Python 3

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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

                           # (etc.)

Note that plain 'for' looping over a dict delivers the keys:

for key in mydict:
    print(key)             # prints a, then b, then c...

review: sorting any container with sorted()

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

namelist = ['jo', 'pete', 'michael', 'zeb', 'avram']

slist = sorted(namelist)          # ['avram', 'jo', 'michael', 'pete', 'zeb']

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

sorted_keys = sorted(bowling_scores)

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

for key in sorted_keys:
    print(f'{key}={bowling_scores[key]}')

sorting a dictionary's keys by its values

A special "sort criteria" argument 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 specify an alternate criteria by which we might sort the keys. The .get() method takes a key and returns a value from the dict, which is what we are asking sorted() to do with each key when sorting by value. However, this complex sorting is more advanced a topic than we cabn cover here.

assign multiple values to individual variables

multi-target assignment performs the assignments in one statement

csv_line = "Haddad's,PA,239.50"

row = csv_line.split(',')        # ["Haddad's", 'PA', '239.50']

codata = ["Haddad's", 'PA', '239.50']

company, state, revenue = codata

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

csv_line = 'jk43:23 Marfield Ln.:Plainview:NY:10024'

stuid, street, city, state, zip = csv_line.split(':')

print(stuid)      # 'jk43'
print(city)       # 'Plainview'

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 = {}                 # initialize an
                               # 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()

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