Data Structures or Derived data Types in Python:

Storing multiple values of same or different types under a single name. They are collection of data.

List
Tuple
String
Dictionary
Set
In this notebook, we’ll discuss indepth on Dictionaries

Dictionary:

Dictionaries are unordered collection of data. They are key-value mapping.
Represented by Curly braces {}

dictionary : {1: 10, 2: 20, 3: 30, 4: 'Hello', 5: -2.0}

Defined by - key : value.
Each key-value map is separated by comma (,)

Keys	1	2	3	4	5
Values	10	20	30	40	50

Defining a dictionary:

’{ }’
dict()

1. ‘{}’

dict1 = {1:10, 2:20, 3:30}

2. dict()

Syntax:

dict(iterable = None)

iterable: any data structures listed above given that elements are present as key value mapping.

IN [1]

d1 = {}
type(d1)

IN [4]

l = [(1, 10), (2, 20), (3, 35)]
d2 = dict(l)
print(d2)
type(d2)

stdout

{1: 10, 2: 20, 3: 35}

Methods or Functions in dictionaries:

There are various methods or functions that are used to work with dictionaries.
1. len
2. str
3. clear
4. copy
5. fromkeys
6. get
8. items
9. keys
10. values
11. update
12. pop 13. popitem 10. setdefault

1. len:

Returns the length of the given iterable.

Syntax:

len(iterable)

IN [5]

d1 = {1: 12, 2: 23, 3: 34}
len(d1)

2. str:

Returns the string format of the given dictionary.

Syntax:

str(dict)

IN [6]

d1 = {1: 12, 2: 23, 3: 34}
str(d1)

3. clear:

Deletes the items in the dictionary.

Syntax:

dict.clear()

IN [7]

d1 = {1: 12, 2: 23, 3: 34}
d1.clear()
d1

4. copy:

Returns the shallow copy of the given dictionary.

Syntax:

dict.copy()

IN [8]

d1 = {1: 12, 2: 23, 3: 34}
d2 = d1.copy()
d2

5. fromkeys:

Returns a new dictionary form the given list of keys. By default the values are set to None.

Syntax:

dict.fromkeys(sep, val = None)

IN [12]

key = ['key1', 'key2', 'key3']
d2 = dict.fromkeys(key)
d2

IN [16]

key = ['key1', 'key2', 'key3']
d2 = dict.fromkeys(key, 0)
d2

6. get:

Returns the value of the given key if present else returns the default value given (None by Default).

Syntax:

dict.get(key, default = None)

IN [17]

d1 = {1: 12, 2: 23, 3: 34}
d1.get(1)

IN [19]

d1 = {1: 12, 2: 23, 3: 34}
print(d1.get(4))

stdout

None

IN [20]

d1 = {1: 12, 2: 23, 3: 34}
d1.get(4, 0)

7. items:

Returns the list of key-value pairs in the dictionary.

Syntax:

dict.items()

IN [25]

d1 = {1: 12, 2: 23, 3: 34}
d1.items()

8. keys:

Returns the list of keys present in the dictionary.

Syntax:

dict.keys()

IN [24]

d1 = {1: 12, 2: 23, 3: 34}
d1.keys()

9. values:

Returns the list of values present on the dictionary.

Syntax:

dict.values()

IN [26]

d1 = {1: 12, 2: 23, 3: 34}
d1.values()

10. update:

Adds the key-value pairs in second dictionary to the first.

Syntax:

dict.update(dict)

IN [27]

d1 = {1: 12, 2: 23, 3: 34}
d2 = {4: 45, 5: 56}
d1.update(d2)
d1

11. pop:

Removes and returns the value of given key. If the key is absent, Error is raised if default is not given.

Syntax:

dict.pop(key, [default])

IN [28]

d1 = {1: 12, 2: 23, 3: 34}
print(d1.pop(2))
d1

stdout

IN [31]

d1 = {1: 12, 2: 23, 3: 34}
print(d1.pop(4))
d1

Error


Traceback (most recent call last):

  File "D:\Programming\Python\Jupyter Notebooks\Python_Programming_Notes\temp.py", line 2, in <module>

    print(d1.pop(4))

KeyError: 4

IN [30]

d1 = {1: 12, 2: 23, 3: 34}
print(d1.pop(4, 30))
d1

stdout

12. popitem:

Removes and returns the last key-value pair of the dictionary.

Syntax:

dict.popitem()

IN [32]

d1 = {1: 12, 2: 23, 3: 34}
print(d1.popitem())
d1

stdout

(3, 34)

13. setdefault:

Sets the default value to key if the key is absent.

Syntax:

dict.setdefault(key, value)

IN [43]

d1 = {1: 12, 2: 23, 3: 34}
d1.setdefault(4, 45)
d1

Accessing elements:

The values in the dictionary are accessed by key.

Syntax:

dictionary[key]

IN [33]

d1 = {1: 12, 2: 23, 3: 34}
d1[2]

IN [35]

d1 = {1: 12, 22: 23, 3: 34}
d1[22]

IN [36]

d1 = {1: 12, 2: 23, 3: 34}
d1[4]

Error


Traceback (most recent call last):

  File "D:\Programming\Python\Jupyter Notebooks\Python_Programming_Notes\temp.py", line 2, in <module>

    d1[4]

KeyError: 4

Adding or Modifying items:

Items can be added or modified by using keys.

If the key already exists, the value is replcaed by the new value. If not, new key-value pair is created

Syntax:

dictaionary[key] = value

IN [38]

d1 = {1: 12, 2: 23, 3: 34}
d1[2] = 24 # modify
d1[4] = 45 # add
d1

Delete Elements:

Elements can be deleted by using the del keyword.

Syntax:

del dict[key]

IN [40]

d1 = {1: 12, 2: 23, 3: 34}
del d1[2], d1[3]
d1

Printing Items:

Use for loop to iterate through the dictionary.

IN [45]

d1 = {1: 12, 2: 23, 3: 34}
for key, val in d1.items():
    print(key, val)

stdout

1 12
2 23
3 34

Sorting a dictionary:

Dictionaries are sorted only by keys.

IN [51]

d1 = {1: 12, 2: 23, 3: 34}
for i in sorted(d1.keys()):
    print(i, d1[i])
print()
for i in sorted(d1):
    print(i, d1[i])

stdout

Nested Dictionaries:

Dictionaries inside a dictionary is nested dictionary.

IN [52]

d1 = {1: 12, 2: 23, 'd': {1: 12, 2: 23, 3: 34}}
for i,j in d1.items():
    print(i,j)

stdout

1 12
2 23
d {1: 12, 2: 23, 3: 34}

Dictionaries and list Comprehension:

Dictionaries can be defined by comprehensions.

Syntax:

dict = {key: value (loops)}

IN [53]

d2 = {x: x**2 for x in range(10)}
d2

Dictionaries for switch case:

Dictionaries can be used as switch cases.

IN [55]

switch = {1: 'One',
          2: 'Two', 
          3: 'Three'}
switch[1]