✅ Comprehensive Analysis of Control Flow in C Language: The if Statement

🌟 1. Single Branch if

📌 Executes if the condition is true, otherwise skips.

#include <stdio.h>
int main() {
    int age;
    printf("Please enter your age:");
    scanf("%d", &age);

    if (age >= 18) {
        printf("Adult, can act independently!\n");
    }

    return 0;
}

🌟 2. if…else (Two-way Structure)

#include <stdio.h>
int main() {
    int num;
    printf("Please enter an integer:");
    scanf("%d", &num);

    if (num % 2 == 0) {
        printf("Even\n");
    } else {
        printf("Odd\n");
    }

    return 0;
}

🌟 3. if…else if…else (Multi-branch Structure)

#include <stdio.h>
int main() {
    float score;
    printf("Please enter your score:");
    scanf("%f", &score);

    if (score >= 90) {
        printf("Excellent (A)\n");
    } else if (score >= 80) {
        printf("Good (B)\n");
    } else if (score >= 70) {
        printf("Average (C)\n");
    } else if (score >= 60) {
        printf("Pass (D)\n");
    } else {
        printf("Fail (F)\n");
    }

    return 0;
}

🌟 4. Nested if (Condition within a Condition)

#include <stdio.h>
int main() {
    int year;
    printf("Please enter a year:");
    scanf("%d", &year);

    if (year % 4 == 0) {
        if (year % 100 != 0) {
            printf("Leap year\n");
        } else {
            if (year % 400 == 0)
                printf("Leap year\n");
            else
                printf("Common year\n");
        }
    } else {
        printf("Common year\n");
    }

    return 0;
}

⚠️ Recommended Writing (More Concise):

if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0)

✅ 5. Comprehensive Example: Multi-condition Judgment

📌 Input height and weight to determine if the weight is standard

• Standard weight ≈ height in cm – 110
• Allowable error ±5kg

#include <stdio.h>
int main() {
    int height, weight, standard;
    printf("Please enter height (cm) and weight (kg):");
    scanf("%d %d", &height, &weight);

    standard = height - 110;

    if (weight > standard + 5) {
        printf("Overweight\n");
    } else if (weight < standard - 5) {
        printf("Underweight\n");
    } else {
        printf("Standard weight\n");
    }

    return 0;
}

🧠 6. Common Errors Summary

Error Writing	Reason
<span>if(x = 5)</span>	<span>=</span> is assignment, should use <span>==</span>
<span>if x > 0</span>	Condition must be enclosed in parentheses <span>()</span>
Multiple statements without <span>{}</span>	May cause logical errors
Confused indentation	Poor readability, prone to errors

✅ Recommendation: Always write <span>{}</span> to cultivate standardized thinking

🧪 7. Classroom Exercises

📝 Exercise 1: Determine if Positive

#include <stdio.h>
int main() {
    int n;
    printf("Please enter an integer:");
    scanf("%d", &n);

    if (n > 0)
        printf("Positive\n");
    else
        printf("Non-positive\n");

    return 0;
}

📝 Exercise 2: Highest Score Judgment (Comparison of Two)

#include <stdio.h>
int main() {
    float a, b;
    printf("Please enter two scores:");
    scanf("%f %f", &a, &b);

    if (a > b)
        printf("First is higher: %.2f\n", a);
    else
        printf("Second is higher: %.2f\n", b);

    return 0;
}

📝 Exercise 3: Triangle Validity Judgment

The sum of any two sides must be greater than the third side 👉 to form a triangle

#include <stdio.h>
int main() {
    int a, b, c;
    printf("Please enter the lengths of three sides:");
    scanf("%d %d %d", &a, &b, &c);

    if (a + b > c && a + c > b && b + c > a)
        printf("Can form a triangle\n");
    else
        printf("Cannot form a triangle\n");

    return 0;
}

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C Language Multi-branch Structure —— Comprehensive Analysis of switch Statement is particularly useful for menu selection and classification judgments ✅