Dictionaries

Dictionaries are mappings. Elements are accessed by a key which may be of any immutable type. Tuples may used as a key, but in that case no elements of the tuple may be mutable. Keys must be unique (no duplication).

The key corresponds to a value. The value may be of any type, including mutable types such as lists. The dictionary consists of all key-value pairs. Dictionaries themselves are mutable and may be of any length up to the limits of the system. Dictionaries can be nested, i.e. the value may itself be a dictionary.

Dictionaries are denoted by curly braces {}. A key-value pair is separated by a colon :.

Creating Dictionaries

• Declaring an empty dictionary
  • D={}
  • Note the similarity to an empty list, but we use curly braces.
• Enumerating the key-value pairs:
  • D={"Alice":2341, "Beth":9102, "Cecil":3258}
  • Another option to create a dictionary with initial entries takes the form of a list of tuples passed to the constructor function dict():
    • D=dict([("Alice",2341),("Beth",9102),("Cecil",3258)])
• Adding an entry:
  • D["Dan"]=5837
  • If the key "Dan" is not present, it will be created and the specified value assigned. If it is present already in the dictionary, the value will be replaced.

Dictionary Operations

• Length of the dictionary:
  • len(D)
  • The length is the number of key-value pairs, i.e. the number of keys.
• Delete the key-value entry:
  • del D[k]
• Delete all entries:
  • D.clear()
• Generate an iterator of keys:
  • keys=list(D.keys())
  • Omit the list constructor if all you need is an iterator.
    • for k in D.keys():
    • In a for loop the keys() can be omitted
      • for k in D:
• Generate an iterator of values
  • D.values()
• Analogous to enumerate for lists, we may use items to loop through a dictionary and obtain both the key and value:
  • for k,v in D.items():
• Copy a dictionary:
  • D2=D1.copy()
    • Equating D2=D1 will not copy the dictionary, but as for other mutable types, will just make an alias.

Quiz

What is the difference between

data=[]
data[0]=12
data[1]=4

and

data={}
data[0]=12
data[1]=4

Are both correct? Is either correct?

More Key Handling Methods

D[key] alone results in an error if the key is not in the dictionary. If you must attempt a lookup and do not know whether the key is present, use get instead.

D.get(key)

This returns None by default if the key is not found. It can be set to return a value with an optional argument.

D.get(key,0)

The in operator may also be used

if key in D

or

if key not in D

depending on what you want to do.

Exercises

Type into Spyder or JupyterLab and run

capitals={"Alabama":"Montgomery"}
capitals["Alaska"]="Juneau"
capitals["Arizona"]="Little Rock"
print(list(capitals.keys()))
print("Virginia" in capitals)
print("Arkansas" in capitals)
newstate="Connecticut"
newcapital="Hartford"
if newstate not in capitals:
    capitals[newstate]=newcapital
for key in capitals:
    print("The capital of",key,"is",capitals[key])

Use the following lists to create a dictionary teams where the key is taken from the first list with the value from the second list. Use your dictionary to print the team located in Chicago. Note that there are two teams in Los Angeles so you must make the values a list, most of which will have only one element. Hint: for neat printing, use the join function to create a string from a list.

cities=["Boston","Brooklyn","New York","Philadelphia","Toronto",
        "San Francisco","Los Angeles","Los Angeles","Phoenix",
        "Sacramento","Chicago","Cleveland","Detroit","Indiana",
        "Milwaukee","Dallas","Houston","Memphis","New Orleans",
        "San Antonio","Atlanta","Charlotte","Miami","Orlando",
        "Washington","Denver","Minnesota","Oklahoma City","Portland" 
        "Salt Lake City"]
mascots=["Celtics","Nets","Knicks","76ers","Raptors","Golden State Warriors",
         "Clippers","Lakers","Suns","Kings","Bulls","Cavaliers","Pistons",
         "Pacers","Bucks","Mavericks","Rockets","Grizzlies","Hornets","Spurs",
         "Hawks","Bobcats","Heat","Magic","Wizards","Nuggets","Timberwolves",
         "Thunder","Trail Blazers","Jazz"]

cities=["Boston","Brooklyn","New York","Philadelphia","Toronto",
        "San Francisco","Los Angeles","Los Angeles","Phoenix",
        "Sacramento","Chicago","Cleveland","Detroit","Indiana",
        "Milwaukee","Dallas","Houston","Memphis","New Orleans",
        "San Antonio","Atlanta","Charlotte","Miami","Orlando",
        "Washington","Denver","Minnesota","Oklahoma City","Portland"
        "Salt Lake City"]
mascots=["Celtics","Nets","Knicks","76ers","Raptors","Golden State Warriors",
         "Clippers","Lakers","Suns","Kings","Bulls","Cavaliers","Pistons",
         "Pacers","Bucks","Mavericks","Rockets","Grizzlies","Hornets","Spurs",
         "Hawks","Bobcats","Heat","Magic","Wizards","Nuggets","Timberwolves",
         "Thunder","Trail Blazers","Jazz"]

teams={}

for i,city in enumerate(cities):
    if city not in teams:
        teams[city]=[mascots[i]]
    else:
        teams[city].append(mascots[i])

print("The NBA team in Chicago is the","".join(teams["Chicago"]))

Resources

Documentation is here.