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19. Relational Operators – Comparing Values is Simple, but Beware of These Errors!

1. Basic Concept of Relational Operators

Relational operators are used to compare the size relationship between two values, returning a boolean value (0 for false, 1 for true):

Operator	Name	Example	Function Description
>	Greater than	a > b	Is a greater than b
<	Less than	a < b	Is a less than b
>=	Greater than or equal to	a >= b	Is a greater than or equal to b
<=	Less than or equal to	a <= b	Is a less than or equal to b
==	Equal to	a == b	Is a equal to b
!=	Not equal to	a != b	Is a not equal to b

2. Basic Usage Examples

1. Numerical Comparison

int a = 10, b = 20;
printf("%d > %d: %d\n", a, b, a > b);   // 0 (false)
printf("%d == %d: %d\n", a, a, a == a); // 1 (true)

2. Character Comparison (Comparing ASCII Codes)

char c1 = 'A', c2 = 'B';
printf("%c < %c: %d\n", c1, c2, c1 < c2);  // 1 (65 < 66)

3. Floating Point Comparison

1. Precision Issues

float x = 0.1 + 0.2;
if (x == 0.3) {  // may be false
    printf("Exactly equal\n");
}

2. Correct Comparison Method

#include <math.h>
#define EPSILON 1e-6
if (fabs(x - 0.3) < EPSILON) {
    printf("Approximately equal\n");
}

4. Operator Precedence

The precedence of relational operators:

1. 1. Arithmetic operators (+ – * / %) have the highest precedence
2. 2. Relational operators (> < >= <=) are next
3. 3. Equality operators (== !=) have the lowest precedence

Example:

int result = 5 + 3 < 10 - 2;  // equivalent to (5+3) < (10-2)

5. Compound Condition Judgments

1. Range Check

int score = 85;
if (score >= 60 && score <= 100) {
    printf("Passed\n");
}

2. Multiple Condition Combinations

char grade = 'B';
if (grade == 'A' || grade == 'B') {
    printf("Excellent\n");
}

6. Common Errors

1. Misusing the Assignment Operator

if (a = 5) {  // Error! Assigning 5 to a, always true
    // ...
}

Correct usage:

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

2. Direct Comparison of Floating Point Numbers

float f = 1.0 / 3.0;
if (f == 0.333333) {  // Dangerous!
    // ...
}

Correct usage:

if (fabs(f - 0.333333) < 1e-5) {
    // ...
}

3. Short-Circuit Evaluation

if (a > 0 && b/a > 1) {  // b/a will not be calculated if a>0 is false
    // ...
}

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