A brief introduction to itemmap

Complete Game Information：

• 1 、How to efficiently extract the first object from multiple arrays to form a new array?
• 2、Common indicators for recommendation systems (continued)MRR, HR, MAP,Fscore
• 3 、What is the use of javascript's map method
• 4
  、What is the reason why Proxy(Object) appears after ref data is deduplicated in Vue3?
• 5、Unlock Python multithreading: From basic to advanced concurrent programming
• 6 、How to convert a JavaScript array to an array object that contains key-value pairs?

How to efficiently extract the first object from multiple arrays to form a new array?

1. Implementation steps Get all array values in the object: Extract all values of the object (i.e. nested arrays) into a new array through Object.values (data). Traverse the array and extract the first element: Use the map () method to traverse each array and get its first object through index [0]. Generate result array: map () will automatically combine all extracted first objects into a new array and return it.

2. For the second array [{ id: 120 },...], Returns { id: 120 }. For the third array [{ id: 130 }], return { id: 130 }.

3. Filter method: Filter out elements from array B that meet the condition it[cid] === item[id]. Expand operator... item: Retain the original attribute (such as id) of the elements in array A and add a new list attribute that contains matching elements filtered out of array B.

4. Method 1: Directly access array elements and use object literals syntax Step description: First access the first element of the array through the index. You then use object literal syntax to extract the required attributes from the accessed elements and create a new object.

5. Method 1: Use Array.prototype.push () Applicable scenario: Add multiple independent objects to an existing empty array or existing array. Principle: The push () method appends one or more elements to the end of the array and returns the new length.

Common indicators for recommendation systems (continued)MRR, HR, MAP,Fscore

Common indicators for recommendation systems (continued): MRR, HR, MAP, F-scoreMRR (Average Reciprocal Rankings) Definition: Mean Reciprocal Rankings (MRR) are used to measure whether items that the user is really interested in are recommended at a higher position in the recommendation system. This indicator emphasizes positional relationships, that is, sequencing.

Eliminate the impact of list length and scoring standards, and support horizontal comparisons across users and lists. Both the quality (relevance) and the rationality of ranking (location) of recommended items are measured. Comparison between NDCG and other recommendation system indicators Commonly used indicators in recommendation systems include Precision, Recall, MAP, MRR, etc. The focus of NDCG is different from them: Precision: the proportion of recommended items that users like.

What is the use of javascript's map method

JavaScript's map () method is mainly used to perform specified operations on each element in an array and return a new array containing the processing results. The following are its core functions and characteristics: Returning a new array map () will traverse the original array, pass each element to the callback function for processing, and store the result in the new array for return. The original array will not be modified, which conforms to the immutability principle of functional programming.

JavaScript's map method is used to convert arrays to generate a new array, and the forEach method is used to traverse the array to perform operations without returning a new array. The specific differences and usage are as follows: The core function of the map method is to accept a callback function and convert each element of the array and return a new array. The return values of the callback function are collected and formed into a new array.

Summarizing JavaScript's map () method is a very useful tool that allows you to perform specific operations on each element in an array and return a new array containing the results. By combining the fetch () method with the map () method, you can easily retrieve data from a JSON file and dynamically display it on a web page.

What is the reason why Proxy(Object) appears after ref data is deduplicated in Vue3?

1. Proxy (object) appears after de-duplication of array objects decorated with ref in Vue3 because ref returns a responsive proxy object. When a proxy object is directly de-duplicated (such as using Set), Set will treat each proxy object as a separate element, resulting in the result containing a Proxy (object).

2. Proxy (Object) data appears in array deduplication in Vue3 because the data created using ref () is not unpacked and directly participates in the operation, resulting in the Proxy object being retained. The following are the specific reasons and solutions: Reason analysis Responsive wrapping of ref (): Vue3's ref () will responsively process the created data and return an object wrapped with a Proxy.

3. When using the responsive array created by ref in Vue3 for deduplication, if you directly manipulate the proxy object, a Proxy Object will appear in the deduplication result. This is because Vue3's responsive system implements data tracking through proxy objects, while data structures such as Sets are compared based on object references rather than content.

4. Proxy (Object) appears after de-duplication of ref data in Vue3 because Vue3's responsive system uses Proxy to wrap objects. Direct de-duplication operations on ref objects (such as using Set) will add the Proxy object itself to the result set, resulting in Proxy (Object) appearing.

5. Proxy (Object) data appears after using ref to create an array in Vue3. The core reason is that the original value of the responsive object is not correctly processed, resulting in the deduplication logic acting on the proxy object rather than the actual data. The following is the specific analysis and solution: Reason analysis The ref or reactive of the responsive proxy mechanism Vue3 responsively wraps the data and generates a Proxy object.

6. In Vue3, the reason why Proxy (Object) data appears after array deduplication is mainly related to Vue3's responsive system and Set's limitations on object array deduplication. The specific analysis is as follows: The responsive data wrapped in ref or reactive is not unpacked. Vue3 uses Proxy to achieve responsive data tracking.

Unlock Python multithreading: From basic to advanced concurrent programming

In programming, multithreading is particularly suitable for I/O-intensive tasks (such as file reading and writing, network requests), because I/O operations block threads, and multithreading can use latency to process other tasks. Basic operation: Create and start threads Python implements multithreading through the threading module. The core steps are as follows: Define threading functions: Write logic that needs to be executed concurrently.

Python multithreaded programming can be achieved through the multiprocessing.dummy module, which is a threaded interface encapsulation of multiprocessing. The underlying layer is based on the threading module and provides an API similar to multiprocess but uses threads to perform tasks.

Start the thread: Call the start () method to start the thread. Wait for thread to complete: Use the join () method to ensure that the main thread waits for all child threads to complete before continuing execution. Multiprocessing performs tasks in parallel by creating multiple processes, each with its own independent memory space. The multiprocessing module in Python provides multiprocessing support.

Concurrent programming: Multi-threading: threading module, thread synchronization (Event/Lock), thread pool (concurrent.futures). Multiprocess: multiprocessing module, inter-process communication (Queue/Pipe). The underlying mechanisms: garbage collection (reference counting/generational collection), namespace and scope (Scope), object comparison and copying (deep copy/shallow copy).

How to convert a JavaScript array to an array object that contains key-value pairs?

1. The core method uses Array.prototype.map () to traverse the original array, generates key-value pair objects for each element through the arrow function, and finally returns the new array. Operation steps define the original array: contains the original data to be converted (such as strings, numbers, etc.). Call the map () method: Pass in the arrow function as a callback, processing each element. Generate key-value pair objects: Return objects in the format { key: value } in the arrow function.

2. In JavaScript, converting a key-value array into an object can be achieved in many ways. The core idea is to pair the key array and the value array according to indexes to generate key-value pairs. The following are the specific implementation methods and considerations: The basic loop implementation traverses the key array through the for loop, and at the same time takes out the value of the corresponding index from the value array, generates a key-value pair and assigns it to the result object.

3. The core method uses Array.prototype.map () to traverse the array, using the arrow function to map each element to an object containing the specified key-value pair.