Understanding Python Collections: Lists, Tuples, and Dictionaries

Data structures in Python such as lists, tuples, and dictionaries play a crucial role in storing and organizing data. Lists allow storing a collection of diverse data items, tuples provide immutability, and dictionaries facilitate key-value pair storage. Learn how to declare, initialize, access, and utilize these collection types efficiently in Python.

• Python Collections
• Data Structures
• Lists
• Tuples
• Dictionaries
• Python Basics

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1. Python Collections ISYS 350

2. Collections: Data Structures to Store Many Data Items List is a collection which is ordered (stored in the order of entering the list and indexed) and changeable(data can be modified). Allows duplicate members. Tuple is a collection which is ordered and unchangeable(data not allowed to change). Allows duplicate members. Typically for storing constants. Dictionary is a collection of key:value pairs which is unordered, changeable and indexed. No duplicate members.

3. List A collection allows you to store a group of data items. Data items do not require to be the same data type. For example, we can have a list with strings and numbers. Data items can be easily accessed using index. Note: Python itself does not have array as in other languages. But array is available in Python libraries such as Pandas and Numpy.

4. Declaring a list In Python lists are written with square brackets, and items separated by commas. mylist = [item1, item2, ...] fruitList = ["apple", "orange", "banana"] Data items may have different data types: employeeList=["peter",30,"paul",35,"mary",28] Print the list: print(employeeList)

5. Initializing list with a repetition operator, * scores = [0] * 5 # test scores = [0, 0, 0, 0, 0]

6. Members are indexed with a positive index and a negative index, and can be accessed by either index temps = [48.0, 30.5, 20.2, 100.0, 42.0] Its positive and negative index values temps[0] temps[-5] # returns 48.0 temps[1] temps[-4] # returns 30.5 temps[2] temps[-3] # returns 20.2 temps[3] temps[-2] # returns 100.0 temps[4] temps[-1] # returns 42.0

7. List len() function The number of elements in a list fruitList = ["apple", "orange", "banana"] print(len(fruitList)) # Show 3 Note: Given any list, the last member has index len(list)-1. >>> fruitList = ["apple", "orange", "banana"] >>> lastMember=fruitList[len(fruitList)-1] >>> print (lastMember) banana

8. Accessing list elements with a for loop https://www.w3schools.com/python/python_for_loops.asp A for loop is used for iterating over a collection. In other language, this is called a foreach loop. Syntax: for item in list: statements Example: fruitList = ["apple", "orange", "banana"] for x in fruitList: print(x)

9. Accessing list elements using index fruitList = ["apple", "orange", "banana"] for i in range(len(fruitList)): print(fruitList[i]) Note: using range(len(fruitList) to generate index from 0 to len(fruitList) 1.

10. Example: Compute the sum and average of numbers in a list myGPA = [2.5, 3.2, 3.4, 2.9, 3.6] sumGPA=0 for item in myGPA: sumGPA += item avgGPA = sumGPA / len(myGPA) print ("Average GPA is: " + str(avgGPA))

11. Python list aggregation functions sum() ,len(), max() and min() myGPA = [2.5, 3.2, 3.4, 2.9, 3.6] sumGPA=sum(myGPA) avgGPA = sumGPA / len(myGPA) best=max(myGPA) lowest=min(myGPA) print ("Average GPA is: " + str(avgGPA)) print ('The best score is {} and the lowest score is {}'.format(best,lowest))

12. Future Value Table with a list of years presentValue=float(input("Enter present value: ")) rate=float(input("Enter interest rate: ")) print() print("\tFuture Value Table") print() print("\tPresent value:\t" +"${:,.2f}".format(presentValue)) print("\tInterest rate:\t" +"{:.2%}".format(rate)) print() print("\tYear \t\t Future Value") print("\t__________ \t _______") print() yearlist=[5,7,8,10,12,20] for year in yearlist: futureValue=presentValue*(1+rate)**year print("\t" + str(year)+ "\t\t " + "${:,.2f}".format(futureValue)) print() Note: The years are not separated by fixed interval.

13. You may declare an empty list and add members later using list append method >>> mylist=[] >>> mylist.append(5) >>> mylist.append(10) >>> print (mylist) [5, 10] >>>

14. List methods for modifying a list Example: fruitList = ["apple", "orange", "banana"] append(item): Add an item to the end of list fruitList.append("cherry") ['apple', 'orange', 'banana', 'cherry'] insert(index, item): Add an item in index position fruitList.insert(2,"mango") ['apple', 'orange', 'mango', 'banana', 'cherry'] remove(item): Remove an item fruitList.remove('cherry') ['apple', 'orange', 'mango', 'banana'] pop(index): remove the item at the index fruitList.pop(1)

15. Note: The remove() function only removes the first occurrence of the element. fruitList = ["apple", "orange", "banana","orange"] fruitList.remove("orange") print(fruitList) ['apple', 'banana', 'orange']

16. Range of Index fruitList=['apple', 'orange', 'mango', 'banana', 'cherry'] We can specify a range of indexes by specifying where to start and where to end the range (the first item is position 0) print(thislist[2:4]) ['mango', 'banana'] #Note that the item in position 4 is NOT included print(fruitList[:4]) ['apple', 'orange', 'mango', 'banana'] #Note that the item in position 4 is NOT included print(fruitList[2:]) ['mango', 'banana', 'cherry'] #Note that the item in position 2 is included

17. Range of Negative Indexes fruitList=['apple', 'orange', 'mango', 'banana', 'cherry'] >>> fruitList[-5] 'apple' >>> fruitList[-1] 'cherry' >>> fruitList[-3:-1] ['mango', 'banana']

18. Sort a list The sort() method sorts the list ascending by default. Example: fruitList=['apple', 'orange', 'mango', 'banana', 'cherry'] fruitList.sort() print(fruitList) ['apple', 'banana', 'cherry', 'mango', 'orange'] Descending order: fruitList.sort(reverse=True) ['orange', 'mango', 'cherry', 'banana', 'apple']

19. Note: After sorting, the original lists order is lost permanently How to make a copy of the original list? Use the copy() method: listOld=list.copy() fruitList=['apple', 'orange', 'mango', 'banana', 'cherry'] fruitListOld=fruitList ### not working fruitListOldCopy=fruitList.copy() ### make a copy fruitList.sort() print(fruitList) print(fruitListOld) print(fruitListOldCopy)

20. Example: Three exam scores are stored in a list. Compute the Weighted Average of the three exams using this formula: 60%*highest score +30%*2nd highest score +10%*lowest score examScores=[79,85,65] examScores.sort() weightedAvg=0.6*examScores[2]+0.3*examScores[1]+0.1*examScores[0] print("The weighted avg is: "+str(weightedAvg))

21. Using sum, max and min to solve the same problem without sorting examScores=[79,85,65] weightedAvg=0.6*max(examScores)+0.3*(sum(examScores)- max(examScores)-min(examScores))+0.1*min(examScores) print("The weighted avg is: "+str(weightedAvg)) Note: This solution works for 3 exams.

22. We can use the keyword in to search if an item exists in a list fruitList = ["apple", "orange", "banana"] if "apple" in fruitList: print('in') else: print('not in') if "kiwi" in fruitList: print('in') else: print('not in')

23. A Python string is stored as a list with each character as a member of the list Example: The indices of the characters in the string coding are labeled as: Index 0 1 2 3 4 5 Character c o d i n g Example: David Chao Index 0 1 2 3 4 5 6 7 8 9 Character D a v i d C h a o

24. Search and count a string To determine if a specified item is present in a list use the in keyword. count(): Return the number of times a value appears in the string.

25. Example of using in and count customerName = input('Enter customer name: ').upper() searchChar=input('Enter search character: ').upper() if searchChar in customerName: countChar=customerName.count(searchChar) print("Yes, there are {} character {} in the name".format(countChar,searchChar)) else: print('{} charactr not in the name'.format(searchChar))

26. Search not limited to single character we can search a string customerName = input('Enter customer name: ').upper() searchStr=input('Enter search string: ').upper() if searchStr in customerName: countStr=customerName.count(searchStr) print("Yes, there are {} {} in the name".format(countStr,searchStr)) else: print('{} string not in the name'.format(searchStr))

27. Count how many times a word found in a paragraph using string s count() function paragraph = input('Enter a paragraph: ').upper() searchWord=input('Enter a search word: ').upper() if searchWord in paragraph: countWord=paragraph.count(searchWord) print("There are {} {} in the paragraph.".format(countWord,searchWord)) else: print('{} not in the paragraph'.format(searchWord))

28. Using Split() method for word frequencies analysis Split a string into a list where each word is a list item. txt = "This is a test, this is only a test." txtList = txt.split() print(txtList) ['This', 'is', 'a', 'test,', 'this', 'is', 'only', 'a', 'test.'] Note : test, with , and test. with .

29. Count the number of words in a string: len(list) txt = "This is a test, this is only a test." wordList = txt.split() print("There are: " + str(len(wordList)) + " words in the text")

30. Count how many times a word found in a paragraph using split method and for loop paragraph = input('Enter a paragraph: ').upper() searchWord=input('Enter a search word: ').upper() wordList=paragraph.split() wordCount=0 for word in wordList: if searchWord in word: ### I use in , not == , why? wordCount+=1 print("There are {} {} in the paragraph.".format(wordCount,searchWord)) Enter a paragraph: this is a test, this is only a test Enter a search word: test There are 2 TEST in the paragraph.

31. A list of objects import employeeClass ### employeeClass module defines employee class myEmp1=employeeClass.employee('e1','peter','m','07/04/2020',7000.00) myEmp2=employeeClass.employee('e2','paul','m','12/25/2018',8000.00) myEmp3=employeeClass.employee('e3','mary','f','03/08/2018',7500.00) empList=[myEmp1,myEmp2,myEmp3] ## list of objects for emp in empList: empOut="empID:{} Name:{} Sex:{} Salary:{} Hire date:{}" print(empOut.format(emp.empID,emp.ename,emp.sex,emp.salary,emp.hireDate))

32. Using index to retrieve list members Example: empList=[myEmp1,myEmp2,myEmp3] Each member has an index: 0, I, 2 Len(empList] returns a value of 3 Range(len(empList)) will return 0,1,2

33. Using index to retrieve members import employeeClass ### employeeClass module defines employee class myEmp1=employeeClass.employee('e1','peter','m','07/04/2020',7000.00) myEmp2=employeeClass.employee('e2','paul','m','12/25/2018',8000.00) myEmp3=employeeClass.employee('e3','mary','f','03/08/2018',7500.00) empList=[myEmp1,myEmp2,myEmp3] for i in range(len(empList)): empOut="empID:{} Name:{} Sex:{} Salary:{} Hire date:{}" print(empOut.format(empList[i].empID,empList[i].ename,empList[i].sex,\ empList[i].salary,empList[i].hireDate)) # \ means line continuation Note: Use the . notation to access a property: empList[i].empID

34. Compute employee average salary import employeeClass ### employeeClass module defines employee class myEmp1=employeeClass.employee('e1','peter','m','07/04/2020',7000.00) myEmp2=employeeClass.employee('e2','paul','m','12/25/2018',8000.00) myEmp3=employeeClass.employee('e3','mary','f','03/08/2018',7500.00) empList=[myEmp1,myEmp2,myEmp3] ## list of objects sumSalary=0 for emp in empList: sumSalary+=emp.salary avgSalary=sumSalary/len(empList) print('We have {} employees, and the averge salary is: ${:,.2f}.'\ .format(len(empList),avgSalary))

35. Compute employee average salary using index import employeeClass ### employeeClass module defines employee class myEmp1=employeeClass.employee('e1','peter','m','07/04/2020',7000.00) myEmp2=employeeClass.employee('e2','paul','m','12/25/2018',8000.00) myEmp3=employeeClass.employee('e3','mary','f','03/08/2018',7500.00) empList=[myEmp1,myEmp2,myEmp3] ## list of objects sumSalary=0 for i in range(len(empList)): sumSalary+=empList[i].salary avgSalary=sumSalary/len(empList) print('We have {} employees, and the averge salary is: ${:,.2f}.'\ .format(len(empList),avgSalary))

36. We may pass a list to a function import employeeClass ### employeeClass module defines employee class def avgSalary(empList): sumSalary=0 for emp in empList: sumSalary+=emp.salary avgSalary=sumSalary/len(empList) return avgSalary myEmp1=employeeClass.employee('e1','peter','m','07/04/2020',7000.00) myEmp2=employeeClass.employee('e2','paul','m','12/25/2018',8000.00) myEmp3=employeeClass.employee('e3','mary','f','03/08/2018',7500.00) empList=[myEmp1,myEmp2,myEmp3] ## list of objects avgSalary=avgSalary(empList) print('We have {} employees, and the averge salary is: ${:,.2f}.'\ .format(len(empList),avgSalary))

37. Tuple A tuple is a collection which is ordered and unchangeable. In Python tuples are written with round brackets. thistuple = ("apple", "banana", "cherry") print(thistuple)

38. Dictionary A dictionary is a collection which is unordered, changeable and indexed. In Python dictionaries are written with curly brackets, and they have keys and values. Example: thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964}

39. Accessing Items We can access the items of a dictionary by referring to its key name, inside square brackets. Example: x = thisdict["model"] >>> thisdict = { "brand": "Ford", "model": "Mustang", "year": 1964} >>> x = thisdict["model"] >>> print(x) Mustang >>>

40. Loop Through a Dictionary To retrieve all the keys: for x in thisdict: print(x) To retrieve all the values: for x in thisdict: print(thisdict[x]) Or: for x in thisdict.values(): print(x) To retrieve all key:value pairs: for x, y in thisdict.items(): print(x, y)

41. built-in class attribute __dict__ present all attributes of an object as a dictionary Note: Two underscores on each side import classModule myEmp=classModule.employee('e1','peter','m','07/04/2020',7000.00) print(myEmp.ename) myEmpDict=myEmp.__dict__ print(myEmpDict["ename"])

42. JSON Object Java Script Object Notation https://www.w3schools.com/js/js_json_intro.asp JSON object: A JSON object contains a set of key-value pairs separated by commas and enclosed within { and } characters. keys must be strings, written with double quotes: {"k1": "value", "k2": 10}