C++ 中的 if 和 if-else 語句用於根據條件控制程序的執行流程，配合邏輯運算符可以構建複雜的判斷邏輯。


一、if / if-else 語句

1. 基本 if 語句

if (條件表達式) {
    // 條件為真（true）時執行的代碼
}

如果條件表達式的值為 非零（即邏輯上為 true），則執行大括號內的語句。

2. if-else 語句

if (條件表達式) {
    // 條件為真時執行
} else {
    // 條件為假（false）時執行
}

3. if-else if-else 多分支結構

if (條件1) {
    // 條件1為真時執行
} else if (條件2) {
    // 條件1為假且條件2為真時執行
} else {
    // 所有條件都為假時執行
}

4.示例

#include<iostream>
using namespace std;

int main() {

    // 統計一個句子或者文章有多少個單詞,多少內容
    string text = "my name is xaye";

    int spaces = 0;
    int total = 0;
    for (int i = 0; i < text.length(); ++i) {
        char current = text[i];
        if (current == ' ') { // 當前 == ' '
            spaces++;
        }
        total++;
    }
    cout << "total = " << total << ", spaces = " << spaces << endl;
    
    return 0;
}
xaye@orange:~/code/dev/19$ ./a.out 
total = 15, spaces = 3

if...else...

#include<iostream>
using namespace std;

int main() {
    //想讓用户輸入一個分數，我們輸出及格和不及格
    // int number;
    // cout << "請輸入一個分數：";
    // cin >> number;

    // 示例1
    // if (number >= 60) {
    //     cout << "及格" << endl;
    // } else {
    //     cout << "不及格" << endl;
    // }

    // 示例2
    // if (number < 30) {
    //     cout << "D" << endl;
    // } else if (number < 60){
    //     cout << "C" << endl;
    // } else if (number < 90) {
    //     cout << "B" << endl;
    // } else {
    //     cout << "A" << endl;
    // }
    // 示例3 (猜數字)
    int favorite_number = 89;

    cout << "Enter a number in the rang 1-100 to find my favorite number:" << endl;
    int number = 0;
    do {
        cin >> number;
        if (number > favorite_number) {
            cout << "Too high -- guess again:" << endl;
        } else if (number < favorite_number) {
            cout << "Too low -- guess again:" << endl;
        } else {
            cout << "is right" << endl;
        }
    } while (number != favorite_number);
    
    return 0;
}
xaye@orange:~/code/dev/19$ ./a.out 
請輸入一個分數：80 
及格
xaye@orange:~/code/dev/19$ ./a.out 
請輸入一個分數：65
B
xaye@orange:~/code/dev/19$ ./a.out 
Enter a number in the rang 1-100 to find my favorite number:
50
Too low -- guess again:
90
Too high -- guess again:
75
Too low -- guess again:
85
Too low -- guess again:
89
is right

二、邏輯運算符

1.示例

或

/*
3.邏輯運算符:|| && !
 || -> or 或者
 && -> and 並且
 !  -> not 非
*/
#include<iostream>
using namespace std;
int main(){
    // 輸入 y yes Y 通過
    // n no N 不通過
    cout << "This program may reformat your hard disk \n and destory all your data.\n Do you wish to continue? <y/n>";
    string user_input;
    cin >> user_input;

    if (user_input == "y" || user_input == "yes" || user_input == "Y") {
        cout << "You were warned!" << endl;
    } else if ((user_input == "n" || user_input == "no" || user_input == "N")) {
        cout << "A wise choice ... bye" << endl;
    } else {
        cout << "輸入有誤" << endl;
    }
    return 0;
}
xaye@orange:~/code/dev/19$ ./a.out 
This program may reformat your hard disk 
 and destory all your data.
 Do you wish to continue? <y/n>y
You were warned!
xaye@orange:~/code/dev/19$ ./a.out 
This program may reformat your hard disk 
 and destory all your data.
 Do you wish to continue? <y/n>n
A wise choice ... bye
xaye@orange:~/code/dev/19$ ./a.out 
This program may reformat your hard disk 
 and destory all your data.
 Do you wish to continue? <y/n>d
輸入有誤

and

/*
3.邏輯運算符:|| && !
 || -> or 或者
 && -> and 並且
 !  -> not 非
*/
#include<iostream>
using namespace std;
int main(){

    int number;
    cout << "請輸入一個分數[0 - 100]：";
    cin >> number;
    
    if (number >= 0 && number < 30) {
        cout << "D" << endl;
    } else if (number >= 30 && number < 60){
        cout << "C" << endl;
    } else if (number >= 60 && number < 90) {
        cout << "B" << endl;
    } else if (number >= 90 && number < 100) {
        cout << "A" << endl;
    } else {
        cout << "錯誤數字" << endl;
    }
    return 0;
}
xaye@orange:~/code/dev/19$ ./a.out 
請輸入一個分數[0 - 100]：99
A
xaye@orange:~/code/dev/19$ ./a.out 
請輸入一個分數[0 - 100]：66
B

非

/*
3.邏輯運算符:|| && !
 || -> or 或者
 && -> and 並且
 !  -> not 非
*/
#include<iostream>
using namespace std;
#include <climits>

bool is_int_range(long number) {
    return number <= INT_MAX && number > INT_MIN;
}
int main(){

    cout << "請輸入一個數字：";
    long number = 0;
    cin >> number;

    if (!is_int_range(number)) { // 是不是超出 int 範圍
        cout << "輸入的數字超出了int類型的範圍" << endl;
    }
    return 0;
}
xaye@orange:~/code/dev/19$ ./a.out 
請輸入一個數字：9999999999
輸入的數字超出了int類型的範圍
xaye@orange:~/code/dev/19$ ./a.out 
請輸入一個數字：22222222