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

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  • Foundations of Data Science 数据科学基础
计算机代写|Java代写|A Closer Look at Variables

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|A Closer Look at Variables


计算机代写|Java代写|Initializing a Variable

变量在第 1 章中介绍过。在这里,我们将仔细研究它们。如前所述,变量是使用这种形式的语句声明的,
类型 var-name;
其中 type 是变量的数据类型,var-name 是它的名字。您可以声明任何有效类型的变量,包括刚刚描述的简单类型,并且每个变量都有一个类型。因此,变量的能力由其类型决定。例如,布尔类型的变量不能用于存储浮点值。此外,变量的类型在其生命周期内不能改变。例如,一个 int 变量不能变成一个 char 变量。
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;
这里,value 是在创建 var 时赋予 var 的值。该值必须与指定的类型兼容。以下是一些示例:
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

尽管前面的示例仅使用常量作为初始化器,但 Java 允许使用在声明变量时有效的任何表达式来动态初始化变量。例如,下面是一个简短的程序,它根据圆柱的底面半径和高度计算圆柱体的体积:
// 演示动态初始化。
类 DynInit {
public static void main(String[] args) {
双半径=4, 高度=5; 卷在运行时动态初始化。
双倍音量=3.1416* 半径 * 半径 * 高度;
尽管前面的示例仅使用常量作为初始化器,但 Java 允许
// 演示动态初始化。
类 DynInit {
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

到目前为止,我们一直在使用的所有变量都是在 main() 方法的开头声明的。但是,Java 允许在任何块中声明变量。正如第 1 章所解释的,一个块以一个左花括号开始,以一个右花括号结束。块定义范围。因此,每次您开始一个新块时,您都在创建一个新范围。范围确定哪些对象对程序的其他部分可见。它还决定了这些对象的生命周期。
一般来说,Java 中的每个声明都有一个范围。因此,Java 定义了一个强大的、细粒度的范围概念。Java 中最常见的两个作用域是由类定义的作用域和由方法定义的作用域。对类作用域(以及在其中声明的变量)的讨论将推迟到本书后面描述类的时候。现在,我们将只检查由方法定义或在方法内定义的范围。




类 scopedemo {
public static void main (string [] args) {
intX一世//main 中的所有代码都知道
如果(X==10)//$s吨一个r吨n和在sC○p和一世n吨$是=20;//$ķn○在n○nl是吨○吨H一世sbl○Cķ$//X$一个nd$是$b○吨Hķn○在nH和r和.s是s吨和米.○在吨.pr一世n吨ln(”$X$一个nd$是:′+X+=′+是)$;$X=是∗2$;//y=100; //和rr○r!是n○吨ķn○在nH和r和←H和r和,是一世s○在吨s一世d和○F一世吨ssC○p和.// \mathrm{x}一世ss吨一世llķn○在nH和r和.小号是s吨和米.○在吨.pr一世n吨ln(“ X一世s”+x)$;

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术语 广义线性模型(GLM)通常是指给定连续和/或分类预测因素的连续响应变量的常规线性回归模型。它包括多元线性回归,以及方差分析和方差分析(仅含固定效应)。



有限元是一种通用的数值方法,用于解决两个或三个空间变量的偏微分方程(即一些边界值问题)。为了解决一个问题,有限元将一个大系统细分为更小、更简单的部分,称为有限元。这是通过在空间维度上的特定空间离散化来实现的,它是通过构建对象的网格来实现的:用于求解的数值域,它有有限数量的点。边界值问题的有限元方法表述最终导致一个代数方程组。该方法在域上对未知函数进行逼近。[1] 然后将模拟这些有限元的简单方程组合成一个更大的方程系统,以模拟整个问题。然后,有限元通过变化微积分使相关的误差函数最小化来逼近一个解决方案。





随机过程,是依赖于参数的一组随机变量的全体,参数通常是时间。 随机变量是随机现象的数量表现,其时间序列是一组按照时间发生先后顺序进行排列的数据点序列。通常一组时间序列的时间间隔为一恒定值(如1秒,5分钟,12小时,7天,1年),因此时间序列可以作为离散时间数据进行分析处理。研究时间序列数据的意义在于现实中,往往需要研究某个事物其随时间发展变化的规律。这就需要通过研究该事物过去发展的历史记录,以得到其自身发展的规律。


多元回归分析渐进(Multiple Regression Analysis Asymptotics)属于计量经济学领域,主要是一种数学上的统计分析方法,可以分析复杂情况下各影响因素的数学关系,在自然科学、社会和经济学等多个领域内应用广泛。


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。



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