Built-in functions and dictionary processing operations

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Contents

• 1. Built-in method len(). Number of items in the dictionary
• 2. Access to a dictionary item by its key D[key]
• 3. Operation del. Delete item by key
• 4. Operation in. Determining the presence of a key in a dictionary
• 5. Operation not in. Determining the absence of a key in a dictionary
• Related topics

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1. Built-in method len().Number of items in the dictionary

The built-in len() function returns the number of items in a given dictionary. According to Python documentation, the general form of using this function is as follows

count = len(D)

where

• D – source dictionary;
• count – the required number of elements.

Example.

# Function len() - get the number of items in the dictionary
# Example 1
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' }
count = len(Months) # count = 12

# Example 2. Using nested dictionaries
Position = { 'Manager': { 'Director',
                          'Deputy Director' },
             'Teacher': { 'Specialist',
                          'Methodist',
                          'Senior Lecturer' },
             'Staff': { 'Cleaner',
                        'Porter',
                        'Watchman' } }

count1 = len(Position) # count1 = 3
count2 = len(Position['Manager']) # count2 = 2
count3 = len(Position['Staff']) # count3 = 3

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2. Access to a dictionary item by its key D[key]

Using the [] operation, you can access the value if the key by which this value is written is known. The access operation [] allows you to both read and write values in the dictionary.

According to the Python documentation, the general form of using the [] operation to read a value is as follows:

item = D[key]

here

• D – specified dictionary;
• key – the key on the basis of which the item is searched in the dictionary. If the key key is not in the dictionary, then a KeyError exception is thrown;
• item – the item that is obtained by the key key.

If you need to write or add the item value to the dictionary using the key key, then the operation [] has the following form of use

D[key] = item

There are two possible cases during recording:

• key is present in the dictionary. In this case, the value of item replaces the previous value;
• key is not in the dictionary. In this case, a new key:item pair is added to the dictionary.

Example.

# Operation [] - access by the key to dictionary items

# Example 1
# Source dictionary
WorkDays = { 1:'Mon', 2:'Tue' }

# Get an item
item = WorkDays[2] # item = 'Tue'

# Instead of 'Tue' write 'Sat'
WorkDays[2] = 'Sat' # WorkDays = {1: 'Mon', 2: 'Sat'}

# Add new pair (6:'Sun')
WorkDays[6] = 'Sun' # WorkDays = {1: 'Mon', 2: 'Sat', 6: 'Sun'}

# Example 2. Using nested dictionaries

SubCat1 = { 1:'A', 2:'B', 3:'C' }
SubCat2 = { 4:'D', 5:'E' }

Category = { 'High':SubCat1,
             'Low' :SubCat2 }

# Get an item
item = Category['Low'] # item = {4: 'D', 5: 'E'}
item2 = Category['Low'][4] # item2 = 'D'

# Add item to dictionary
Category['Middle'] = { 6:'J', 7:'К' }

# Change the item in dictionary
Category['High'][3] = 'F' # Category = {'High': {1:'A', 2:'B', 3:'F'},
                          # 'Low': {4:'D', 5:'E'},
                          # 'Middle': {6:'J', 7:'K'}}

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3. Operation del. Delete item by key

The del operation is intended to remove an element from the dictionary based on the given key key. The general form of using the operation is as follows

del D[key]

where

• D – given dictionary;
• key – The key in the dictionary whose item you want to delete.

If you specify a key that does not exist, a KeyError exception will be thrown.

Example.

# Operation del - delete item from dictionary

# Source dictionary
Salary = { 'Director': 120800.0,
           'Secretary': 101150.25,
           'Locksmith': 188200.00 }

# Delete item by key 'Secretary'
del Salary['Secretary'] # Salary = {'Director': 120800.0,
                        # 'Locksmith': 188200.0}

# Attempting to delete a key that does not exist
# del Salary[5] - this is impossible, a KeyError exception is thrown: 5
# del Salary['None'] - also forbidden

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4. Operation in. Determining the presence of a key in a dictionary

To determine if a given key exists in a dictionary, Python uses the in operation. The general form of using the in operation is as follows

f_is = key in D

where

• D – source dictionary;
• key – the key whose presence in the dictionary D needs to be determined;
• f_is – the result of a logical type. If f_is = True, then the key key is present in the dictionary. If f_is = False, then the key is not in the dictionary.

Example. The example uses the in operation to determine if there is a Salary key in the dictionary that needs to be deleted. The operation is used in the conditional if statement.

# Operation in - determining the presence of a key in a dictionary

# Source dictionary
Salary = { 'Director': 120800.0,
           'Secretary': 101150.25,
           'Locksmith': 188200.00 }

# Delete item by key 'Secretary' with check
key = 'Secretary'
if key in Salary:
    del Salary['Secretary'] # Salary = {'Director': 120800.0,
                            # 'Locksmith': 188200.0}

# Attempting to delete a key that does not exist
# if there is no key, then a KeyError exception is not thrown
key2 = 5
if key2 in Salary:
    del Salary[key2]

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5. Operation not in. Determining the absence of a key in a dictionary

The not in operation returns always the opposite of the in operation. The general form of the not in operation is as follows:

f_is = key not in D

where

• D – source dictionary;
• key – the key whose presence in the dictionary D needs to be determined;
• f_is – result of logical type. If f_is = True, then the key key is not in the dictionary D. If f_is = False, then the key key is present in the dictionary D.

Example. The example demonstrates the use of the not in operation to determine if the key that was entered from the keyboard is present in the dictionary.

# Operation not in - determination of the absence
# of a key in the dictionary

# Formation of a dictionary of words with their numerical equivalent

# 1. Create an empty dictionary
Words = dict() # Words = {}

# 2. Enter the number of words in the dictionary
count = int(input("Number of words: "))

# 3. The cycle of adding words
i=0

while i<count:
    print("Input word:")
    wrd = str(input("Text:"))
    value = int(input("Value: "))

    # If the wrd key is not in the dictionary, then add the [wrd:value] pair
    if wrd not in Words:
        Words[wrd] = value
    i=i+1

# Display the generated dictionary
print("\nYou enter the following information:")
print(Words)

The result of the program:

Number of words: 3
Input word:
Text:wrd1
Value: 100
Input word:
Text:wrd2
Value: 200
Input word:
Text:wrd3
Value: 250

You enter the following information:
{'wrd1': 100, 'wrd2': 200, 'wrd3': 250}

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Related topics

• Dictionaries. Basic concepts. Specifications. Creating dictionaries. Accessing dictionary values
• Methods of working with dictionaries
• Working with dictionaries. Build-in functions list(), zip(). Dictionary bypass. Dictionary generators. Sorting. Combinations of dictionaries with sets

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