258 lines
5.8 KiB
Markdown
258 lines
5.8 KiB
Markdown
---
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title: Conditional Statements
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localeTitle: 条件陈述
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---
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# C中的条件语句
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条件语句也称为分支语句。他们之所以这么称呼，是因为该计划选择跟随一个或另一个分支。
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## 如果声明
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这是条件语句的最简单形式。它由一个布尔表达式后跟一个或多个语句组成。如果布尔表达式的计算结果为**true** ，那么将执行'if'语句中的代码块。如果布尔表达式的计算结果为**false** ，那么将执行'if'语句结束后（在结束大括号之后）的第一组代码。
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C编程语言**_将任何非零和非空值假定为true_** ，如果它**_为零或null，则将其假定为false_**值。
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#### 句法
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```C
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if(boolean_expression)
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{
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//Block of Statements executed when boolean_expression is true
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}
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```
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#### 例
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```C
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int a = 100;
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if(a < 200)
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{
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printf("a is less than 200\n" );
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}
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```
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#### 结果
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`a is less than 200`
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## 2.如果......其他声明
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如果布尔表达式的计算结果为**true** ，则执行if块，否则执行else块。
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#### 句法
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```C
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if(boolean_expression)
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{
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//Block of Statements executed when boolean_expression is true
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}
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else
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{
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//Block of Statements executed when boolean_expression is false
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}
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```
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#### 例
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```C
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int a = 300;
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if(a < 200)
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{
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printf("a is less than 200\n" );
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}
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else
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{
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printf("a is more than 200\n");
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}
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```
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#### 结果
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`a is more than 200`
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## 3\. if ... else if else注释
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当使用if ... else if..else语句时，要记住几点 -
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* 一个**if**可以有**0或者其他**的， **如果是的话** ，它**必须在其他任何地方之后** 。
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* 一个**if**可以**有零到多个，如果**是，他们**必须在其他之前** 。
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* 一旦**else**成功，其余的其他if或者其他都将被测试。
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#### 句法
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```C
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if(boolean_expression_1)
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{
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//Block of Statements executed when boolean_expression_1 is true
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}
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else if(boolean_expression_2)
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{
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//Block of Statements executed when boolean_expression_1 is false and boolean_expression_2 is true
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}
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else if(boolean_expression_3)
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{
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//Block of Statements executed when both boolean_expression_1 and boolean_expression_2 are false and boolean_expression_3 is true
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}
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else
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{
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//Block of Statements executed when all boolean_expression_1, boolean_expression_2 and boolean_expression_3 are false
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}
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```
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#### 例
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```C
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int a = 300;
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if(a == 100)
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{
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printf("a is equal to 100\n" );
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}
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else if(a == 200)
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{
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printf("a is equal to 200\n");
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}
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else if(a == 300)
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{
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printf("a is equal to 300\n");
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}
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else
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{
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printf("a is more than 300\n");
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}
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```
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#### 结果
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`a is equal to 300`
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## 4.嵌套if语句
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嵌套if-else语句在C编程中总是合法的，这意味着你可以在另一个if或else if语句中使用if或else if语句。
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#### 句法
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```C
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if(boolean_expression_1)
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{
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//Executed when boolean_expression_1 is true
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if(boolean_expression_2)
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{
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//Executed when both boolean_expression_1 and boolean_expression_2 are true
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}
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}
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```
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#### 例
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```C
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int a = 100;
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int b = 200;
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if(a == 100)
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{
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printf("a is equal to 100\n" );
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if(b == 200)
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{
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printf("b is equal to 200\n");
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}
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}
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```
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#### 结果
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```bash
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a is equal to 100
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b is equal to 200
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```
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## 5\. Switch Case Statement
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如果... else（并非总是），switch语句通常比嵌套更快。此外，switch语句的语法更清晰，易于理解。
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### 开关盒的语法
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```
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switch (n)
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{
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case constant1:
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// code to be executed if n is equal to constant1;
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break;
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case constant2:
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// code to be executed if n is equal to constant2;
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break;
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.
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.
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.
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default:
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// code to be executed if n doesn't match any constant
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}
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```
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当找到与switch表达式匹配的case常量时，程序的控制将传递给与该case相关联的代码块。
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在上面的伪代码中，假设n的值等于constant2。编译器将执行与case语句关联的代码块，直到switch块结束，或者直到遇到break语句。
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break语句用于防止代码进入下一种情况。
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### 例：
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```
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// Program to create a simple calculator
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// Performs addition, subtraction, multiplication or division depending the input from user
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# include <stdio.h>
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int main()
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{
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char operator;
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double firstNumber,secondNumber;
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printf("Enter an operator (+, -, *, /): ");
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scanf("%c", &operator);
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printf("Enter two operands: ");
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scanf("%lf %lf",&firstNumber, &secondNumber);
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switch(operator)
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{
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case '+':
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printf("%.1lf + %.1lf = %.1lf",firstNumber, secondNumber, firstNumber+secondNumber);
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break;
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case '-':
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printf("%.1lf - %.1lf = %.1lf",firstNumber, secondNumber, firstNumber-secondNumber);
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break;
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case '*':
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printf("%.1lf * %.1lf = %.1lf",firstNumber, secondNumber, firstNumber*secondNumber);
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break;
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case '/':
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printf("%.1lf / %.1lf = %.1lf",firstNumber, secondNumber, firstNumber/secondNumber);
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break;
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// operator is doesn't match any case constant (+, -, *, /)
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default:
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printf("Error! operator is not correct");
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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 an operator (+, -, *,): -
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Enter two operands: 32.5
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12.4
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32.5 - 12.4 = 20.1
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```
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用户输入的“ - ”运算符存储在运算符变量中。并且，两个操作数32.5和12.4分别存储在变量firstNumber和secondNumber中。
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然后，程序的控制跳转到
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```
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printf("%.1lf / %.1lf = %.1lf",firstNumber, secondNumber, firstNumber/firstNumber);
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```
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最后，break语句结束switch语句。
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如果未使用break语句，则执行正确大小写之后的所有情况。 |