 500] bar_width = 5 plt.bar(left_edges, heights, bar_width) plt.show() Plotting a Bar Chart (3 of 6) The bar function has a color parameter that you can use to change the colors of the bars. The argument that you pass into this parameter is a tuple containing a series of color codes. Color Code Corresponding Color 'b' Blue 'g' Green 'r' Red 'c' Cyan 'm' Magenta 'y' Yellow 'k' Black 'w' White Plotting a Bar Chart (4 of 6) Example of how to pass a tuple of color codes as a keyword argument: plt.bar(left_edges, heights, color=('r', 'g', 'b', 'w', 'k')) The colors of the bars in the resulting bar chart will be as follows: The first bar will be red. The second bar will be green. The third bar will be blue. The fourth bar will be white. The fifth bar will be black. Plotting a Bar Chart (5 of 6) Example of how to pass a tuple of color codes as a keyword argument: plt.bar(left_edges, heights, color=('r', 'g', 'b', 'w', 'k')) The colors of the bars in the resulting bar chart will be as follows: The first bar will be red. The second bar will be green. The third bar will be blue. The fourth bar will be white. The fifth bar will be black. Plotting a Bar Chart (6 of 6) Use the xlabel and ylabel functions to add labels to the X and Y axes. Use the xticks function to display custom tick mark labels along the X axis. Use the yticks function to display custom tick mark labels along the Y axis. Plotting a Pie Chart (1 of 4) You use the pie function in the matplotlib.pyplot module to create a pie chart. When you call the pie function, you pass a list of values as an argument. The sum of the values will be used as the value of the whole. Each element in the list will become a slice in the pie chart. The size of a slice represents that element's value as a percentage of the whole. Plotting a Pie Chart (2 of 4) Example values = [20, 60, 80, 40] plt.pie(values) plt.show() Plotting a Pie Chart (3 of 4) The pie function has a labels parameter that you can use to display labels for the slices in the pie chart. The argument that you pass into this parameter is a list containing the desired labels, as strings. Plotting a Pie Chart (4 of 4) Example sales = [100, 400, 300, 600] slice_labels = ['1st Qtr', '2nd Qtr', '3rd Qtr', '4th Qtr'] plt.pie(sales, labels=slice_labels) plt.title('Sales by Quarter') plt.show() Plotting a Pie Chart The pie function automatically changes the color of the slices, in the following order: blue, green, red, cyan, magenta, yellow, black, and white. You can specify a different set of colors, however, by passing a tuple of color codes as an argument to the pie function’s colors parameter: plt.pie(values, colors=('r', 'g', 'b', 'w', 'k')) When this statement executes, the colors of the slices in the resulting pie chart will be red, green, blue, white, and black. Summary This chapter covered: Lists, including: Repetition and concatenation operators Indexing Techniques for processing lists Slicing and copying lists List methods and built-in functions for lists Two-dimensional lists Tuples, including: Immutability Difference from and advantages over lists Plotting charts and graphs with the matplotlib Package