Python, Theory

Python. Working with dictionaries. Built-in functions list(), zip(). Dictionary bypass. Dictionary generators. Sorting. Combination of dictionaries with sets




Working with dictionaries. Built-in functions list(), zip(). Dictionary bypass. Dictionary generators. Sorting. Combination of dictionaries with sets

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1. Built-in list() function. Example of using with dictionaries

Using the built-in list() function allows you to get lists of values, keys and pairs (key: value). To create an appropriate list using list() you need to use one of the following methods:

  • the values() method allows you to get a list of dictionary values;
  • the keys() method allows you to get a list of dictionary keys;
  • the items() method allows you to get a list of pairs (key:value).

More details about using the values(), keys(), items() methods are described here.

Example.

# Work with dictionaries
# Built-in list() method

# Source dictionary
Figures = { 1:'Rectangle', 2:'Triangle', 3:'Circle' }

# 1. Get a list of dictionary values
values = list(Figures.values())
print(values) # ['Rectangle', 'Triangle', 'Circle']

# 2. Get a list of dictionary keys
keys = list(Figures.keys()) # keys = [1, 2, 3, 4]
print(keys)

# 3. Get a list of pairs (key:value)
pairs = list(Figures.items())
print(pairs) # [(1,'Rectangle'),(2,'Triangle'),(3,'Circle')]

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2. Built-in zip() function. Example of creation a dictionary

The zip() function allows you to create a dictionary by combining lists of keys and values.

Example.

# Dictionaries. Function zip()

# Creating a dictionary from lists of keys and values
Numbers = dict(zip([1, 2, 3], ['One', 'Two', 'Three']))
print(Numbers) # {1: 'One', 2: 'Two', 3: 'Three'}

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3. Bypassing the dictionary using for loop. Example

The example demonstrates bypassing a dictionary using a for loop and displaying all pairs (key: value).

Example.

# Work with dictionaries
# Bypassing the dictionary using a for loop

# Source dictionary
Months = { 1:'Jan', 2:'Feb', 3:'Mar',
           4:'Apr', 5:'May', 6:'Jun',
           7:'Jul', 8:'Aug', 9:'Sep',
           10:'Oct', 11:'Nov', 12:'Dec' }

# The loop for of bypass the dictionary
# in loop: mn - key, Months[mn] - value
for mn in Months:
    print(mn, ': ', Months[mn])

The result of the program

1 : Jan
2 : Feb
3 : Mar
4 : Apr
5 : May
6 : Jun
7 : Jul
8 : Aug
9 : Sep
10 : Oct
11 : Nov
12 : Dec

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4. Using dictionaries for sparse data structures. Example

Dictionaries can be used effectively to represent sparse data structures. Two-dimensional arrays in which only a few elements have certain values can be examples of such data.

Example. The example demonstrates the representation of a sparse matrix of numbers in the form of a dictionary. Let a matrix be given

Sparse matrix of numbers,

which must be presented in the form of a dictionary. The solution to the problem is as follows:

# Dictionaries. Representation of sparse data structures

# Matrix M
M = {} # create an empty dictionary
M[(0, 1)] = 1 # key=(0,1): value=1
M[(1, 2)] = 3 # pair (key:value) = ((1,2):3)
M[(2, 0)] = 2 # pair (key:value) = ((2,0):2)

print(M) # {(0, 1): 1, (1, 2): 3, (2, 0): 2}

In the above example, dictionary keys are tuples that contain the coordinates of non-empty elements. So, the key value (2, 0) means row 2, column 0.

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5. Creating dictionaries using dictionary generators. Examples

Dictionaries can be created using dictionary generators. In this case, one of the following methods is used:

  • generate a dictionary based on a given expression;
  • generate a dictionary based on an iterated object;
  • initialize the dictionary from key lists;
  • generate a dictionary using the zip() function.

The example demonstrates the use of all methods.

Example.

# Dictionaries. Dictionary generators

# 1. Generate a dictionary based on a given expression
# Pairs are generated (key: value) = (i: 5*i)
A = { i: i*5 for i in [10,20,30,40] }
print(A) # {10: 50, 20: 100, 30: 150, 40: 200}

# 2. Generate a dictionary by an iterable object (string)
s = 'Hello'
B = { sym: sym*3 for sym in s } # пара (sym:sym*3)
print(B) # {'H': 'HHH', 'e': 'eee', 'l': 'lll', 'o': 'ooo'}

numbers = [ 15, 25, 30 ]
C = { num: str(num) for num in numbers }
print(C) # {15: '15', 25: '25', 30: '30'}

# 3. Initializing a Dictionary from a Key List.
# Fill the list of keys with the value
# entered from the keyboard

value = int(input('Input value: '))
listKeys = [ 1, 2, 3, 4 ]
D = { lk:value for lk in listKeys }
print(D)

# Fill key list with None value
value = None
listKeys = [ 'a', 'b', 'c' ]
E = { lk:value for lk in listKeys }
print(E) # {'a': None, 'b': None, 'c': None}

# 4. Generate a dictionary using the zip() function
WMonths = [ 'Dec', 'Jan', 'Feb']
NumMonths = [ 12, 1, 2 ]
F = { nm:wm for (nm,wm) in zip(NumMonths,WMonths) }
print(F) # {12: 'Dec', 1: 'Jan', 2: 'Feb'}

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6. Sorting in dictionary by keys. Example

As you know, the keys in the dictionary are stored in random order. If it becomes necessary to sort the dictionary by keys, then you can use the sort() method, which is used for lists. To do this, you first need to convert the representation of the keys into a list.

Example.

# Dictionaries. Sorting by keys

# Source dictionary
A = { 'f':10, 'а':2, 'с':17 }

# Sorting by keys
# 1. Get a keys view
ak = A.keys() # ak = dict_keys(['f', 'а', 'с'])

# 2. Convert ak to list
list_ak = list(ak) # list_ak = ['f', 'а', 'с']

# 3. Sort a list of keys - sort() function
list_ak.sort() # ['а', 'с', 'f']

# 4. Print the list of (key: value)
# in sorted order by keys,
# form a new dictionary
B = {}
for k in list_ak:
    print('(', k, ': ', A[k], ')')
    B[k] = A[k]

print(B) # {'a': 2, 'c': 17, 'f': 10}

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7. An example of using dictionary key lists in set operations

Based on the keys of the dictionary, you can create lists. These key lists can be used in operations on sets that form a new set. In other words, you can mix different mapping objects (sets, dictionaries) with each other using the appropriate operations.

In order for the keys to be used in operations on sets, you must first convert them to lists using the keys() method. More details about using the keys() method are described here.

Example. The example demonstrates the interaction of dictionaries and sets using operations on the sets |, &, ^.

# Dictionaries. Combining dictionaries and sets

# Source dictionary
A = { 1:'One', 2:'Two', 3:'Three', 4:'Four', 5:'Five' }

# Combining key mapping and sets,
# we get a new set

# 1. Get key mapping
keys = A.keys() # keys = dict_keys([1, 2, 3, 4, 5])

# 2. Generate a new set of keys that contains the number 2
B = keys & {2} # B = {2} - a new set

# 3. Generate a new set of keys that contains the numbers 3, 5
C = keys {3, 5} # {1, 2, 4}

# 4. Add the numbers 8, 11 to the set of keys
D = keys | { 8, 11 } # D = {1, 2, 3, 4, 5, 8, 11}

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Related topics

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