Python - Dictionary Questions

Question 1

Dictionary?

Answer 1

In python, dictionary is similar to hash or maps in other languages. It consists of key value pairs. The value can be accessed by unique key in the dictionary.

Question 2

Create a new dictionary

Answer 2

# Create a new dictionary 
d = dict() 

# or you can do 
d = {}

Question 3

Add a key - value pairs to dictionary

Answer 3

# Add a key - value pairs to dictionary
d['xyz'] = 123
d['abc'] = 345

returns {‘xyz’: 123, ‘abc’: 345}

Question 4

print the whole dictionary

Answer 4

# print the whole dictionary
print(d)

Question 5

print only the keys

Answer 5

# print only the keys
print(d.keys())

returns ['key1', 'key2',...]
#notice the parens around the keys

Question 6

print only values

Answer 6

# print only values
print(d.values())

returns [value1, value2]
#notice the lack of parens around the values

Question 7

iterate over dictionary

Answer 7

# iterate over dictionary
for i in d :
    print("%s  %d" %( i, d[i] ) )

Question 8

another way of iteration over a dictionary

Answer 8

# another method of iteration
for index, value in enumerate(d):
    print (index, value , d[value])

Question 9

check if key exist

Answer 9

# check if key exist
print('xyz' in d)

Question 10

delete the key-value pair

Answer 10

# delete the key-value pair
del d['xyz']

Question 11

Dict.cmp()

Answer 11

Dict.cmp(): Compares elements of both dict.

Question 12

Dict.len()

Answer 12

Dict.len(): Gives the total length of the dictionary.

Question 13

Dict.str()

Answer 13

Dict.str(): Produces a printable string representation of a dictionary.

Question 14

Dict.type()

Answer 14

Dict.type(): Returns the type of the passed variable

Question 15

Dict.clear()

Answer 15

Dict.clear(): Removes all elements of dictionary dict

Question 16

Dict.copy()

Answer 16

Dict.copy(): Returns a shallow copy of dictionary dict

Question 17

Dict.fromkeys()

Answer 17

Dict.fromkeys(): Create a new dictionary with keys from seq and values set to value.

Question 18

Dict.get()

Answer 18

Dict.get(): For key, returns value or default if key not in dictionary

Dict.get(key, default=None)

Question 19

Dict.has_key()

Answer 19

Dict.has_key(): Returns true if key in dictionary dict, false otherwise

Question 20

Dict.items()

Answer 20

Dict.items(): Returns a list of dict’s (key, value) tuple pairs

Question 21

Dict.keys()

Answer 21

Dict.keys(): Returns list of dictionary dict’s keys

Question 22

Dict.setdefault()

Answer 22

Dict.setdefault(): Set dict[key]=default if key is not already in dict

Question 23

Dict.update()

Answer 23

Dict.update(): Adds dictionary dict2’s key-values pairs to dict

Question 24

Dict.values()

Answer 24

Dict.values(): Returns list of dictionary dict’s values

Question 25

dic = {“A”:1, “B”:2}

How would we access the above values?

Answer 25

dic = {“A”:1, “B”:2}
print(dic.get(“A”))
print(dic.get(“C”))
print(dic.get(“C”,”Not Found ! “))

Question 26

Given three arrays sorted in non-decreasing order, print all common elements in these arrays.

ar1 = [1, 5, 10, 20, 40, 80]
ar2 = [6, 7, 20, 80, 100]
ar3 = [3, 4, 15, 20, 30, 70, 80, 120]

Answer 26

First convert all three lists into dictionaries having elements as keys and their frequencies as value, using Counter() method.

Now perform intersection operation for three dictionaries, this will result us dictionary having common elements among three array list with their frequencies.

# Function to find common elements in three
# sorted arrays
from collections import Counter

def commonElement(ar1,ar2,ar3):
     # first convert lists into dictionary
     ar1 = Counter(ar1)
     ar2 = Counter(ar2)
     ar3 = Counter(ar3)

     # perform intersection operation
     resultDict = dict(ar1.items() & ar2.items() & ar3.items())
     common = []

 # iterate through resultant dictionary
 # and collect common elements
 for (key,val) in resultDict.items():
      for i in range(0,val):
           common.append(key)

 print(common)

# Driver program
if \_\_name\_\_ == "\_\_main\_\_":
    ar1 = [1, 5, 10, 20, 40, 80]
    ar2 = [6, 7, 20, 80, 100]
    ar3 = [3, 4, 15, 20, 30, 70, 80, 120]
    commonElement(ar1,ar2,ar3)

#Output
[80, 20]

Question 27

How to merge two dictionaries using update() ?

Answer 27

Using the method update()

By using the method update() in Python, one list can be merged into another. But in this, the second list is merged into the first list and no new list is created. It returns None.
Example:
# Python code to merge dict using update() method
def Merge(dict1, dict2):
    return(dict2.update(dict1))

# Driver code
dict1 = {'a': 10, 'b': 8}
dict2 = {'d': 6, 'c': 4}

# This return None
print(Merge(dict1, dict2))

# changes made in dict2
print(dict2)

Question 28

How to merge two dictionaries using ** ?

Answer 28

Using ** in Python

# Python code to merge dict using a single 
# expression

dict1 = {'a':1, 'b':2}
dict2 = {'c':3, 'd':4}

dict3 = {**dict1, **dict2}
_________________

This is generally considered a trick in Python where a single expression is used to merge two dictionaries and stored in a third dictionary. The single expression is **. This does not affect the other two dictionaries. ** implies that the argument is a dictionary. Using ** [double star] is a shortcut that allows you to pass multiple arguments to a function directly using a dictionary. For more information refer **kwargs in Python. Using this we first pass all the elements of the first dictionary into the third one and then pass the second dictionary into the third. This will replace the duplicate keys of the first dictionary.