2.2 几种分支结构

使用if语句

if语句用来检验表达式的真假，如果为真则执行某些代码。

#include<iostream>
using namespace std;

int main()
{
    if(true)
    cout << "This is always displayed!" << endl;
    cout << endl;
    if(false)
    cout << "This is never displayed!" << endl << endl;
    
    int score = 1000;
    if(score)
    cout << "At least you didn't score zero!" << endl << endl;
    if(score>=250)
    cout << "You scored 250 or more!" << endl;
    if(score>=500)
    {
    cout << "You scored 500 or more.Nice!" << endl;
    if(score >= 1000)
    cout << "You scored 1000 or more.Impressive!" << endl;
    }

    return 0;
}

运行结果为：

 注意：在if语句中验证的表达式的括号后面没有分号。如果加上分号，会形成一个与if语句成对的一个空语句，使if语句无效。

if语句的嵌套

if可以让程序执行一条语句或一个包含多条语句的代码块。

这个代码块可以包含其他if语句。

if语句中包含另一个if语句称为嵌套。

如果嵌套太深的话，代码将很难读懂，最好限制在基层以内。

使用else子句

在if语句中加入else子句可以引入只有当被验证的表达式位false是才执行的代码。

形式：

if(expression)

    statement;

else

     statement;

// Score Rater 2.0
// Demonstrates an else clause

#include <iostream>
using namespace std;

int main() 
{
    int score;
    cout << "Enter your score: ";
    cin >> score;

    if (score >= 1000)
    {
        cout << "You scored 1000 or more. Impressive!
";
    }
    else
    {
        cout << "You scored less than 1000.
";
    }

    return 0;
}

 

 使用带else子句的if语句序列

if (expression1)
    statement1;
else if (expression2)
    statement2;

.....

else if (expressionN)
     statementN;
else
     statementN+1;

// Score Rater 3.0
// Demonstrates if else-if else suite

#include <iostream>
using namespace std;

int main() 
{
    int score;
    cout << "Enter your score: ";
    cin >> score;

    if (score >= 1000)
    {
        cout << "You scored 1000 or more. Impressive!
";
    }
    else if (score >= 500)
    {
        cout << "You scored 500 or more. Nice.
";
    }
    else if (score >= 250)
    {
        cout << "You scored 250 or more. Decent.
";
    }
    else
    {
        cout << "You scored less than 250. Nothing to brag about.
";
    }

    return 0;
}

 使用switch语句

switch (choice)
{
   case value1:
           statement1;
           break;
   case value2:
           statement2;
           break;
           .
           .
   case valueN:
           statementN;
           break;
   default:
           statementN+1;
}

当程序运行到break语句时，会退出switch结构。

break和default的使用是可选的。

注意：switch语句只能用来比较int型（或其他可以当作int型处理的值，如char型或枚举型），switch语句不能用于其他任何类型。

// Menu Chooser
// Demonstrates the switch statement

#include <iostream>
using namespace std;

int main() 
{
    cout << "Difficulty Levels

";
    cout << "1 - Easy
";
    cout << "2 - Normal
";
    cout << "3 - Hard

";

    int choice;
    cout << "Choice: ";
    cin >> choice;

    switch (choice)
    {
        case 1:    
            cout << "You picked Easy.
";
            break;
        case 2:    
            cout << "You picked Normal.
";
            break;
        case 3:    
            cout << "You picked Hard.
";
            break;
        default:
            cout << "You made an illegal choice.
";
    }

    return 0;
}

 几乎会在每种情况结尾添加break。