Python Dynamic Data type List & Dictionaries

Editor's Notes

• #2: In this session you will learn to introduction to list , understand the list operation and methods of list , parsing of the lines , object and values and list of arguments. You will also learn to use dictionary , function and methods of dictionary and also difference between lists and dictionaries.
• #3: List is a most versatile datatype available in python. It is defined as a sequence of values. List contain values with comma separated between two square brackets. All the items of the list should be of the same type. List indices start at 0. There are several ways to create a new list. The simplest is to enclose the elements in square brackets. Example >>>list1=[10,20,30,40] >>>list2=['Harish','Suresh','Kunal','Girish] List that contains no elements is called an empty list. We can create an empty list using empty brackets []. Example >>>empty1=[] Here empty1 is the empty list which does not contain any elements. List are mutable because we can change the order of the items. We can reassign an element in a list.
• #4: The common way to traverse the elements of a list with for loop.
• #5: List respond to the "+" operator much like strings they mean concatenation. Example >>>a=[1,2,3] >>>b=[4,5,6] >>>c=a+b >>>print(c) [1,2,3,4,5,6] In the above example when list a and b has been added and stored in c. Then c has displayed the elements of a and b by concatenating. List respond to the "*" operator for repeats a list a given number of times. Example >>>a=[10,20,30] >>>a*4 [10,20,30,10,20,30,10,20,30,10,20,30] In the above example when list a is multiplied by 4 then the elements of list a is printed for four times. Slice operator also work on lists. Using this operator we can see certain range of characters also. >>> alpha=['a','b','c','d','e','f'] >>> alpha[1:3] ['b', 'c'] >>> alpha[:4] ['a', 'b', 'c', 'd'] >>> alpha[3:] ['d', 'e', 'f'] >>> alpha[:] ['a', 'b', 'c', 'd', 'e', 'f'] In the above example first alpha[1:3] will displayed the 1st index to until 3rd index. Then in the next alpha[:4] will show the elements from 0th index to until 4th index. Then in the next alpha[3:] will displayed last three elements of the list. And in alpha[:] will display whole list.
• #6: Python provides methods that operate on lists. The following are some of the methods which are used for manipulating the list. append()- This method is used to add a new element to the end of a list. Example >>> mylist=['A','B','C'] >>> mylist.append('D') >>> print (mylist) ['A', 'B', 'C', 'D'] >>> extend() Method - This method is used to take a list as an argument and appends all the elements. Example >>> mylist1=['A','B','C'] >>> mylist2=['D','E'] >>> mylist1.extend(mylist2) >>> print(mylist1) ['A', 'B', 'C', 'D', 'E'] >>> sort() -This method is used to arrange the elements of the list from low to high. Example >>> mylist1=['D','C','E','B','A'] >>> mylist1.sort() >>> print(mylist1) ['A', 'B', 'C', 'D', 'E']
• #7: There are several ways to delete elements from a list. Using pop() method - If we know the index of the element we want we can use pop(). Example >>> mylist1=['A','B','C'] >>> element=mylist1.pop(1) >>> print(mylist1) ['A', 'C'] >>> print(element) B >>> pop() modifies the list and returns the element that was removed. If we don't provide an index value then it deletes the last element and returns the value. Using del operator - If we do not want to retain the value which we have deleted using an index then we need to use del operator. Example >>> mylist1=['A','B','C'] >>> del mylist1[1] >>> print(mylist1) ['A', 'C'] >>> In the above example we have deleted the 1st index element i.e 'B' from the list. Using remove() method- If we know the element to be removed then we can use remove() method. Example >>> mylist1=['A','B','C'] >>> mylist1.remove('B') >>> print(mylist1) ['A', 'C'] In the above example we have used remove() method to remove the element 'B' from list.
• #8: There are number of built in functions that can be used on lists that allow you to quickly look through a list without writing your own loops. The below is a small example of using these built in functions. Examples: >>> numbers=[10,20,30,40,50] >>> print(len(numbers)) 5 >>> print(max(numbers)) 50 >>> print(min(numbers)) 10 >>> print(sum(numbers)) 150 >>> print(sum(numbers)/len(numbers)) 30.0 >>> From these functions sum() function will work with only numbers. But other functions like max(),len(),min() etc will work with list of strings and other types that can be comparable
• #9: A string is sequence of characters.A list is sequence of values of any type. list() method is used to convert a string to list of characters. Example >>> str='NIIT' >>> ele=list(str) >>> print ele ['N', 'I', 'I', 'T'] split() method is used to break the string into list of words Example >>> str='I am studying in NIIT' >>> t=str.split() >>> print(t) ['I', 'am', 'studying', 'in', 'NIIT'] split() method takes and optional argument called delimiter which specifies the word boundaries. A delimiter is a character which differentiate between one word to another in the below example the character "," is separating the words. Example >>> str='Harish,Sridhar,Kishore' >>> delimiter=',' >>> str.split(delimiter) ['Harish', 'Sridhar', 'Kishore'] join() method is the inverse of split. It takes a list of strings and concatenates the elements.join() is a string method. Example >>> str=['Delhi','Mumbai','Chennai','Kolkata'] >>> delimiter='-' >>> delimiter.join(str) 'Delhi-Mumbai-Chennai-Kolkata' In the above code str list contains four city names with comma separated. Using join() method it will.This method will be executed using a delimiter.
• #10: Usually when we are reading a file we want to do something to the lines other than just printing the whole line. Often we want to find the a criteria based line and then parse the line to find some interesting part of the line. For example if we wanted to print out the day of the week from those lines that start with “From ”? From [email protected] Sat Jan 5 09:14:16 2008 The split method is very effective when faced with this kind of problem. We can write a small program that looks for lines where the line starts with “From ”, split those lines, and then print out the third word in the line: fhand = open(‘MyText.txt') for line in fhand: line = line.rstrip() if not line.startswith('From ') : continue words = line.split() print words[2] The program produces the following output: Sat
• #11: Solution: In order to do this activity, you need to perform the following tasks: Create a Python empty project. Use while() to create the menu. Use append(), remove(), extend(), sort(), reverse() to perform list operation. Task 1: Create a Python empty project: Step 1: Open NetBeans 8.0.2. Step 2: Click File menu. Step 3: Select New Project. Step 4: Select Python from Categories list and Python Project from Projects list. Step 5: Click Next button. Step 6: On New Python Project dialog box set the Project Name as CR_Session08_Activity02. Step 7: Select project Location by clicking Browse button. Step 8: Click Finish button. Task 2: Use while() to create the menu, use append(), remove(), extend(), sort(), reverse() to perform list operation: Step 1: Type the following code: def insertList(nameList): item=raw_input("Enter an item to the list: ") nameList.append(item) def removeList(nameList): item=raw_input("Enter an item you want to remove from the list: ") nameList.remove(item) def sortList(nameList): nameList.sort() def showList(nameList): for item in nameList: print(item) def extendList(nameList): nameList1=[23,34,35] nameList.extend(nameList1) def reverseList(nameList): nameList.reverse() def quitList(): exit() nameList=[] while("true"): print("1. Insert an item into List.") print("2. Remove an item from List.") print("3. Sort items of List.") print("4. Extend List.") print("5. Reverse List.") print("6. Display items from List.") print("7. Quit.") numC=raw_input("Enter your option: ") num=int(numC) if(num==1): insertList(nameList) elif(num==2): removeList(nameList) elif(num==3): sortList(nameList) elif(num==4): extendList(nameList) elif(num==5): reverseList(nameList) elif(num==6): showList(nameList) elif(num==7): quit() Step 2: Run the program.
• #12: We know that obj1 and obj2 both refer to a string, but we don't know whether they refer to the same string. To check whether two variables refer to the same object we need to use is operator. To check whether two variables refer to the same object, we can use the is operator. In the above program we have used the same and we received the output as True. Hence both the objects are pointing to the same string so they are identical
• #13: In this case we would say that the two lists are equivalent, because they have the same elements, but not identical, because they are not the same object. If two objects are identical, they are also equivalent, but if they are equivalent, they are not necessarily identical. Until now, we have been using “object” and “value” interchangeably, but it is more precise to say that an object has a value. If you execute obj1 = [10,20,30], obj1 refers to a list object whose value is a particular sequence of elements. If another list has the same elements, we would say it has the same value.
• #14: Aliased object is mutable then changes made with one alias affect the other. This behavior can be useful at the same time error-prone. It is safer to avoid aliasing when we are working with mutable object. String is an immutable object where aliasing is not a problem.
• #15: When you pass a list to a function, the function gets a reference to the list. If the function modifies a list parameter, the caller sees the change. In the above example, delete_head() method removes the first element from a list. The parameter val and the variable mylist are aliases for the same object.
• #16: Solution: Split
• #17: Solution: Split
• #18: A dictionary is like a list, but more general. In a list, the index positions have to be integer but in a dictionary, the indices can be (almost) any type. We can think of a dictionary as a mapping between a set of indices (which are called keys) and a set of values. Each key maps to a value. The association of a key and a value is called a key-value pair or sometimes an item. The function dict() creates a new dictionary with no items. Because dict() is the name of a built-in function, we should avoid using it as a variable name.
• #20: The curly brackets, {}, represent an empty dictionary. To add items to the dictionary, we can use square brackets. In the above program we have “jjj” variable is a dictionary which is having 3 values. And “ooo” variable does not have any values and hence an empty dictionary
• #21: The function dict creates a new dictionary with no items. Because dict is the name of a built-in function, you should avoid using it as a variable name. >>>eng2sp=dict() >>>print eng2sp The curly brackets, {}, represent an empty dictionary. To add items to the dictionary, you can use square brackets: >>>eng2sp['one']='uno' This line creates an item that maps from the key ’one’ to the value 'uno'. If we print the dictionary again, we see a key-value pair with a colon between the key and value: >>>print eng2sp {'one':'uno'} This output format is also an input format. For example, you can create a new dictionary with three items: >>>eng2sp={'one':'uno','two':'dos','three':'tres'} When we print the may be the following output >>>print eng2sp {'one':'uno','three':'tres','two':'dos'} The order of the key-value pairs is not the same. If we type the same example on another computer we might get other results. So in general the order of items of dictionary is unpredictable.
• #22: The above slide shows the different methods for working with dictionary. Example copy() Method The following example shows the usage of copy() method. >>>dict1={'Name':'Harish','Age':29} >>>dict2=dict1.copy() >>>print('New Dictionary:',str(dict2)) The output will be displayed like the following New Dictionary: {'Age':29,'Name':'Harish'}
• #23: Properties of Dictionary Keys Dictionary values have no restrictions. They can be any arbitrary python object either standard objects or user-defined objects. There are two important points to remember about dictionary keys. 1.More than one entry per key not allowed. Which means no duplicate key is allowed. When duplicate keys encountered during assignment. 2.Keys must be immutable. Which means you can use strings, numbers or tuples as dictionary keys but something like ['key'] is not allowed. keys() method - This method keys() returns a list of all the available keys in the dictionary. values() method - The method values() returns a list of all the values available in a given dictionary.
• #24: items() method The method items() returns a list of dicts (key,value) tuple pairs. The above program will display the key and value pair of the dictionary.
• #25: The above is a sample example to access a particular element of dictionary using a key.
• #26: In the above example it has shown len() function uses. This function will work on dictionaries. This will return the number of key and value pair. There is a snippet of code which displays the same.
• #27: The in operator works differently for dictionaries than for other sequences. The in operator works on dictionaries. It tells whether something appears as a key in the dictionary. The in operator can be used to check whether a value existing in dictionary. To check the values existing in a dictionary we need use values() method. The values() method will return as a List.
• #28: The above example explaining how in operator works on dictionary. Here the in operator is used to check whether a particular key or value exist in the dictionary or not. In operator will return true or false value.
• #29: Get() Method The method get() returns a value for the given key. If key is not available then returns default value None. Syntax : dict.get(key,default=None) The parameter explained in below. key -This is the Key to be searched in the dictionary. default -This is the Value to be returned in case key does not exist. The above snippet of code displays the values by giving the key value using get() method.
• #30: Even though dictionaries are not stored in order, we can write a for loop that goes through all the entries in a dictionary - actually it goes through all of the keys in the dictionary and looks up the values. If a dictionary uses in a for statement, it traverses the keys of the dictionary. In the above code snippet the for loop prints each key and the corresponding value.
• #31: It is an error to reference a key which is not in the dictionary. We can use the in operator to see if a key is in the dictionary. The above examples show how an error comes when we are trying to print a value based on key value which is not existing in a empty dictionary.
• #32: The above slide shows the difference between the list and dictionary. The slides shows the two snippet of code which show the difference between the list and dictionary. The snippet of code shows how to add elements to the list and dictionary and display it. The snippet code also shows how to access a particular element of a list or dictionary.
• #33: Solution: True
• #34: Solution: True
• #35: Solution: In order to do this activity, you need to perform the following tasks: Create a Python empty project. Create a mapping of state to abbreviation, Create a basic set of states and some cities in them and Add some more cities. Task 1: Create a Python empty project: Step 1: Open NetBeans 8.0.2. Step 2: Click File menu. Step 3: Select New Project. Step 4: Select Python from Categories list and Python Project from Projects list. Step 5: Click Next button. Step 6: On New Python Project dialog box set the Project Name as CR_Session10_Exercise01. Step 7: Select project Location by clicking Browse button. Step 8: Click Finish button. Task 2: Create a mapping of state to abbreviation, create a basic set of states and some cities in them, add some more cities: Step 1: Type the following code: # create a mapping of state to abbreviation states = { 'Oregon': 'OR', 'Florida': 'FL', 'California': 'CA', 'New York': 'NY', 'Michigan': 'MI' } # create a basic set of states and some cities in them cities = { 'CA': 'San Francisco', 'MI': 'Detroit', 'FL': 'Jacksonville' } # add some more cities cities['NY'] = 'New York' cities['OR'] = 'Portland' # print out some cities print '-' * 10 print "NY State has: ", cities['NY'] print "OR State has: ", cities['OR'] # print some states print '-' * 10 print "Michigan's abbreviation is: ", states['Michigan'] print "Florida's abbreviation is: ", states['Florida'] # do it by using the state then cities dict print '-' * 10 print "Michigan has: ", cities[states['Michigan']] print "Florida has: ", cities[states['Florida']] # print every state abbreviation print '-' * 10 for state, abbrev in states.items(): print "%s is abbreviated %s" % (state, abbrev) # print every city in state print '-' * 10 for abbrev, city in cities.items(): print "%s has the city %s" % (abbrev, city) Step 2: Run the program.