### 计算机代写|Java代写|A Closer Look at Variables

Java是一种广泛使用的计算机编程语言，拥有跨平台、面向对象、泛型编程的特性，广泛应用于企业级Web应用开发和移动应用开发。

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• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 计算机代写|Java代写|Initializing a Variable

Variables were introduced in Chapter 1. Here, we will take a closer look at them. As you learned earlier, variables are declared using this form of statement,
type var-name;
where type is the data type of the variable, and var-name is its name. You can declare a variable of any valid type, including the simple types just described, and every variable will have a type. Thus, the capabilities of a variable are determined by its type. For example, a variable of type boolean cannot be used to store floating-point values. Furthermore, the type of a variable cannot change during its lifetime. An int variable cannot turn into a char variable, for example.
All variables in Java must be declared prior to their use. This is necessary because the compiler must know what type of data a variable contains before it can properly compile any statement that uses the variable. It also enables Java to perform strict type checking.

In general, you must give a variable a value prior to using it. One way to give a variable a value is through an assignment statement, as you have already seen. Another way is by giving it an initial value when it is declared. To do this, follow the variable’s name with an equal sign and the value being assigned. The general form of initialization is shown here:
type var = value;
Here, value is the value that is given to var when var is created. The value must be compatible with the specified type. Here are some examples:
int count $=10 ; / /$ give count an initial value of char ch $=\mathrm{X}^{\prime} ; / /$ initialize ch with the letter $\mathrm{X}$ float $\mathrm{f}=1.2 \mathrm{~F} ; / / \mathrm{f}$ is initialized with $1.2$
When declaring two or more variables of the same type using a comma-separated list, you can give one or more of those variables an initial value. For example:
int $a, b=8, c=19, d ; / / b$ and $c$ have initializations
In this case, only $\mathbf{b}$ and $\mathbf{c}$ are initialized.

## 计算机代写|Java代写|Dynamic Initialization

Although the preceding examples have used only constants as initializers, Java allows variables to be initialized dynamically, using any expression valid at the time the variable is declared. For example, here is a short program that computes the volume of a cylinder given the radius of its base and its height:
// Demonstrate dynamic initialization.
class DynInit {
public static void main(String[] args) {
double radius $=4$, height $=5$; volume is dynamically initialized at run time.
$/ /$ dynamically initialize volume
double volume $=3.1416$ * radius * radius * height;
Although the preceding examples have used only constants as initializers, Java allows variables
to be initialized dynamically, using any expression valid at the time the variable is declared.
For example, here is a short program that computes the volume of a cylinder given the radius
of its base and its height:
// Demonstrate dynamic initialization.
class DynInit {
public static void main (String [] args) double radius $=4$, height $=5$; volume is dynamically initialized at run time.
// dynamically initialize volume
double volume $=3.1416$ radius * radius * height;
System.out.println(“Volume is ” + volume);
System.out.println(“Volume is ” + volume);
}
}
Here, three local variables – radius, height, and volume – are declared. The first two, radius and height, are initialized by constants. However, volume is initialized dynamically to the volume of the cylinder. The key point here is that the initialization expression can use any element valid at the time of the initialization, including calls to methods, other variables, or literals.

## 计算机代写|Java代写|The Scope and Lifetime of Variables

So far, all of the variables that we have been using were declared at the start of the main( ) method. However, Java allows variables to be declared within any block. As explained in Chapter 1, a block is begun with an opening curly brace and ended by a closing curly brace. A block defines a scope. Thus, each time you start a new block, you are creating a new scope. A scope determines what objects are visible to other parts of your program. It also determines the lifetime of those objects.
In general, every declaration in Java has a scope. As a result, Java defines a powerful, finely grained concept of scope. Two of the most common scopes in Java are those defined by a class and those defined by a method. A discussion of class scope (and variables declared within it) is deferred until later in this book, when classes are described. For now, we will examine only the scopes defined by or within a method.

The scope defined by a method begins with its opening curly brace. However, if that method has parameters, they too are included within the method’s scope. A method’s scope ends with its closing curly brace. This block of code is called the method body.

As a general rule, variables declared inside a scope are not visible (that is, accessible) to code that is defined outside that scope. Thus, when you declare a variable within a scope, you are localizing that variable and protecting it from unauthorized access and/or modification.
Indeed, the scope rules provide the foundation for encapsulation. A variable declared within a block is called a local variable.

Scopes can be nested. For example, each time you create a block of code, you are creating a new, nested scope. When this occurs, the outer scope encloses the inner scope. This means

that objects declared in the outer scope will be visible to code within the inner scope. However, the reverse is not true. Objects declared within the inner scope will not be visible outside it. To understand the effect of nested scopes, consider the following program:
$/ /$ Demonstrate block scope.
class scopedemo {
public static void main (string [] args) {
int $x_{i} / /$ known to all code within main
$x=10 ;$
if $(x==10) \quad{/ /$ start new scope
int $y=20 ; / /$ known only to this block
$/ / x$ and $y$ both known here.
system.out. println (” $x$ and $y: \prime+x+=\prime+y)$;
$x=y * 2$;
}
$/ / y=100 ; / /$ Error! y not known here ←_Here, $y$ is outside of its scope.
$/ / \mathrm{x}$ is still known here.
System. out. println( $” x$ is ” $+x)$;
}
}

## 计算机代写|Java代写|Initializing a Variable

Java 中的所有变量都必须在使用前声明。这是必要的，因为编译器必须知道变量包含什么类型的数据，然后才能正确编译使用该变量的任何语句。它还使 Java 能够执行严格的类型检查。

type var = value;

int count=10;//给 count 一个初始值 char ch=X′;//用字母初始化 chX漂浮F=1.2 F;//F初始化为1.2

## 计算机代写|Java代写|Dynamic Initialization

// 演示动态初始化。

public static void main(String[] args) {

//动态初始化音量

// 演示动态初始化。

public static void main (String [] args) 双半径=4， 高度=5; 卷在运行时动态初始化。
// 动态初始化音量

System.out.println(“音量为” + 音量);
System.out.println(“音量为” + 音量);
}
}

## 计算机代写|Java代写|The Scope and Lifetime of Variables

//演示块范围。

public static void main (string [] args) {
intX一世//main 中的所有代码都知道
X=10;

}
}

## 有限元方法代写

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## MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。