C++ if/else語句

在C++編程中，if語句用於測試條件。 在C++中有多種類型的if語句，它們分別如下所示 -

• if語句
• if-else語句
• 嵌套if語句
• if-else-if階梯

1. C++ if語句

C++ if語句測試條件。 如果條件為真，則執行。

if(condition){
    //code to be executed
}

C++ if語句的執行流程圖如下所示 -

C++ if語句示例

#include <iostream>
using namespace std;

int main () {
    int num = 10;
    if (num % 2 == 0)
    {
        cout<<"It is even number";
    }
    return 0;
}

執行上面代碼，得到以下結果 -

It is even number

2. C++ if-else語句

C++ if-else語句也測試條件。 如果if條件為真，則執行if塊中的代碼，否則執行else塊中的代碼。

if(condition){
    //code if condition is true
}else{
    //code if condition is false
}

if-else執行的流程如下圖中所示 -

C++ if-else示例

#include <iostream>
using namespace std;
int main () {
   int num = 11;
   if (num % 2 == 0)
   {
       cout<<"It is even number";
   }
   else
   {
       cout<<"It is odd number";
   }
   return 0;
}

執行上面代碼得到以下結果 -

It is odd number

C++ if-else示例：帶有來自用戶的輸入

#include <iostream>
using namespace std;
int main () {
    int num;
    cout<<"Enter a Number: ";
    cin>>num;
    if (num % 2 == 0)
    {
        cout<<"It is even number"<<endl;
    }
    else
    {
        cout<<"It is odd number"<<endl;
    }
   return 0;
}

執行上面代碼得到以下結果 -

[zaixian@localhost cpp]$ g++ ifelse.cpp && ./a.out
Enter a Number: 43
It is odd number
[zaixian@localhost cpp]$ g++ ifelse.cpp && ./a.out
Enter a Number: 22
It is even number
[zaixian@localhost cpp]$

3. C++ if-else-if梯形語句

C++ if-else-if梯形語句用於從多個語句中執行一個條件。

if(condition1){
    //code to be executed if condition1 is true
}else if(condition2){
    //code to be executed if condition2 is true
}
else if(condition3){
    //code to be executed if condition3 is true
}
...
else{
    //code to be executed if all the conditions are false
}

if-else-if語句的執行流程如下圖所示 -

C++ if-else-if示例

#include <iostream>
using namespace std;
int main () {
       int num;
       cout<<"Enter a number to check grade:";
       cin>>num;
       if (num <0 || num >100)
       {
           cout<<"wrong number\n";
       }
       else if(num >= 0 && num < 50){
           cout<<"Fail\n";
       }
       else if (num >= 50 && num < 60)
       {
           cout<<"D Grade\n";
       }
       else if (num >= 60 && num < 70)
       {
           cout<<"C Grade\n";
       }
       else if (num >= 70 && num < 80)
       {
           cout<<"B Grade\n";
       }
       else if (num >= 80 && num < 90)
       {
           cout<<"A Grade\n";
       }
       else if (num >= 90 && num <= 100)
       {
           cout<<"A+ Grade\n";
       }
    return 0;
}

執行上面示例代碼，得到以下結果 -

[zaixian@localhost cpp]$ g++ if-else-if.cpp && ./a.out
Enter a number to check grade:99
A+ Grade
[zaixian@localhost cpp]$ g++ if-else-if.cpp && ./a.out
Enter a number to check grade:59
D Grade
[zaixian@localhost cpp]$ g++ if-else-if.cpp && ./a.out
Enter a number to check grade:49
Fail
[zaixian@localhost cpp]$