Dictionary

 

Dictionary is a collection of related data PAIRS. For instance, if we want to store the username and age of 5 users, we can store them in a dictionary.

 

To declare a dictionary, you write dictionaryName = {dictionary key : data}, with the requirement that dictionary keys must be unique (within one dictionary). That is, you cannot declare a dictionary like this
myDictionary = {“Peter”:38, “John”:51, “Peter”:13}.

 

This is because “Peter” is used as the dictionary key twice. Note that we use curly brackets { } when declaring a dictionary. Multiple pairs are separated by a comma.

 

Example:

userNameAndAge = {“Peter”:38, “John”:51, “Alex”:13, “Alvin”:“Not Available”}

 

You can also declare a dictionary using the dict( ) method. To declare the userNameAndAge dictionary above, you write

 

userNameAndAge = dict(Peter = 38, John = 51, Alex = 13, Alvin = “Not Available”)

 

When you use this method to declare a dictionary, you use round brackets ( ) instead of curly brackets { } and you do not put quotation marks for the dictionary keys.

 

To access individual items in the dictionary, we use the dictionary key, which is the first value in the {dictionary key : data} pair. For instance, to get John’s age, you write userNameAndAge[“John”]. You’ll get the value 51.

 

To modify items in a dictionary, we write dictionaryName[dictionary key of item to be modified] = new data. For instance, to modify the “John”:51 pair, we write userNameAndAge[“John”] = 21. Our dictionary now becomes userNameAndAge = {“Peter”:38, “John”:21, “Alex”:13, “Alvin”:“Not Available”}.

 

We can also declare a dictionary without assigning any initial values to it. We simply write dictionaryName = { }. What we have now is an empty dictionary with no items in it.

 

To add items to a dictionary, we write dictionaryName[dictionary key] = data. For instance, if we want to add “Joe”:40 to our dictionary, we write userNameAndAge[“Joe”] = 40. Our dictionary now becomes userNameAndAge = {“Peter”:38, “John”:21, “Alex”:13, “Alvin”:“Not Available”, “Joe”:40}

 

To remove items from a dictionary, we write del dictionaryName[dictionary key]. For instance, to remove the “Alex”:13 pair, we write del userNameAndAge[“Alex”]. Our dictionary now becomes userNameAndAge = {“Peter”:38, “John”:21, “Alvin”:“Not Available”, “Joe”:40}

 

Run the following program to see all these in action.

 

#declaring the dictionary, dictionary keys and data can be of different data types

myDict = {“One”:1.35, 2.5:”Two Point Five”, 3:”+”, 7.9:2}

 

#print the entire dictionary

print(myDict)                              

#You’ll get {2.5: 'Two Point Five', 3: '+', 'One': 1.35, 7.9: 2}

#Note that items in a dictionary are not stored in the same order as the way you declare them.

 

#print the item with key = “One”.

print(myDict[“One”])                        

#You’ll get 1.35

 

#print the item with key = 7.9.

print(myDict[7.9])      

#You’ll get 2

 

#modify the item with key = 2.5 and print the updated dictionary

myDict[2.5] = “Two and a Half”

print(myDict)                              

#You’ll get {2.5: 'Two and a Half', 3: '+', 'One': 1.35, 7.9: 2}

            

#add a new item and print the updated dictionary

myDict[“New item”] = “I’m new”            

print(myDict)                              

#You’ll get {'New item': 'I’m new', 2.5: 'Two and a Half', 3: '+', 'One': 1.35, 7.9: 2}

 

#remove the item with key = “One” and print the updated dictionary

del myDict[“One”]                              

print(myDict)      

#You’ll get {'New item': 'I’m new', 2.5: 'Two and a Half', 3: '+', 7.9: 2}

 

For more examples and sample codes of working with a dictionary, you can refer to Appendix D.