Improve Row Count Estimates for Table Variables without Changing Code

Table variables can cause performance issues with joins when they contain a large number of rows. In SQL Server 2019, Microsoft has improved how the optimizer works with table variables which can improve performance without making changes to your code. In this article, Greg Larsen explains how this feature works and if it really does make a difference.

You probably have heard that table variables work fine when the table variable only contains a small number of records, but when the table variable contains a large number of records it doesn’t perform all that well. A solution for this problem has been implemented in version 15.x of SQL Server (Azure SQL Database and SQL Server 2019) with the rollout of a feature called Table Variable Deferred Compilation

_Table Variable Deferred Compilation _is one of many new features to improve performance that was introduced in the Azure SQL Database and SQL Server 2019. This new feature was included in the _Intelligent Query Processing _(IQP). See Figure 1 for a diagram that shows all the IQP features introduced in Azure SQL Database and SQL Server 2019, as well as features that originally were part of the Adaptive Query Processing feature included in the older generation of Azure SQL Database and SQL Server 2017.

Figure 1: Intelligent Query Processing

In releases of SQL Server prior to 15.x, the database engine used a wrong assumption on the number of rows that were in a table variable. Because of this bad assumption, the execution plan that was generated didn’t work too well when a table variable contained lots of rows. With the introduction of SQL Server 2019, the database engine now defers the compilation of a query that uses a table variable until the table variable is used the first time. By doing this, the database engine can more accurately identify cardinality estimates for table variables. By having more accurate cardinality numbers, queries that have large numbers of rows in a table variable will perform better. Those queries will need to be running against a database with a database compatibility level set to 150 (version 15.x of SQL Server) to take advantage of this feature. To better understand how deferred compilation improves the performance of table variables that contain a large number of rows, I’ll run through an example, but first, I’ll discuss what is the problem with table variables in versions of SQL Server prior to version 15.x.

What is the Problem with Table Variables?

A table variable is defined using a DECLARE statement in a batch or stored procedure. Table variables don’t have distribution statistics and don’t trigger recompiles. Because of this, SQL Server is not able to estimate the number of rows in a table variable like it does for normal tables. When the optimizer compiles code that contains a table variable, prior to 15.x, it assumes a table is empty. This assumption causes the optimizer to compile the query using an expected row count of 1 for the cardinality estimate for a table variable. Because the optimizer only thinks a table variable contains a single row, it picks operators for the execution plan that work well with a small set of records, like the NESTED LOOPS operator for a JOIN operation. The operators that work well on a small number of records do not always scale well when a table variable contains a large number of rows. Microsoft documented this problem and recommends that temp tables might be a better choice than using a table variable that contains more than 100 rows. Additionally, Microsoft even recommends that if you are joining a table variable with other tables that you consider using the query hint RECOMPILE to make sure that table variables get the correct cardinality estimates. Without the proper cardinality estimates queries with large table variables are known to perform poorly.

With the introduction of version 15.x and the _Table Variable Deferred Compilation _feature, the optimizer delays the compilation of a query that uses a table variable until just before it is used the first time. This allows the optimizer to know the correct cardinality estimates of a table variable. When the optimizer has an accurate cardinality estimate, it has a good chance at picking execution plan operators that perform well for the number of rows in a table variable. In order for the optimizer to defer the compilation, the database must have its compatibility level set to 150. To show how deferred compilation of table variables work, I’ll show an example of this new feature in action.

#homepage #performance #performance monitor

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How to Use Hotwire Rails

Introduction

We are back with another exciting and much-talked-about Rails tutorial on how to use Hotwire with the Rails application. This Hotwire Rails tutorial is an alternate method for building modern web applications that consume a pinch of JavaScript.

Rails 7 Hotwire is the default front-end framework shipped with Rails 7 after it was launched. It is used to represent HTML over the wire in the Rails application. Previously, we used to add a hotwire-rails gem in our gem file and then run rails hotwire: install. However, with the introduction of Rails 7, the gem got deprecated. Now, we use turbo-rails and stimulus rails directly, which work as Hotwire’s SPA-like page accelerator and Hotwire’s modest JavaScript framework.

What is Hotwire?

Hotwire is a package of different frameworks that help to build applications. It simplifies the developer’s work for writing web pages without the need to write JavaScript, and instead sending HTML code over the wire.

Introduction to The Hotwire Framework:

1. Turbo:

It uses simplified techniques to build web applications while decreasing the usage of JavaScript in the application. Turbo offers numerous handling methods for the HTML data sent over the wire and displaying the application’s data without actually loading the entire page. It helps to maintain the simplicity of web applications without destroying the single-page application experience by using the below techniques:

Turbo Frames: Turbo Frames help to load the different sections of our markup without any dependency as it divides the page into different contexts separately called frames and updates these frames individually.
Turbo Drive: Every link doesn’t have to make the entire page reload when clicked. Only the HTML contained within the tag will be displayed.
Turbo Streams: To add real-time features to the application, this technique is used. It helps to bring real-time data to the application using CRUD actions.

2. Stimulus

It represents the JavaScript framework, which is required when JS is a requirement in the application. The interaction with the HTML is possible with the help of a stimulus, as the controllers that help those interactions are written by a stimulus.

3. Strada

Not much information is available about Strada as it has not been officially released yet. However, it works with native applications, and by using HTML bridge attributes, interaction is made possible between web applications and native apps.

Simple diagrammatic representation of Hotwire Stack:

Prerequisites For Hotwire Rails Tutorial

As we are implementing the Ruby on Rails Hotwire tutorial, make sure about the following installations before you can get started.

• Ruby on Rails
• Hotwire gem
• PostgreSQL/SQLite (choose any one database)
• Turbo Rails
• Stimulus.js

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Create a new Rails Project

Find the following commands to create a rails application.

mkdir ~/projects/railshotwire
cd ~/projects/railshotwire
echo "source 'https://rubygems.org'" > Gemfile
echo "gem 'rails', '~> 7.0.0'" >> Gemfile
bundle install  
bundle exec rails new . --force -d=postgresql

Now create some files for the project, up till now no usage of Rails Hotwire can be seen.
Fire the following command in your terminal.

• For creating a default controller for the application

echo "class HomeController < ApplicationController" > app/controllers/home_controller.rb
echo "end" >> app/controllers/home_controller.rb

• For creating another controller for the application

echo "class OtherController < ApplicationController" > app/controllers/other_controller.rb
echo "end" >> app/controllers/home_controller.rb

• For creating routes for the application

echo "Rails.application.routes.draw do" > config/routes.rb
echo '  get "home/index"' >> config/routes.rb
echo '  get "other/index"' >> config/routes.rb
echo '  root to: "home#index"' >> config/routes.rb
echo 'end' >> config/routes.rb

• For creating a default view for the application

mkdir app/views/home
echo '<h1>This is Rails Hotwire homepage</h1>' > app/views/home/index.html.erb
echo '<div><%= link_to "Enter to other page", other_index_path %></div>' >> app/views/home/index.html.erb

• For creating another view for the application

mkdir app/views/other
echo '<h1>This is Another page</h1>' > app/views/other/index.html.erb
echo '<div><%= link_to "Enter to home page", root_path %></div>' >> app/views/other/index.html.erb

• For creating a database and schema.rb file for the application

bin/rails db:create
bin/rails db:migrate

• For checking the application run bin/rails s and open your browser, your running application will have the below view.

Additionally, you can clone the code and browse through the project. Here’s the source code of the repository: Rails 7 Hotwire application

Now, let’s see how Hotwire Rails can work its magic with various Turbo techniques.

Hotwire Rails: Turbo Drive

Go to your localhost:3000 on your web browser and right-click on the Inspect and open a Network tab of the DevTools of the browser.

Now click on go to another page link that appears on the home page to redirect from the home page to another page. In our Network tab, we can see that this action of navigation is achieved via XHR. It appears only the part inside HTML is reloaded, here neither the CSS is reloaded nor the JS is reloaded when the navigation action is performed.

By performing this action we can see that Turbo Drive helps to represent the HTML response without loading the full page and only follows redirect and reindeer HTML responses which helps to make the application faster to access.

Hotwire Rails: Turbo Frame

This technique helps to divide the current page into different sections called frames that can be updated separately independently when new data is added from the server.
Below we discuss the different use cases of Turbo frame like inline edition, sorting, searching, and filtering of data.

Let’s perform some practical actions to see the example of these use cases.

Make changes in the app/controllers/home_controller.rb file

#CODE

class HomeController < ApplicationController
   def turbo_frame_form
   end
   
   def turbo_frame submit
      extracted_anynumber = params[:any][:anynumber]
      render :turbo_frame_form, status: :ok, locals: {anynumber: extracted_anynumber,      comment: 'turbo_frame_submit ok' }
   end
end

Add app/views/home/turbo_frame_form.html.erb file to the application and add this content inside the file.

#CODE

<section>

    <%= turbo_frame_tag 'anyframe' do %>
            
      <div>
          <h2>Frame view</h2>
          <%= form_with scope: :any, url: turbo_frame_submit_path, local: true do |form| %>
              <%= form.label :anynumber, 'Type an integer (odd or even)', 'class' => 'my-0  d-inline'  %>
              <%= form.text_field :anynumber, type: 'number', 'required' => 'true', 'value' => "#{local_assigns[:anynumber] || 0}",  'aria-describedby' => 'anynumber' %>
              <%= form.submit 'Submit this number', 'id' => 'submit-number' %>
          <% end %>
      </div>
      <div>
        <h2>Data of the view</h2>
        <pre style="font-size: .7rem;"><%= JSON.pretty_generate(local_assigns) %></pre> 
      </div>
      
    <% end %>

</section>

Make some adjustments in routes.rb

#CODE

Rails.application.routes.draw do
  get 'home/index'
  get 'other/index'

  get '/home/turbo_frame_form' => 'home#turbo_frame_form', as: 'turbo_frame_form'
  post '/home/turbo_frame_submit' => 'home#turbo_frame_submit', as: 'turbo_frame_submit'


  root to: "home#index"
end

• Next step is to change homepage view in app/views/home/index.html.erb

#CODE

<h1>This is Rails Hotwire home page</h1>
<div><%= link_to "Enter to other page", other_index_path %></div>

<%= turbo_frame_tag 'anyframe' do %>        
  <div>
      <h2>Home view</h2>
      <%= form_with scope: :any, url: turbo_frame_submit_path, local: true do |form| %>
          <%= form.label :anynumber, 'Type an integer (odd or even)', 'class' => 'my-0  d-inline'  %>
          <%= form.text_field :anynumber, type: 'number', 'required' => 'true', 'value' => "#{local_assigns[:anynumber] || 0}",  'aria-describedby' => 'anynumber' %>
          <%= form.submit 'Submit this number', 'id' => 'submit-number' %>
      <% end %>
  <div>
<% end %>

After making all the changes, restart the rails server and refresh the browser, the default view will appear on the browser.

Now in the field enter any digit, after entering the digit click on submit button, and as the submit button is clicked we can see the Turbo Frame in action in the below screen, we can observe that the frame part changed, the first title and first link didn’t move.

Hotwire Rails: Turbo Streams

Turbo Streams deliver page updates over WebSocket, SSE or in response to form submissions by only using HTML and a series of CRUD-like operations, you are free to say that either

• Update the piece of HTML while responding to all the other actions like the post, put, patch, and delete except the GET action.
• Transmit a change to all users, without reloading the browser page.

This transmit can be represented by a simple example.

• Make changes in app/controllers/other_controller.rb file of rails application

#CODE

class OtherController < ApplicationController

  def post_something
    respond_to do |format|
      format.turbo_stream {  }
    end
  end

   end

Add the below line in routes.rb file of the application

#CODE

post '/other/post_something' => 'other#post_something', as: 'post_something'

Superb! Rails will now attempt to locate the app/views/other/post_something.turbo_stream.erb template at any moment the ‘/other/post_something’ endpoint is reached.

For this, we need to add app/views/other/post_something.turbo_stream.erb template in the rails application.

#CODE

<turbo-stream action="append" target="messages">
  <template>
    <div id="message_1">This changes the existing message!</div>
  </template>
</turbo-stream>

This states that the response will try to append the template of the turbo frame with ID “messages”.

Now change the index.html.erb file in app/views/other paths with the below content.

#CODE

<h1>This is Another page</h1>
<div><%= link_to "Enter to home page", root_path %></div>

<div style="margin-top: 3rem;">
  <%= form_with scope: :any, url: post_something_path do |form| %>
      <%= form.submit 'Post any message %>
  <% end %>
  <turbo-frame id="messages">
    <div>An empty message</div>
  </turbo-frame>
</div>

• After making all the changes, restart the rails server and refresh the browser, and go to the other page.

• Once the above screen appears, click on the Post any message button

This action shows that after submitting the response, the Turbo Streams help the developer to append the message, without reloading the page.

Another use case we can test is that rather than appending the message, the developer replaces the message. For that, we need to change the content of app/views/other/post_something.turbo_stream.erb template file and change the value of the action attribute from append to replace and check the changes in the browser.

#CODE

<turbo-stream action="replace" target="messages">
  <template>
    <div id="message_1">This changes the existing message!</div>
  </template>
</turbo-stream>

When we click on Post any message button, the message that appear below that button will get replaced with the message that is mentioned in the app/views/other/post_something.turbo_stream.erb template

Stimulus

There are some cases in an application where JS is needed, therefore to cover those scenarios we require Hotwire JS tool. Hotwire has a JS tool because in some scenarios Turbo-* tools are not sufficient. But as we know that Hotwire is used to reduce the usage of JS in an application, Stimulus considers HTML as the single source of truth. Consider the case where we have to give elements on a page some JavaScript attributes, such as data controller, data-action, and data target. For that, a stimulus controller that can access elements and receive events based on those characteristics will be created.

Make a change in app/views/other/index.html.erb template file in rails application

#CODE

<h1>This is Another page</h1>
<div><%= link_to "Enter to home page", root_path %></div>

<div style="margin-top: 2rem;">
  <%= form_with scope: :any, url: post_something_path do |form| %>
      <%= form.submit 'Post something' %>
  <% end %>
  <turbo-frame id="messages">
    <div>An empty message</div>
  </turbo-frame>
</div>

<div style="margin-top: 2rem;">
  <h2>Stimulus</h2>  
  <div data-controller="hello">
    <input data-hello-target="name" type="text">
    <button data-action="click->hello#greet">
      Greet
    </button>
    <span data-hello-target="output">
    </span>
  </div>
</div>

Make changes in the hello_controller.js in path app/JavaScript/controllers and add a stimulus controller in the file, which helps to bring the HTML into life.

#CODE

import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
  static targets = [ "name", "output" ]

  greet() {
    this.outputTarget.textContent =
      `Hello, ${this.nameTarget.value}!`
  }
}

Go to your browser after making the changes in the code and click on Enter to other page link which will navigate to the localhost:3000/other/index page there you can see the changes implemented by the stimulus controller that is designed to augment your HTML with just enough behavior to make it more responsive.

With just a little bit of work, Turbo and Stimulus together offer a complete answer for applications that are quick and compelling.

Using Rails 7 Hotwire helps to load the pages at a faster speed and allows you to render templates on the server, where you have access to your whole domain model. It is a productive development experience in ROR, without compromising any of the speed or responsiveness associated with SPA.

Conclusion

We hope you were satisfied with our Rails Hotwire tutorial. Write to us at service@bacancy.com for any query that you want to resolve, or if you want us to share a tutorial on your query.

For more such solutions on RoR, check out our Ruby on Rails Tutorials. We will always strive to amaze you and cater to your needs.

Original article source at: https://www.bacancytechnology.com/

#rails #ruby 

Find the Array Length in Bash

Bash supports both numeric and associative arrays. The total number of elements of these types of arrays can be calculated in multiple ways in Bash. The length of the array can be counted using the “#” symbol or loop, or using a command like “wc” or “grep”.  The different ways of counting the array length in Bash are shown in this tutorial.

Find the Array Length Using “#”

Using the “#” symbol is the simplest way to calculate the array length. The methods of counting the total number of elements of the numeric and associative array is shown in this part of the tutorial.

Example 1: Count the Length of a Numeric Array Using “#”

Create a Bash file with the following script that counts and prints the length of a numeric array using the “#” symbol. The “@” and “*” symbols are used here to denote all elements of the array.

#Declare a numeric array

items=("Shirt" "T-Shirt" "Pant" "Panjabi" "Shoe")

#Count array length using '#'

echo "Array length using '#' with '@':  ${#items[@]}"

echo "Array length using '#' with '*':  ${#items[*]}"

The following output appears after executing the script. The array contains five string values and the same output is shown for both “@” and “*” symbols:

Example 2: Count the Length of an Associative Array Using “#”

Create a Bash file with the following script that counts and prints the length of an associative array using the “#” symbol. The “@” and “*” symbols are used here to denote all elements of the array.

#Declare an associative array

declare -A items=([6745]="Shirt (M)" [2345]="Shirt (L)" [4566]="Pant (36)")

#Count array length using '#'

echo "Associative array length using '#' with '@':  ${#items[@]}"

echo "Associative array length using '#' with '*':  ${#items[*]}"

The following output appears after executing the script. The array contains three string values and the same output is shown for both the “@” and “*” symbols:

Find the Array Length Using a Loop

Using a loop is another way to count the total number of elements in the array. The length of an array is counted using a while loop in the following example:

Example: Count the Length of an Array Using a Loop

Create a Bash file with the following script that counts the total number of elements using a “while” loop. A numeric array of four string values is declared in the script using the “declare” command. The “for” loop is used to iterate and print the values of the array. Here, the $counter variable is used to count the length of the array that is incremented in each iteration of the loop.

#Declare an array

declare -a items=("Shirt(M)" "Shirt(L)" "Panjabi(42)" "Pant(38)")

echo "Array values are:"

#Count array length using loop

counter=0

#Iterate the array values

for val in ${items[@]}

do

  #Print the array value

     echo $val

     ((counter++))

  done

  echo "The array length using loop is $counter."

The following output appears after executing the script. The array values and the length of the array are printed in the output:

Find the Array Length Using the “Wc” Command

The length of the array can be counted using some commands. The “wc” command is one of them. But this command does not return the correct output if the array contains the string value of multiple words. The method of counting the total number of elements of an array and comparing the array length value that is counted by the “#” symbol and “wc” command is shown in the following example.

Example: Count the Length of an Array Using the “Wc” Command

Create a Bash file with the following script that counts the total number of elements using the “wc” command. A numeric array of five string values is declared in the script. The “wc” command with the -w option is used to count the length of two arrays of 5 elements. One array contains a string of one word and another array contains a string of two words. The length of the second arrays is counted using the “#” symbol and the “wc” command.

#Declare a numeric array of a single word of the string

items=("Shirt" "T-Shirt" "Pant" "Panjabi" "Shoe")

echo "Array values: ${items[@]}"

#Count array length using 'wc'

len=`echo ${items[@]} | wc -w`

echo "Array length using 'wc' command: $len"

 

#Declare a numeric array of multiple words of the string

items2=("Shirt (XL)" "T-Shirt (L)" "Pant (34)" "Panjabi (38)" "Shoe (9)")

echo "Array values: ${items2[@]}"

echo "Array length using '#': ${#items2[@]}"

#Count array length using 'wc'

len=`echo ${items2[@]} | wc -w`

echo "Array length using 'wc' command: $len"

The following output appears after executing the script. According to the output, the “wc” command generates the wrong output for the array that contains a string value of two words:

Conclusion

The methods of counting the length of an array using the “#” symbol, loop, and the “wc” command are shown in this tutorial.

Original article source at: https://linuxhint.com/

#bash #array 

Static Code Analysis: What It Is? How to Use It?

Static code analysis refers to the technique of approximating the runtime behavior of a program. In other words, it is the process of predicting the output of a program without actually executing it.

Lately, however, the term “Static Code Analysis” is more commonly used to refer to one of the applications of this technique rather than the technique itself — program comprehension — understanding the program and detecting issues in it (anything from syntax errors to type mismatches, performance hogs likely bugs, security loopholes, etc.). This is the usage we’d be referring to throughout this post.

“The refinement of techniques for the prompt discovery of error serves as well as any other as a hallmark of what we mean by science.”

• J. Robert Oppenheimer

Outline

We cover a lot of ground in this post. The aim is to build an understanding of static code analysis and to equip you with the basic theory, and the right tools so that you can write analyzers on your own.

We start our journey with laying down the essential parts of the pipeline which a compiler follows to understand what a piece of code does. We learn where to tap points in this pipeline to plug in our analyzers and extract meaningful information. In the latter half, we get our feet wet, and write four such static analyzers, completely from scratch, in Python.

Note that although the ideas here are discussed in light of Python, static code analyzers across all programming languages are carved out along similar lines. We chose Python because of the availability of an easy to use ast module, and wide adoption of the language itself.

How does it all work?

Before a computer can finally “understand” and execute a piece of code, it goes through a series of complicated transformations:

As you can see in the diagram (go ahead, zoom it!), the static analyzers feed on the output of these stages. To be able to better understand the static analysis techniques, let’s look at each of these steps in some more detail:

Scanning

The first thing that a compiler does when trying to understand a piece of code is to break it down into smaller chunks, also known as tokens. Tokens are akin to what words are in a language.

A token might consist of either a single character, like (, or literals (like integers, strings, e.g., 7, Bob, etc.), or reserved keywords of that language (e.g, def in Python). Characters which do not contribute towards the semantics of a program, like trailing whitespace, comments, etc. are often discarded by the scanner.

Python provides the tokenize module in its standard library to let you play around with tokens:

Python

1

import io

2

import tokenize

3

4

code = b"color = input('Enter your favourite color: ')"

5

6

for token in tokenize.tokenize(io.BytesIO(code).readline):

7

    print(token)

Python

1

TokenInfo(type=62 (ENCODING),  string='utf-8')

2

TokenInfo(type=1  (NAME),      string='color')

3

TokenInfo(type=54 (OP),        string='=')

4

TokenInfo(type=1  (NAME),      string='input')

5

TokenInfo(type=54 (OP),        string='(')

6

TokenInfo(type=3  (STRING),    string="'Enter your favourite color: '")

7

TokenInfo(type=54 (OP),        string=')')

8

TokenInfo(type=4  (NEWLINE),   string='')

9

TokenInfo(type=0  (ENDMARKER), string='')

(Note that for the sake of readability, I’ve omitted a few columns from the result above — metadata like starting index, ending index, a copy of the line on which a token occurs, etc.)

#code quality #code review #static analysis #static code analysis #code analysis #static analysis tools #code review tips #static code analyzer #static code analysis tool #static analyzer

Найдите длину массива в Bash

Bash поддерживает как числовые, так и ассоциативные массивы. Общее количество элементов этих типов массивов может быть вычислено несколькими способами в Bash. Длину массива можно подсчитать с помощью символа « # » или цикла, или с помощью команды типа « wc» или «grep ». В этом руководстве показаны различные способы подсчета длины массива в Bash.

Найдите длину массива, используя «#»

Использование символа « # » — самый простой способ вычислить длину массива. В этой части руководства показаны способы подсчета общего количества элементов числового и ассоциативного массива.

Пример 1. Подсчет длины числового массива с использованием «#»

Создайте файл Bash со следующим сценарием, который подсчитывает и печатает длину числового массива, используя символ «#». Здесь используются символы «@» и «* » для обозначения всех элементов массива.

#Declare a numeric array

items=("Shirt" "T-Shirt" "Pant" "Panjabi" "Shoe")

#Count array length using '#'

echo "Array length using '#' with '@':  ${#items[@]}"

echo "Array length using '#' with '*':  ${#items[*]}"

Следующий вывод появляется после выполнения скрипта. Массив содержит пять строковых значений, и для символов « @» и «* » отображается один и тот же результат :

Пример 2. Подсчет длины ассоциативного массива с использованием «#»

Создайте файл Bash со следующим сценарием, который подсчитывает и печатает длину ассоциативного массива, используя символ «#». Здесь используются символы «@» и «* » для обозначения всех элементов массива.

#Declare an associative array

declare -A items=([6745]="Shirt (M)" [2345]="Shirt (L)" [4566]="Pant (36)")

#Count array length using '#'

echo "Associative array length using '#' with '@':  ${#items[@]}"

echo "Associative array length using '#' with '*':  ${#items[*]}"

Следующий вывод появляется после выполнения скрипта. Массив содержит три строковых значения, и для символов « @» и «* » отображается один и тот же результат :

Найдите длину массива с помощью цикла

Использование цикла — еще один способ подсчета общего количества элементов в массиве. Длина массива подсчитывается с помощью цикла while в следующем примере:

Пример. Подсчет длины массива с использованием цикла

Создайте файл Bash со следующим сценарием, который подсчитывает общее количество элементов с помощью цикла « пока ». Числовой массив из четырех строковых значений объявляется в скрипте с помощью команды « объявить ». Цикл for используется для повторения и печати значений массива. Здесь переменная $counter используется для подсчета длины массива, который увеличивается на каждой итерации цикла.

#Declare an array

declare -a items=("Shirt(M)" "Shirt(L)" "Panjabi(42)" "Pant(38)")

echo "Array values are:"

#Count array length using loop

counter=0

#Iterate the array values

for val in ${items[@]}

do

  #Print the array value

     echo $val

     ((counter++))

  done

  echo "The array length using loop is $counter."

Следующий вывод появляется после выполнения скрипта. Значения массива и длина массива печатаются в выводе:

Найдите длину массива с помощью команды «Wc»

Длину массива можно подсчитать с помощью некоторых команд. Команда « wc » — одна из них. Но эта команда не возвращает правильный вывод, если массив содержит строковое значение из нескольких слов. В следующем примере показан метод подсчета общего количества элементов массива и сравнения значения длины массива, подсчитываемого символом « # » и командой « wc ».

Пример: подсчет длины массива с помощью команды «Wc»

Создайте файл Bash со следующим сценарием, который подсчитывает общее количество элементов с помощью команды « wc ». В скрипте объявлен числовой массив из пяти строковых значений. Команда « wc » с параметром -w используется для подсчета длины двух массивов по 5 элементов. Один массив содержит строку из одного слова, а другой массив содержит строку из двух слов. Длина вторых массивов подсчитывается с помощью символа « # » и команды « wc ».

#Declare a numeric array of a single word of the string

items=("Shirt" "T-Shirt" "Pant" "Panjabi" "Shoe")

echo "Array values: ${items[@]}"

#Count array length using 'wc'

len=`echo ${items[@]} | wc -w`

echo "Array length using 'wc' command: $len"

 

#Declare a numeric array of multiple words of the string

items2=("Shirt (XL)" "T-Shirt (L)" "Pant (34)" "Panjabi (38)" "Shoe (9)")

echo "Array values: ${items2[@]}"

echo "Array length using '#': ${#items2[@]}"

#Count array length using 'wc'

len=`echo ${items2[@]} | wc -w`

echo "Array length using 'wc' command: $len"

Следующий вывод появляется после выполнения скрипта. Судя по выводу, команда « wc » выдает неверный вывод для массива, содержащего строковое значение из двух слов:

Заключение

В этом руководстве показаны методы подсчета длины массива с помощью символа « # », цикла и команды « wc ».

Оригинальный источник статьи: https://linuxhint.com/

#bash #array 

在 Bash 中查找数组长度

Bash 支持数字数组和关联数组。在 Bash 中可以通过多种方式计算这些类型数组的元素总数。可以使用“ # ”符号或循环，或使用“ wc”或“grep ”等命令来计算数组的长度。本教程展示了在 Bash 中计算数组长度的不同方法。

使用“#”查找数组长度

使用“ # ”符号是计算数组长度的最简单方法。本教程的这一部分显示了计算数值和关联数组元素总数的方法。

示例 1：使用“#”计算数值数组的长度

使用以下脚本创建一个 Bash 文件，该脚本使用“#”符号计算并打印数字数组的长度。这里使用“@”和“* ”符号来表示数组的所有元素。

#Declare a numeric array

items=("Shirt" "T-Shirt" "Pant" "Panjabi" "Shoe")

#Count array length using '#'

echo "Array length using '#' with '@':  ${#items[@]}"

echo "Array length using '#' with '*':  ${#items[*]}"

执行脚本后出现以下输出。该数组包含五个字符串值，“ @”和“* ”符号显示相同的输出：

示例 2：使用“#”计算关联数组的长度

使用以下脚本创建一个 Bash 文件，该脚本使用“#”符号计算并打印关联数组的长度。这里使用“@”和“* ”符号来表示数组的所有元素。

#Declare an associative array

declare -A items=([6745]="Shirt (M)" [2345]="Shirt (L)" [4566]="Pant (36)")

#Count array length using '#'

echo "Associative array length using '#' with '@':  ${#items[@]}"

echo "Associative array length using '#' with '*':  ${#items[*]}"

执行脚本后出现以下输出。该数组包含三个字符串值，“ @”和“* ”符号显示相同的输出：

使用循环查找数组长度

使用循环是计算数组中元素总数的另一种方法。在以下示例中使用 while 循环计算数组的长度：

示例：使用循环计算数组的长度

使用以下脚本创建一个 Bash 文件，该脚本使用“ while ”循环计算元素总数。使用“ declare ”命令在脚本中声明了一个包含四个字符串值的数字数组。“ for ”循环用于迭代和打印数组的值。这里，$counter 变量用于计算在循环的每次迭代中递增的数组长度。

#Declare an array

declare -a items=("Shirt(M)" "Shirt(L)" "Panjabi(42)" "Pant(38)")

echo "Array values are:"

#Count array length using loop

counter=0

#Iterate the array values

for val in ${items[@]}

do

  #Print the array value

     echo $val

     ((counter++))

  done

  echo "The array length using loop is $counter."

执行脚本后出现以下输出。数组值和数组长度打印在输出中：

使用“Wc”命令查找数组长度

可以使用一些命令来计算数组的长度。“ wc ”命令就是其中之一。但是如果数组包含多个单词的字符串值，则此命令不会返回正确的输出。通过“ # ”符号和“ wc ”命令统计数组元素总数并比较数组长度值的方法如下例所示。

示例：使用“Wc”命令计算数组的长度

使用以下脚本创建一个 Bash 文件，该脚本使用“ wc ”命令计算元素总数。脚本中声明了一个包含五个字符串值的数字数组。带有 -w 选项的“ wc ”命令用于计算两个 5 元素数组的长度。一个数组包含一个单词的字符串，另一个数组包含两个单词的字符串。使用“ # ”符号和“ wc ”命令计算第二个数组的长度。

#Declare a numeric array of a single word of the string

items=("Shirt" "T-Shirt" "Pant" "Panjabi" "Shoe")

echo "Array values: ${items[@]}"

#Count array length using 'wc'

len=`echo ${items[@]} | wc -w`

echo "Array length using 'wc' command: $len"

 

#Declare a numeric array of multiple words of the string

items2=("Shirt (XL)" "T-Shirt (L)" "Pant (34)" "Panjabi (38)" "Shoe (9)")

echo "Array values: ${items2[@]}"

echo "Array length using '#': ${#items2[@]}"

#Count array length using 'wc'

len=`echo ${items2[@]} | wc -w`

echo "Array length using 'wc' command: $len"

执行脚本后出现以下输出。根据输出，“ wc ”命令为包含两个单词的字符串值的数组生成错误输出：

结论

本教程展示了使用“ # ”符号、循环和“ wc ”命令计算数组长度的方法。

文章原文出处：https: //linuxhint.com/

#bash #array