C 语言中的决策结构

简介

在本实验中，你将学习如何在 C 编程中构建决策结构。你将从理解 if 语句的基本概念开始，该语句允许你的程序根据特定条件做出决策。接着，你将探索不等运算符、链式 if-else 语句、使用逻辑运算符组合表达式，并练习条件逻辑。通过本实验的学习，你将掌握如何在 C 程序中创建决策结构，从而编写出更复杂和智能的代码。

Skills Graph

理解 If 语句

在本步骤中，你将学习 C 编程中 if 语句的基本概念，这对于在代码中创建决策结构至关重要。if 语句允许你的程序根据特定条件做出决策并执行不同的代码块。

让我们从一个简单的 C 程序开始，演示基本的 if 语句语法。打开 VSCode 编辑器，在 ~/project 目录下创建一个名为 if_statement.c 的新文件：

cd ~/project
touch if_statement.c

#include <stdio.h>

int main() {
    int temperature = 25;

    if (temperature > 30) {
        printf("It's a hot day!\n");
    }

    if (temperature <= 30) {
        printf("The temperature is comfortable.\n");
    }

    return 0;
}

编译并运行程序：

gcc if_statement.c -o if_statement
./if_statement

示例输出：

The temperature is comfortable.

让我们分解一下 if 语句：

• if 关键字后跟一个用括号 () 括起来的条件
• 条件是一个逻辑表达式，其结果为真（true）或假（false）
• 如果条件为真，则执行花括号 {} 内的代码块
• 如果条件为假，则跳过该代码块

现在，让我们创建一个更具交互性的示例，展示 if 语句如何根据用户输入做出决策：

#include <stdio.h>

int main() {
    int age;

    printf("Enter your age: ");
    scanf("%d", &age);

    if (age >= 18) {
        printf("You are eligible to vote.\n");
    }

    if (age < 18) {
        printf("You are not old enough to vote.\n");
    }

    return 0;
}

编译并运行程序：

gcc voting_eligibility.c -o voting_eligibility
./voting_eligibility

示例输出：

Enter your age: 20
You are eligible to vote.

需要记住的关键点：

• if 语句允许你的程序做出决策
• 条件会被评估为真或假
• 只有条件为真的代码块会被执行
• 你可以使用比较运算符，如 >、<、>=、<=、==、!=

探索不等运算符

在本步骤中，你将通过探索不等运算符深入学习 C 编程，这些运算符对于创建更复杂的条件语句至关重要。不等运算符允许你比较值并根据它们的关系做出决策。

让我们在 ~/project 目录下创建一个名为 inequality_operators.c 的新文件，以演示不同的不等运算符：

cd ~/project
touch inequality_operators.c

#include <stdio.h>

int main() {
    int x = 10;
    int y = 20;

    // 等于 (==) 运算符
    if (x == y) {
        printf("x is equal to y\n");
    } else {
        printf("x is not equal to y\n");
    }

    // 不等于 (!=) 运算符
    if (x != y) {
        printf("x is different from y\n");
    }

    // 大于 (>) 运算符
    if (y > x) {
        printf("y is greater than x\n");
    }

    // 小于 (<) 运算符
    if (x < y) {
        printf("x is less than y\n");
    }

    // 大于或等于 (>=) 运算符
    if (y >= x) {
        printf("y is greater than or equal to x\n");
    }

    // 小于或等于 (<=) 运算符
    if (x <= y) {
        printf("x is less than or equal to y\n");
    }

    return 0;
}

编译并运行程序：

gcc inequality_operators.c -o inequality_operators
./inequality_operators

示例输出：

x is not equal to y
x is different from y
y is greater than x
x is less than y
y is greater than or equal to x
x is less than or equal to y

让我们创建另一个示例，展示如何使用不等运算符处理用户输入：

#include <stdio.h>

int main() {
    int score;

    printf("Enter your exam score: ");
    scanf("%d", &score);

    if (score >= 90) {
        printf("Excellent! You got an A.\n");
    } else if (score >= 80) {
        printf("Good job! You got a B.\n");
    } else if (score >= 70) {
        printf("Not bad. You got a C.\n");
    } else if (score >= 60) {
        printf("You passed. You got a D.\n");
    } else {
        printf("Sorry, you failed the exam.\n");
    }

    return 0;
}

编译并运行程序：

gcc grade_calculator.c -o grade_calculator
./grade_calculator

示例输出：

Enter your exam score: 85
Good job! You got a B.

需要记住的关键点：

• 不等运算符用于比较值
• == 检查值是否相等
• != 检查值是否不相等
• > 检查左侧值是否大于右侧值
• < 检查左侧值是否小于右侧值
• >= 检查左侧值是否大于或等于右侧值
• <= 检查左侧值是否小于或等于右侧值

链式 If-Else 语句

在本步骤中，你将学习如何通过链式 if-else 语句在 C 编程中创建更复杂的决策结构。链式 if-else 语句允许你处理多个条件，并在之前的条件未满足时提供替代的代码路径。

让我们在 ~/project 目录下创建一个名为 weather_advisor.c 的文件，以演示链式 if-else 语句：

cd ~/project
touch weather_advisor.c

#include <stdio.h>

int main() {
    int temperature;
    char weather_type;

    printf("Enter the temperature: ");
    scanf("%d", &temperature);

    printf("Enter the weather type (S for sunny, R for rainy, C for cloudy): ");
    scanf(" %c", &weather_type);

    if (temperature > 30 && weather_type == 'S') {
        printf("It's a hot and sunny day. Stay hydrated and use sunscreen!\n");
    } else if (temperature > 30 && weather_type == 'R') {
        printf("It's hot and rainy. Be careful of potential thunderstorms.\n");
    } else if (temperature > 30 && weather_type == 'C') {
        printf("It's hot and cloudy. Wear light clothing.\n");
    } else if (temperature >= 20 && temperature <= 30) {
        printf("The temperature is comfortable.\n");
    } else if (temperature < 20) {
        printf("It's a bit cool today. Consider wearing a light jacket.\n");
    } else {
        printf("Invalid input. Please check your temperature and weather type.\n");
    }

    return 0;
}

编译并运行程序：

gcc weather_advisor.c -o weather_advisor
./weather_advisor

示例输出：

Enter the temperature: 35
Enter the weather type (S for sunny, R for rainy, C for cloudy): S
It's a hot and sunny day. Stay hydrated and use sunscreen!

让我们创建另一个示例，展示链式 if-else 语句的更实际用途：

#include <stdio.h>

int main() {
    char category;
    double price, discount = 0.0;

    printf("Enter product category (A/B/C): ");
    scanf(" %c", &category);

    printf("Enter product price: ");
    scanf("%lf", &price);

    if (category == 'A') {
        if (price > 1000) {
            discount = 0.2;  // 20% discount for high-value A category
        } else {
            discount = 0.1;  // 10% discount for A category
        }
    } else if (category == 'B') {
        if (price > 500) {
            discount = 0.15;  // 15% discount for high-value B category
        } else {
            discount = 0.05;  // 5% discount for B category
        }
    } else if (category == 'C') {
        if (price > 200) {
            discount = 0.1;  // 10% discount for high-value C category
        } else {
            discount = 0.0;  // No discount for low-value C category
        }
    } else {
        printf("Invalid category!\n");
        return 1;
    }

    double discounted_price = price * (1 - discount);
    printf("Original price: $%.2f\n", price);
    printf("Discount: %.0f%%\n", discount * 100);
    printf("Discounted price: $%.2f\n", discounted_price);

    return 0;
}

编译并运行程序：

gcc discount_calculator.c -o discount_calculator
./discount_calculator

示例输出：

Enter product category (A/B/C): B
Enter product price: 600
Original price: $600.00
Discount: 15%
Discounted price: $510.00

需要记住的关键点：

• 链式 if-else 语句允许你处理多个条件
• 每个条件按从上到下的顺序检查
• 只有第一个匹配条件的代码块会被执行
• else 块在没有条件满足时提供默认操作
• 你可以嵌套 if-else 语句以创建更复杂的决策结构

使用逻辑运算符组合表达式

在本步骤中，你将学习如何在 C 编程中使用逻辑运算符组合多个条件。逻辑运算符允许你通过连接多个表达式来创建更复杂的条件语句。

让我们在 ~/project 目录下创建一个名为 logical_operators.c 的文件，以演示三种主要的逻辑运算符：AND (&&)、OR (||) 和 NOT (!)：

cd ~/project
touch logical_operators.c

#include <stdio.h>

int main() {
    int age, income;
    char is_student;

    printf("Enter your age: ");
    scanf("%d", &age);

    printf("Enter your annual income: ");
    scanf("%d", &income);

    printf("Are you a student? (Y/N): ");
    scanf(" %c", &is_student);

    // AND (&&) 运算符：两个条件都必须为真
    if (age >= 18 && income < 30000) {
        printf("You qualify for a basic credit card.\n");
    }

    // OR (||) 运算符：至少一个条件必须为真
    if (age < 25 || is_student == 'Y') {
        printf("You may be eligible for a student discount.\n");
    }

    // 组合多个条件
    if ((age >= 18 && income >= 30000) || (is_student == 'Y')) {
        printf("You qualify for a premium credit card.\n");
    }

    // NOT (!) 运算符：否定条件
    if (!(age < 18)) {
        printf("You are an adult.\n");
    }

    return 0;
}

编译并运行程序：

gcc logical_operators.c -o logical_operators
./logical_operators

示例输出：

Enter your age: 22
Enter your annual income: 25000
Are you a student? (Y/N): Y
You qualify for a basic credit card.
You may be eligible for a student discount.
You qualify for a premium credit card.
You are an adult.

让我们创建另一个示例，展示更复杂的逻辑条件：

#include <stdio.h>

int main() {
    char membership_type;
    int visits_per_month, total_spending;

    printf("Enter your membership type (B for Bronze, S for Silver, G for Gold): ");
    scanf(" %c", &membership_type);

    printf("Enter number of visits per month: ");
    scanf("%d", &visits_per_month);

    printf("Enter total monthly spending: ");
    scanf("%d", &total_spending);

    // 使用多个逻辑运算符的复杂条件
    if ((membership_type == 'G') ||
        (membership_type == 'S' && visits_per_month >= 10) ||
        (membership_type == 'B' && total_spending > 500)) {
        printf("You are eligible for a special promotion!\n");
    }

    // 使用 NOT 运算符的复杂条件
    if (!(membership_type == 'B' && visits_per_month < 5)) {
        printf("You can access additional membership benefits.\n");
    }

    return 0;
}

编译并运行程序：

gcc membership_conditions.c -o membership_conditions
./membership_conditions

示例输出：

Enter your membership type (B for Bronze, S for Silver, G for Gold): S
Enter number of visits per month: 12
Enter total monthly spending: 300
You are eligible for a special promotion!
You can access additional membership benefits.

需要记住的关键点：

• && (AND) 运算符：两个条件都必须为真
• || (OR) 运算符：至少一个条件必须为真
• ! (NOT) 运算符：否定条件
• 你可以组合多个条件以创建复杂逻辑
• 括号可以帮助明确求值顺序

实践条件逻辑

在最后的步骤中，你将通过创建一个综合程序来应用所学的 C 语言决策结构知识，该程序将展示多种条件逻辑技术。

让我们在 ~/project 目录下创建一个名为 personal_finance_advisor.c 的多功能程序，帮助用户做出财务决策：

cd ~/project
touch personal_finance_advisor.c

#include <stdio.h>

int main() {
    double income, expenses, savings;
    char employment_status, age_group;

    // 输入财务信息
    printf("Enter your monthly income: $");
    scanf("%lf", &income);

    printf("Enter your monthly expenses: $");
    scanf("%lf", &expenses);

    printf("Enter your current savings: $");
    scanf("%lf", &savings);

    printf("Employment status (F for Full-time, P for Part-time, U for Unemployed): ");
    scanf(" %c", &employment_status);

    printf("Age group (Y for Young (18-35), M for Middle-aged (36-55), S for Senior (56+)): ");
    scanf(" %c", &age_group);

    // 计算储蓄率
    double savings_rate = (savings / income) * 100;
    double disposable_income = income - expenses;

    // 使用复杂条件逻辑提供财务建议
    printf("\n--- Financial Analysis ---\n");

    // 储蓄建议
    if (savings_rate < 10) {
        printf("Warning: Your savings rate is low (%.2f%%).\n", savings_rate);
    } else if (savings_rate >= 10 && savings_rate < 20) {
        printf("Good start! Your savings rate is moderate (%.2f%%).\n", savings_rate);
    } else {
        printf("Excellent! Your savings rate is strong (%.2f%%).\n", savings_rate);
    }

    // 投资建议
    if ((employment_status == 'F' && disposable_income > 500) ||
        (age_group == 'Y' && savings > 5000)) {
        printf("Recommendation: Consider starting an investment portfolio.\n");
    }

    // 应急基金建议
    if (savings < (expenses * 3)) {
        printf("Advice: Build an emergency fund covering at least 3 months of expenses.\n");
    }

    // 支出控制
    if (expenses > (income * 0.7)) {
        printf("Alert: Your expenses are too high compared to your income.\n");
    }

    // 特殊条件
    if ((age_group == 'Y' && employment_status == 'F') ||
        (savings > (income * 2))) {
        printf("You are in a good financial position!\n");
    }

    // 详细的财务健康评估
    if (!(disposable_income < 0)) {
        printf("Your disposable income is: $%.2f\n", disposable_income);
    } else {
        printf("Warning: Your expenses exceed your income.\n");
    }

    return 0;
}

编译并运行程序：

gcc personal_finance_advisor.c -o personal_finance_advisor
./personal_finance_advisor

示例输出：

Enter your monthly income: $5000
Enter your monthly expenses: $3500
Enter your current savings: $10000
Employment status (F for Full-time, P for Part-time, U for Unemployed): F
Age group (Y for Young (18-35), M for Middle-aged (36-55), S for Senior (56+)): Y

--- Financial Analysis ---
Good start! Your savings rate is moderate (40.00%).
Recommendation: Consider starting an investment portfolio.
You are in a good financial position!
Your disposable income is: $1500.00

让我们创建一个额外的挑战程序，进一步练习条件逻辑：

#include <stdio.h>

int main() {
    int math_score, science_score, english_score;
    char extra_credit;

    printf("Enter Math score (0-100): ");
    scanf("%d", &math_score);

    printf("Enter Science score (0-100): ");
    scanf("%d", &science_score);

    printf("Enter English score (0-100): ");
    scanf("%d", &english_score);

    printf("Did you complete extra credit? (Y/N): ");
    scanf(" %c", &extra_credit);

    // 计算平均分
    double average = (math_score + science_score + english_score) / 3.0;

    // 综合成绩评定
    if (average >= 90 ||
        (extra_credit == 'Y' && average >= 85)) {
        printf("Congratulations! You earned an A grade.\n");
    } else if (average >= 80 ||
               (extra_credit == 'Y' && average >= 75)) {
        printf("Great job! You earned a B grade.\n");
    } else if (average >= 70 ||
               (extra_credit == 'Y' && average >= 65)) {
        printf("You passed. You earned a C grade.\n");
    } else {
        printf("You need to improve. Consider retaking the courses.\n");
    }

    return 0;
}

编译并运行程序：

gcc grade_calculator.c -o grade_calculator
./grade_calculator

示例输出：

Enter Math score (0-100): 85
Enter Science score (0-100): 88
Enter English score (0-100): 92
Did you complete extra credit? (Y/N): N
Great job! You earned a B grade.

需要记住的关键点：

• 使用逻辑运算符组合多个条件
• 使用嵌套的 if-else 语句进行复杂决策
• 考虑边缘情况并提供全面的逻辑
• 将复杂问题分解为更小、可管理的条件

总结

在本实验中，你学习了 C 编程中 if 语句的基本概念，这对于在代码中创建决策结构至关重要。你探索了 if 语句如何允许程序根据特定条件做出决策并执行不同的代码块。你还学习了不等运算符、链式 if-else 语句、使用逻辑运算符组合表达式，以及通过练习条件逻辑在 C 程序中构建更复杂的决策结构。

本实验涵盖的关键点包括理解 if 语句的基本语法、将逻辑表达式评估为真或假，以及根据条件执行适当的代码块。你还学习了如何使用 if 语句根据用户输入做出决策，这是构建交互式应用程序的重要技能。