Dictionaries

Consider the following data:

Ahmad    0771234567
Lina     0777777777
Omar     0795556666
Sara     0784443333
Salim    0778889999
...

This data represents a phone book, where each person’s name is associated with their phone number. Using this data, we would like to be able to look up a person’s phone number by their name, and to add, update, or delete entries in the phone book.

In Python, we can achieve this using what is called a dictionary. A dictionary is a collection of key-value pairs, where each key is unique and is used to access its corresponding value. In our phone book example, the names are the keys, and the phone numbers are the values.

Creating a Dictionary

The following code creates a dictionary representing the phone book:

phone_book = {
    "Ahmad": "0771234567",
    "Lina" : "0777777777",
    "Omar" : "0795556666",
    "Sara" : "0784443333",
    "Salim": "0778889999"
}

Accessing Values

You can access a value in the dictionary by using its key inside square brackets:

print(phone_book["Lina"])  # Output: 0777777777
print(phone_book["Omar"])  # Output: 0795556666
print(phone_book["Ali"])   # ERROR: 'Ali' is not in the dictionary

print(phone_book[0])       # ERROR: 0 is not in the dictionary
print(phone_book[1:4])     # Keys in dictionaries are not indexed like lists
                           # Dictionaries do not support slicing

Common Dictionary Operations

Here are some common operations you can perform on dictionaries:

# Adding a new entry
phone_book["Huda"] = "0782223333"

# Updating an existing entry
phone_book["Ahmad"] = "0779998888"

# Deleting an entry
del phone_book["Salim"]

# Checking if a key exists
if "Lina" in phone_book:
    print("Lina's number is:", phone_book["Lina"])

# The in operator does not check for values
if "0777777777" in phone_book:
    print("This will not print because values are not checked.")

# Iterating over keys:
for name in phone_book:
    print(name, "->", phone_book[name])

Example 1: Counting Character Frequencies

Define a function char_frequency(s) that takes a string s as an argument and prints the frequency of each character in the string.

def char_frequency(s):
    frequency = {}

    for char in s:
        if char in frequency:
            frequency[char] += 1
        else:
            frequency[char] = 1

    for char in frequency:
        print(char, ":", frequency[char])

The above code goes through each character in the string s. If the character is already a key in the frequency dictionary, it increments its count by 1. If not, it adds the character to the dictionary with a count of 1.

The code can be made more concise using the get method of dictionaries:

    for char in s:
        frequency[char] = frequency.get(char, 0) + 1

The method get(key, default) returns the value associated with key if it exists; otherwise, it returns default. In our example, if char is not in frequency, get returns 0, so we add 1 to it.

Example 2

Define a function that receives a string and returns a dictionary mapping word lengths to the words in the string that have that length.

Example

input: "I like programming in Python very much"
output: 
{
    1:  ['I'], 
    4:  ['like', 'very', 'much'], 
    11: ['programming'], 
    2:  ['in'], 
    6:  ['Python']
}

Solution

def words_by_length(s):
    dictionary = {}
    words = s.split()

    for word in words:
        n = len(word)
        if n not in dictionary:
            dictionary[n] = []
        dictionary[n].append(word)
        
    return dictionary