小编Die*_*nut的帖子

看看下面的代码

testList = ["This", "Is", "A", "Test"]
dummyList = testList
dummyList = dummyList + ["Hello"]

我期待更改testList的内容,因为dummyList引用testList,我们只是添加了一个新元素,但这没有发生.现在看下面的代码.

testList = ["This", "Is", "A", "Test"]
dummyList = testList
dummyList[0] = 'Hello'

在此代码中,testList中发生了更改.为什么对dummyList的更改会改变第二块代码中testList的内容而不是第一块？

我有以下代码，称之为代码#1

Scanner keyboard = new Scanner(System.in);
String s = keyboard.nextLine();
int x = keyboard.nextInt();
double y = keyboard.nextDouble();
System.out.println("String: "+s);
System.out.println("Double: "+y);
System.out.println("Int: "+x);

如果我输入

Hello World
12.1
12 

输出将是

String: Hello World
Double: 12.1
Int: 12

但是，如果我重新排列我的代码，请将其称为 code#2，如

Scanner keyboard = new Scanner(System.in);
int x = keyboard.nextInt();
double y = keyboard.nextDouble();
String s = keyboard.nextLine();
System.out.println("String: "+s);
System.out.println("Double: "+y);
System.out.println("Int: "+x);

我输入

12
12.1

编译器跳过字符串输入和输出

String:
Double: 12.1
Int: 12 

这对我来说很奇怪。我被告知编译器总是从上到下阅读。我想象编译器将 code#2 读取为

int x = keyboard.nextInt();

首先等待用户输入一个整数，以便将其分配给 x。然后读 …

我有以下列表:

A = [['Computer Science', 'Gender- Male', 'Race Ethnicity- Hispanic', 'Race Ethnicity- White'],
    ['Computer Science', 'Gender- Female', 'Race Ethnicity- White'],
    ['History', 'Gender-Female', 'Race Ethnicity- Black'],
    ['Mechanical Engineering', 'Geder- Male', 'Race Ethnicity- American Indian or Alaskan Native', 'Race Ethnicity- Hispanic']]

我只想保留涉及种族和民族的元素.这就是我想要的结果:

B = [['Race Ethnicity- Hispanic', 'Race Ethnicity- White'],
    ['Race Ethnicity- White'],
    ['Race Ethnicity- Black'],
    ['Race Ethnicity- American Indian or Alaskan Native', 'Race Ethnicity- Hispanic']]

以下类型的工作但不保留列表结构列表

[y for x in test for y in x if "Race Ethnicity" in y]

我怎样才能做到这一点？