“From today on, study hard and make progress every day.”

Repetition is the best method for memory; spend one minute each day to remember the basics of C language.

“C Language Beginner’s Essential Knowledge Notes Series of 100 Articles”“

25. if Statement: The Most Common Conditional Judgment

1. Basic Structure of if Statement

The if statement is the most fundamental conditional control structure in C language, with the following syntax:

if (condition_expression) {
    // Code block executed when condition is true
}

Execution Flow

1. 1. Calculate the value of the condition expression
2. 2. If the result is true (non-zero), execute the code block
3. 3. If the result is false (zero), skip the code block

2. Simple Examples

1. Basic Usage

int age = 18;
if (age >= 18) {
    printf("Adult\n");
}

2. Single Statement Short Form

if (x > 0) printf("Positive\n");  // Can omit {} when there is only one statement

3. if-else Structure

1. Complete Syntax

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

2. Practical Application

int score = 75;
if (score >= 60) {
    printf("Pass\n");
} else {
    printf("Fail\n");
}

4. Multiple Conditional Judgments

1. else-if Ladder

if (condition1) {
    // Condition1 is true
} else if (condition2) {
    // Condition2 is true
} else {
    // All conditions are false
}

2. Grade Level 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("D\n");
}

5. Detailed Explanation of Condition Expressions

1. Common Condition Forms

if (a == b)    // Equality check
if (a != 0)    // Inequality check
if (ptr)       // Non-null pointer check
if (flag)      // Boolean value check

2. Compound Conditions

if (age >= 18 && age <= 60)  // AND
if (score < 60 || score > 90) // OR
if (!is_holiday)             // NOT

6. Common Errors

1. Confusion Between Assignment and Comparison

if (x = 5) {  // Error! Assigning 5 to x
    // ...
}

Correct approach:

if (x == 5) {  // Correct comparison
    // ...
}

2. Floating Point Comparison

if (f == 0.3) {  // Dangerous! Floating point precision issue
    // ...
}

Correct:

#include <math.h>
if (fabs(f - 0.3) < 1e-6) {  // Allowable error
    // ...
}</math.h>

7. Advanced Usage Techniques

1. Simplifying Multiple Conditions

if (10 <= x && x <= 20)  // Mathematical range check

2. Function Return Value Check

#include <string.h>
if (strcmp(s1, s2) == 0)  // String equality</string.h>

3. Initialization Check

FILE *fp;
if ((fp = fopen("file.txt", "r")) == NULL) {
    // Error handling
}

4. Common Condition Precedence

// High-frequency conditions placed first
if (likely_case) {
    // ...
} else {
    // ...
}

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