What is a dictionary in Python?

Python is a very popular programming language, especially in the fields of data science and artificial intelligence. One of the very important data structures is the dictionary. This article will introduce what a dictionary in Python is, how to use it, and some practical applications.

1. What is a dictionary?

A dictionary is a mutable, unordered, and iterable collection data type. It consists of some keys and corresponding values. The dictionary is created by using curly brackets {} and assigning values ​​through key:value. For example:

my_dict = {'name': 'John', 'age': 30, 'city': 'New York'}

This dictionary has three key-value pairs, namely 'name':'John', 'age':30 and 'city':'New York'. A key-value pair is separated by a colon, and each key-value pair is separated by a comma.

2. How to use dictionary?

The keys in the dictionary must be immutable objects, such as strings, numbers, or tuples. The value can be any data type, or even another dictionary. To access a value in a dictionary, use the corresponding key. For example:

print(my_dict['name'])  # 输出 John

If you want to add a new key-value pair to the dictionary, you can directly assign the value:

my_dict['job'] = 'programmer'

This adds a new key 'job' and the corresponding key to the dictionary. Value 'programmer'.

To delete a key-value pair in the dictionary, you can use the del statement:

del my_dict['city']

This deletes the key 'city' and the corresponding value 'New York'. In addition, you can also use some dictionary methods, such as keys(), values(), items(), etc. For example:

print(my_dict.keys())    # 输出 ['name', 'age', 'job']
print(my_dict.values())  # 输出 ['John', 30, 'programmer']
print(my_dict.items())   # 输出 [('name', 'John'), ('age', 30), ('job', 'programmer')]

Among them, the keys() method returns all keys, the values() method returns all values, and the items() method returns all key-value pairs.

3. Practical applications of dictionaries

Dictionaries are widely used in Python. Here are some common scenarios.

(1) Counter

Counter is a common scenario, such as counting the number of occurrences of each character in a string. This is where you can use a dictionary. For example:

my_str = 'hello world'
counts = {}
for char in my_str:
    if char in counts:
        counts[char] += 1
    else:
        counts[char] = 1
print(counts)

The output result is: {'h': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'w': 1, ' r': 1, 'd': 1}, indicating the number of occurrences of each character.

(2) Data processing

In data processing, it is often necessary to use dictionaries to store and operate data. For example, after collecting large amounts of data, the data needs to be aggregated and analyzed.

The following is a simple example, assuming there is a list storing the age of each person, we need to divide them into three age groups: 0-18, 18-60 and 60 and above. You can use a dictionary to achieve this functionality.

ages = [16, 25, 34, 42, 50, 68, 70, 80, 90]
age_counts = {'0-18': 0, '18-60': 0, '60+': 0}
for age in ages:
    if age <= 18:
        age_counts['0-18'] += 1
    elif age <= 60:
        age_counts['18-60'] += 1
    else:
        age_counts['60+'] += 1
print(age_counts)

The output result is: {'0-18': 1, '18-60': 4, '60 ': 4}, indicating how many people there are in each age group.

(3) API call

When using some APIs, the returned data is usually a dictionary. For example, suppose we use a weather API and obtain the local weather conditions:

weather = {'location': 'New York', 'temperature': 15, 'humidity': 0.6, 'condition': 'sunny'}

Then we need to extract some information from it. We can directly use the dictionary key to obtain the corresponding value:

print(weather['location'])    # 输出 New York
print(weather['temperature']) # 输出 15

4. Summary

Dictionary is a very important data structure in Python. Its variable, unordered, and iterable characteristics make it widely used in many scenarios. This article introduces the basic usage of dictionaries and some practical applications. For Python beginners, mastering the use of dictionaries is very helpful to improve programming skills, and it can also bring great convenience to actual development.

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