Introduction to Python

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Dictionaries: Rankings

dictionary rankings

Dictionaries can be sorted by value to produce a ranking.

• dictionaries, particularly ones that have numeric values, can be sorted by value
• if the value is a quantitative measure for each key, the dict can be used as a ranking
• examples are: students and their grade point averages, companies and their market caps, sports teams and their wins, competing products and their prices

loop through dict keys and values

We loop through keys and then use subscripting to get values.

mydict = {'a': 1, 'b': 2, 'c': 3, 'd': 4}

for key in mydict:         # a
    val =  mydict[key]

    print(key)             # a
    print(val)             # 1
    print()
                           # b
                           # 2

                           # (continues with 'c' and 'd')

• Note that plain 'for' looping over a dict delivers the keys.
• We must then use the key to query the dict to get the value.
• Loop through the dict and receiving each key, we use a subscript to get the value.

Ex. 6.8

review: sorting any container with sorted()

With any container or iterable (list, tuple, file), sorted() returns a list of sorted items.

namelist = ['banana', 'apple', 'dates', 'cherry']

slist = sorted(namelist, reverse=True)

print(slist)          # ['dates', 'cherry', 'banana', 'apple']

Remember that no matter what container is passed to sorted(), the function returns a list. Also remember that the reverse=True argument to sorted() can be used to sort the items in reverse order.

sorting a dict (sorting its keys)

sorted() returns a sorted list of a dict's keys.

bowling_scores = {'bob': 123, 'zeb': 98, 'mike': 202, 'alice': 184}

sorted_keys = sorted(bowling_scores)

print(sorted_keys)       # [ 'alice', 'bob', 'mike', 'zeb' ]

Ex. 6.9

sorting a dictionary's keys by its values

A special argument to sorted() can cause Python to sort a dict's keys by its values.

bowling_scores = {'jeb': 123, 'zeb': 98, 'mike': 202, 'alice': 184}

sorted_keys = sorted(bowling_scores, key=bowling_scores.get)

print(sorted_keys)                 # ['zeb', 'jeb', 'alice', 'mike']

for player in sorted_keys:
    print(f"{player} scored {bowling_scores[player]}")

        ##  zeb scored 98
        ##  jeb scored 123
        ##  alice scored 184
        ##  mike scored 202

The key= argument allows us to instruct sorted() how to sort the keys. The sorting works in part by use of the dict .get() method (discussed later). Passing .get to sorted() causes it to sort by value instead of by key. Ex. 6.10

assign multiple values to individual variables

multi-target assignment allows us to "unpack" the values in a container.

If the container on the right has 3 values, we may unpack them to three named variables.

company, state, revenue = ["Haddad's", 'PA', '239.50']

print(company)      # Haddad's
print(revenue)      # 239.50

But if the values we want are in a CSV line we can split them to a list -- and then assign them using multi-target assignment.

csv_line = "Haddad's,PA,239.50"

company, state, revenue = csv_line.split(',')

print(company)      # Haddad's
print(state)        # PA

Ex. 6.14

build up a dict from two fields in a file

As with all containers, we loop through a data source, select and add to a dict.

ids_names = {}                 # empty dict

fh = open('student_db.txt')

for line in fh:
    stuid, street, city, state, zip = line.split(':')

    ids_names[stuid] = state   # key id is paired to
                               # student's state


print("here is the state for student 'jb29':  ")
print(ids_names['jb29'])        #  NJ

fh.close()

ex. 6.15