192 lines
4.5 KiB
Markdown
192 lines
4.5 KiB
Markdown
---
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title: If-Else Statement
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localeTitle: If-Else声明
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---
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## If-Else声明有什么作用？
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* If-Else语句是简单If语句的扩展。
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* 在简单的If语句中，如果测试表达式的值为false，那么我们跳过块的代码并继续我们的下一个语句。
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* 但很多时候，如果test表达式的值为false，我们希望执行某些步骤。
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* 在这种情况下，我们使用if-else语句。
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### If-Else声明的一般形式
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```cpp
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if (test expression)
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{
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//statements that run if the test expression is true
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}
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else
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{
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//statements that run if the test expression is false
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}
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```
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### If-Else语句的示例
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如果test表达式为true：
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```cpp
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int a=10;
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if (a < 20) // This expression is true, so...
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{
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//...the code in this block gets executed, and...
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}
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else
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{
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//...the code in this block gets skipped.
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}
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//program continues
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```
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如果test表达式为false：
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```cpp
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int a=10;
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if (a>20) // This expression is false, so this time...
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{
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//...this code gets skipped...
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}
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else
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{
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//...and this code executes instead.
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}
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//program continues
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```
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### C ++中的示例：
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```cpp
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//Program to check whether number entered by user is positive or negative
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#include <iostream>
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using namespace std;
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int main()
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{
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int no;
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cout << "Enter a number: " << endl;
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cin >> no;
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// condition to check if number is positive or negative
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if (no >= 0) // positive
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{
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// block if value is true
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cout << "You entered a positive number: " << no << endl;
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}
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else // negative
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{
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// block if value is false
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cout << "You entered a negative number: " << no << endl;
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}
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// program continues
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cout << "This step is always printed" << endl;
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return 0;
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}
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```
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#### 产量
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* 输入正数时：
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```
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Enter a number:
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4
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You entered a positive number: 4
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This step is always printed
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```
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* 输入负数时：
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```
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Enter a number:
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-200
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You entered a negative number: -200
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This step is always printed
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```
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[亲自尝试一下代码](https://repl.it/MzBq)
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# **随意在FreeCodeCamp的GitHub页面或[FreeCodeCamp的论坛](https://forum.freecodecamp.org/)上询问任何问题[。](https://forum.freecodecamp.org/)**
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[亲自尝试一下代码](https://repl.it/MzBq)
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### 使用if ... else if ... else梯形图
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如果我们必须使用if else基于多个条件做出决策。如果条件如下，我们使用其他 -
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```cpp
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#include<iostream>
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int main()
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{
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int score;
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std::cout<<"Enter your score: \n";
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std::cin>>score;
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if(score>=90)
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std::cout<<"Top performance.";
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else if(score<90 && score>=70)
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std::cout<<"Good performance";
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else if(score<70 && score>=45)
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std::cout<<"Average performance";
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else if(score<45 && score>=30)
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std::cout<<"You can improve it.";
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return 0;
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}
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```
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#### 产量
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```
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Enter your score:
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85
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Good performance
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```
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### if ... else if ... else ladder的另一个例子
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假设我们有用户输入两个数字，我们将显示两个数字是否大于另一个数字。如果两者都不大于另一个，那么我们打印声明“两者都是平等的”。
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在这个scinerio中，我们需要一个if ... else if ... else梯形图。该程序将如下所示：
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```
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#include<iostream>
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using namespace std;
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int main()
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{
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int number1,number2;
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cout << "Enter first number: \n";
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cin >> number1;
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cout << "Enter second number: \n";
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cin >> number2;
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if(number1 > number2) // Checks if the first number is greater than the second number
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{
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cout << "Number 1 is greater.";
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}
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else if(number2 > number1) // Checks if the second number is greater than the first number
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{
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cout << "Number 2 is greater.";
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}
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else // If both of the above cases return false, then both numbers are equal
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{
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cout << "Both the numbers are equal.";
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}
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return 0;
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}
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```
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#### 产量
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```
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Enter first number:
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85
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Enter second number:
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86
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Number 2 is greater.
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```
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* 请注意，只有在不满足初始“if”条件时，程序才会检查“else if”条件。如果这些条件都不满足，则执行最后一个'else'块，打印语句：“两个数字相等。”。
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* if ... else if ... else梯形图的大小可能会有所不同，具体取决于程序试图解决的问题以及需要检查的条件数。
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**祝大家好运**
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**快乐的编码！ :)**
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**欢迎在freeCodeCamp.org的GitHub页面或[freeCodeCamp.org论坛](https://forum.freecodecamp.org/)上询问任何问题** 。 |