C Language Special: 9. Conditional Statements and Branch Control (if, else, switch)

In C language, conditional judgment statements such as if, else if, else, and switch-case allow different code paths to be executed based on various conditions. This is the foundation for writing logical control programs.

Before diving into the main content, let’s first review the definition and structure of functions in C language.

1. Function Definition in C Language

Basic Syntax:

ReturnType FunctionName(ParameterList) {
    // Function body
    return ReturnValue;
}

Example:

int add(int a, int b) {
    return a + b;
}

A function is a set of statements that can be called repeatedly, taking inputs through parameters and returning results via return.

2. if, else Statements (Basic Branch Structure)

2.1 Basic Syntax

if (condition) {
    // Execute when condition is true
} else {
    // Execute when condition is false
}

2.2 Example

int x = 10;
if (x > 0) {
    printf("x is positive\n");
} else {
    printf("x is non-positive\n");
}

3. if, else if Multiple Branch Structure

3.1 Basic Syntax

if (condition1) {
    // Condition1 is true
} else if (condition2) {
    // Condition2 is true
} else {
    // Other cases
}

• It will evaluate from top to bottom, executing the corresponding code block as soon as a condition is met;
• else can be omitted, and else if can have multiple instances.

3.2 Example

int score = 85;

if (score >= 90) {
    printf("A\n");
} else if (score >= 80) {
    printf("B\n");
} else if (score >= 70) {
    printf("C\n");
} else {
    printf("Fail\n");
}

4. switch-case Multi-value Matching Structure

4.1 Basic Syntax

switch (expression) {
    case value1:
        // Execute when value1 matches
        break;
    case value2:
        // Execute when value2 matches
        break;
    ...
    default:
        // Execute when no case matches
        break;
}

• switch is suitable for discrete value judgments (such as integers, characters);
• Each case must be followed by break, otherwise it will fall through;
• default is optional.

4.2 Example

int day = 3;

switch (day) {
    case 1:
        printf("Monday\n");
        break;
    case 2:
        printf("Tuesday\n");
        break;
    case 3:
        printf("Wednesday\n");
        break;
    default:
        printf("Unknown day\n");
        break;
}

5. Differences and Recommendations between if and switch

Comparison Point	if/else if/else	switch/case
Applicable Scenarios	Range judgment (e.g., x > 0 && x < 100)	Discrete constant matching (e.g., 1, 2, 3…)
Expression Type	Any type with relational operations	Only integers/characters (C99 supports enum)
Readability	Difficult to read when conditions are complex	Concise and clear, suitable for menu-like selection structures
Performance Differences	Compiler optimizes automatically (no significant difference)	Some compilers optimize switch better

6. Nested Usage Example (Comprehensive)

void checkValue(int x) {
    if (x > 0) {
        if (x % 2 == 0) {
            printf("Positive Even\n");
        } else {
            printf("Positive Odd\n");
        }
    } else if (x == 0) {
        printf("Zero\n");
    } else {
        printf("Negative\n");
    }
}

7. Advanced Usage of switch: Merging Multiple Cases

char grade = 'B';

switch (grade) {
    case 'A':
    case 'B':
    case 'C':
        printf("Pass\n");
        break;
    case 'D':
    case 'F':
        printf("Fail\n");
        break;
    default:
        printf("Invalid grade\n");
}

8. Traps and Precautions

Issue	Example	Correct Approach
Forgot `<span>break</span>`	`<span>case 1: ... case 2: ...</span>`	Add `<span>break</span>` after each `<span>case</span>`
Multiple if conditions are true simultaneously	`<span>if (x > 0) else if (x > -1)</span>`	Order logically or use an `<span>else if</span>` chain
`<span>switch</span>` does not support range judgment	`<span>case x > 10:</span>` is illegal	Use `<span>if (x > 10)</span>` instead
Incorrect brackets `<span>{}</span>`	`<span>if (x > 0) printf(); else ...</span>`	It is recommended to develop good indentation + bracket habits

9. Summary Comparison Table

Syntax Structure	Usage Description	Supports Multiple Branches	Supports Range Judgment
`<span>if ... else</span>`	General syntax for conditional judgment	Yes	Yes
`<span>else if</span>`	Multi-condition branch judgment	Yes	Yes
`<span>switch-case</span>`	Discrete constant branch judgment	Yes	No

10. Conclusion

• if/else is the most basic and flexible form of conditional control;
• switch is clearer and more efficient in multi-value branch scenarios (such as menus, state handlers);
• Choosing the right control structure can make the code logical and maintainable;
• When writing conditional statements, good indentation, adding brackets, and clear logical order are key programming habits.