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

20. Logical Operators: AND, OR, NOT (&&, ||, !), very simple, but there are two mistakes that most beginners will 100% make!

1. Basic Concepts of Logical Operators

The C language provides three logical operators for combining or negating conditional expressions:

Operator	Name	Example	Description
&&	Logical AND	a && b	True if both a and b are true
||	Logical OR	a || b	True if at least one of a or b is true
!	Logical NOT	!a	Negates the truth value of a

2. Detailed Truth Tables

1. Logical AND (&&)

a	b	a && b
0	0	0
0	1	0
1	0	0
1	1	1

2. Logical OR (||)

a	b	a || b
0	0	0
0	1	1
1	0	1
1	1	1

3. Logical NOT (!)

a	!a
0	1
1	0

3. Basic Usage Examples

1. Conditional Combination

int age = 25, score = 85;
if (age >= 18 && score >= 60) {
    printf("Pass\n");
}

2. Range Check

int x = 15;
if (x < 0 || x > 100) {
    printf("Out of range\n");
}

3. State Negation

int isReady = 0;
if (!isReady) {
    printf("System not ready\n");
}

4. Short-Circuit Evaluation Characteristics

1. Short-Circuit of Logical AND

int a = 0, b = 1;
if (a && b++) {  // a is false, b++ will not execute
    // Will not execute
}
printf("%d\n", b);  // Outputs 1 (not incremented)

2. Short-Circuit of Logical OR

int x = 1, y = 0;
if (x || y++) {  // x is true, y++ will not execute
    // Will execute
}
printf("%d\n", y);  // Outputs 0 (not incremented)

5. Operator Precedence

1. 1. Logical NOT (!) has the highest precedence
2. 2. Arithmetic operators > Relational operators > Logical AND > Logical OR
3. 3. Use parentheses to clarify precedence

Example:

if ((a > b) && (c < d))  // Clarifies precedence

6. Common Mistakes

1. Misusing Bitwise Operators

if (a & b) {  // Bitwise AND, not logical check
    // ...
}

Correct approach:

if (a && b) {  // Logical AND
    // ...
}

2. Confusing = and ==

if (a = 0) {  // Assignment operation, always false
    // ...
}

Correct approach:

if (a == 0) {  // Comparison operation
    // ...
}

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