C Notes

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Logical Operators in C Programming

Learn logical operators in C: AND (&&), OR (||), and NOT (!). Understand short-circuit evaluation, truth tables, operator precedence, and practical examples for decision-making.

Logical operators allow you to combine multiple conditions into a single expression. When a simple "is x greater than 5?" isn't enough and you need "is x greater than 5 AND less than 100?", logical operators are what you reach for. They're essential for building complex decision-making logic in any real-world C program.

The Three Logical Operators

C provides three logical operators:

Operator	Name	Description	Example
`&&`	Logical AND	True if BOTH operands are true	`(a > 0 && b > 0)`
`		`	Logical OR	True if EITHER operand is true	`(a > 0	b > 0)`
`!`	Logical NOT	Inverts the truth value	`!(a > 0)`

Just like relational operators, logical operators return 1 for true and 0 for false.

Logical AND (&&)

The AND operator returns true only when both conditions are true. If either condition is false, the entire expression is false.

Truth Table for AND:

A	B	A && B
0	0	0
0	1	0
1	0	0
1	1	1

c example

#include <stdio.h>

int main() {
    int age = 25;
    float salary = 55000;

    // Both conditions must be true
    if (age >= 21 && salary >= 50000) {
        printf("Eligible for premium credit card\n");
    }

    // Range checking
    int score = 85;
    if (score >= 80 && score <= 90) {
        printf("Score %d falls in B+ range\n", score);
    }

    // Multiple AND conditions
    int x = 15;
    if (x > 0 && x < 100 && x % 5 == 0) {
        printf("%d is positive, less than 100, and divisible by 5\n", x);
    }

    return 0;
}

Output

Eligible for premium credit card
Score 85 falls in B+ range
15 is positive, less than 100, and divisible by 5

Logical OR (||)

The OR operator returns true when at least one condition is true. It returns false only when both conditions are false.

Truth Table for OR:

A	B	A \
0	0	0
0	1	1
1	0	1
1	1	1

Example

#include <stdio.h> int main() { char grade = 'B'; // Either condition can be true if (grade == 'A' || grade == 'B') { printf("Excellent performance!\n"); } // Checking for weekends int day = 6; // 1=Mon, ..., 7=Sun if (day == 6 || day == 7) { printf("It's the weekend!\n"); } // Vowel check char ch = 'e'; if (ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o' || ch == 'u') { printf("'%c' is a vowel\n", ch); } return 0; }

Output

Excellent performance!
It's the weekend!
'e' is a vowel

Logical NOT (!)

The NOT operator is a unary operator that inverts the truth value of its operand. If the expression is true (non-zero), ! makes it false (0), and vice versa.

c example

#include <stdio.h>

int main() {
    int loggedIn = 0;  // 0 = false, not logged in

    if (!loggedIn) {
        printf("Please log in first\n");
    }

    // Inverting conditions
    int age = 15;
    if (!(age >= 18)) {
        printf("Not an adult (age: %d)\n", age);
    }

    // NOT with values
    printf("!0 = %d\n", !0);
    printf("!1 = %d\n", !1);
    printf("!5 = %d\n", !5);     // Any non-zero becomes 0
    printf("!(-3) = %d\n", !(-3)); // Negative non-zero also becomes 0

    return 0;
}

Output

Please log in first
Not an adult (age: 15)
!0 = 1
!1 = 0
!5 = 0
!(-3) = 0

Short-Circuit Evaluation

This is one of the most important concepts related to logical operators in C. Short-circuit evaluation means that C stops evaluating a logical expression as soon as the result is determined:

For &&: If the left operand is false, the right operand is NOT evaluated (because false AND anything = false)
For ||: If the left operand is true, the right operand is NOT evaluated (because true OR anything = true)

Example

#include <stdio.h> int main() { int a = 0, b = 5; // Short-circuit with AND // Since a is 0 (false), (b/a > 2) is never evaluated // This prevents division by zero! if (a != 0 && b / a > 2) { printf("This won't print\n"); } else { printf("Short-circuit prevented division by zero!\n"); } // Short-circuit with OR int x = 10; // Since x > 5 is true, the second condition is skipped if (x > 5 || ++x > 11) { printf("x = %d (not incremented due to short-circuit)\n", x); } return 0; }

Output

Short-circuit prevented division by zero!
x = 10 (not incremented due to short-circuit)

Demonstrating Side Effects with Short-Circuit

Example

#include <stdio.h> int main() { int a = 1, b = 2, c = 3; // With AND: second part executes because first is true if (a > 0 && ++b > 2) { printf("AND: b was incremented to %d\n", b); } // With OR: second part does NOT execute because first is true c = 3; if (a > 0 || ++c > 3) { printf("OR: c was NOT incremented, c = %d\n", c); } return 0; }

Output

AND: b was incremented to 3
OR: c was NOT incremented, c = 3

Warning: Never rely on side effects (like ++) in short-circuited expressions. It makes code hard to read and debug.

Combining Logical Operators

You can build complex conditions by combining AND, OR, and NOT:

Example

#include <stdio.h> int main() { int age = 25; int hasLicense = 1; int hasInsurance = 1; int isSuspended = 0; // Complex condition for driving eligibility if (age >= 18 && hasLicense && hasInsurance && !isSuspended) { printf("You are eligible to drive\n"); } // Login validation char username[] = "admin"; int passwordCorrect = 1; int accountLocked = 0; if ((passwordCorrect && !accountLocked) || (age > 18 && hasLicense)) { printf("Access granted\n"); } return 0; }

Output

You are eligible to drive
Access granted

Operator Precedence

The precedence of logical operators relative to other operators:

Priority	Operator	Description
Higher	`!`	Logical NOT
...	Arithmetic, Relational	(evaluated first)
	`&&`	Logical AND
Lower	`		`	Logical OR

This means && has higher precedence than ||:

Example

#include <stdio.h> int main() { int a = 1, b = 0, c = 1; // This is evaluated as: a || (b && c) // NOT as: (a || b) && c int result1 = a || b && c; printf("a || b && c = %d\n", result1); // 1 || (0 && 1) = 1 || 0 = 1 // Be explicit with parentheses for clarity int result2 = (a || b) && c; printf("(a || b) && c = %d\n", result2); // (1 || 0) && 1 = 1 && 1 = 1 // Different case a = 0; b = 0; c = 1; printf("0 || 0 && 1 = %d\n", a || b && c); // 0 || (0 && 1) = 0 || 0 = 0 printf("(0 || 0) && 1 = %d\n", (a || b) && c); // same result here return 0; }

Output

a || b && c = 1
(a || b) && c = 1
0 || 0 && 1 = 0
(0 || 0) && 1 = 0

Practical Example: Leap Year Check

A year is a leap year if it's divisible by 4, except for century years which must be divisible by 400:

Example

#include <stdio.h> int main() { int year; printf("Enter a year: "); scanf("%d", &year); if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)) { printf("%d is a leap year\n", year); } else { printf("%d is not a leap year\n", year); } return 0; }

Output

Enter a year: 2024
2024 is a leap year

Practical Example: Triangle Validity

Example

#include <stdio.h> int main() { int a = 3, b = 4, c = 5; // Triangle inequality: sum of any two sides must be greater than the third if (a + b > c && b + c > a && a + c > b) { printf("Sides %d, %d, %d form a valid triangle\n", a, b, c); // Check type of triangle if (a == b && b == c) { printf("It's an equilateral triangle\n"); } else if (a == b || b == c || a == c) { printf("It's an isosceles triangle\n"); } else { printf("It's a scalene triangle\n"); } } else { printf("Not a valid triangle\n"); } return 0; }

Output

Sides 3, 4, 5 form a valid triangle
It's a scalene triangle

Difference Between Logical and Bitwise Operators

Don't confuse logical operators with bitwise operators:

Logical	Bitwise	Difference
`&&`	`&`	`&&` short-circuits; `&` evaluates both and works bit-by-bit
`		`	`	`	`	`short-circuits;`	` evaluates both and works bit-by-bit
`!`	`~`	`!` returns 0 or 1; `~` flips all bits

Example

#include <stdio.h> int main() { int a = 2, b = 3; printf("Logical: %d && %d = %d\n", a, b, a && b); // 1 (both non-zero) printf("Bitwise: %d & %d = %d\n", a, b, a & b); // 2 (bit operation) printf("Logical: %d || %d = %d\n", a, b, a || b); // 1 printf("Bitwise: %d | %d = %d\n", a, b, a | b); // 3 return 0; }

Output

Logical: 2 && 3 = 1
Bitwise: 2 & 3 = 2
Logical: 2 || 3 = 1
Bitwise: 2 | 3 = 3

Interview Questions

Q1: What is short-circuit evaluation in C? Give an example where it prevents a crash.

Short-circuit evaluation means C stops evaluating a logical expression once the result is determined. For &&, if the left side is false, the right side is skipped. Example: if (ptr != NULL && ptr->value > 0) — if ptr is NULL, the dereference ptr->value is never executed, preventing a segmentation fault.

Q2: What is the output of printf("%d", !0 + !0 + !0)?

The output is 3. !0 evaluates to 1 (NOT of false is true). So the expression becomes 1 + 1 + 1 = 3.

Q3: Is && the same as & in C?

No. && is the logical AND operator — it evaluates truthiness and returns 0 or 1 with short-circuit behavior. & is the bitwise AND operator — it performs bit-by-bit AND on the binary representation and evaluates both operands regardless.

Q4: What is the precedence order of !, &&, and ||?

! has the highest precedence (it's a unary operator), followed by &&, and then || has the lowest precedence among the three. So !a && b || c is evaluated as ((!a) && b) || c.

Q5: Can you use integers other than 0 and 1 with logical operators?

Yes. In C, any non-zero value is considered true, and zero is false. So 5 && 3 evaluates to 1, and 0 || -7 evaluates to 1. The NOT operator !5 gives 0, and !0 gives 1.

Summary

C has three logical operators: && (AND), || (OR), and ! (NOT)
They always return 1 (true) or 0 (false) as integers
Short-circuit evaluation is a key feature — the second operand may not be evaluated
&& has higher precedence than ||, so use parentheses for clarity
Don't confuse logical operators (&&, ||) with bitwise operators (&, |)
Any non-zero value is treated as true in logical expressions
Short-circuit evaluation can be used to guard against null pointers and division by zero

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Logical Operators in C Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, operators, logical, logical operators in c programming

Continue learning this concept

OperatorsArithmetic Operators in C ProgrammingLearn all arithmetic operators in C including addition, subtraction, multiplication, division, and modulus. Understand integer division traps, operator precedence, and type casting with practical examples.OperatorsAssignment Operators in C ProgrammingLearn all assignment operators in C including simple assignment, compound assignment operators (+=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=). Understand their syntax, behavior, and practical usage.OperatorsBitwise Operators in C ProgrammingComplete guide to bitwise operators in C: AND, OR, XOR, NOT, left shift, and right shift. Learn practical applications like flag manipulation, masking, and power-of-2 operations with examples.OperatorsIncrement and Decrement Operators in CMaster increment (++) and decrement (--) operators in C. Understand the difference between prefix and postfix, common pitfalls, undefined behavior, and practical usage in loops and expressions.OperatorsRelational Operators in C ProgrammingMaster relational (comparison) operators in C: equal to, not equal, greater than, less than, and their combinations. Learn how they return 0 or 1, common mistakes, and usage in conditions.OperatorsTernary Operator in C ProgrammingLearn the ternary (conditional) operator (?:) in C. Understand syntax, nested ternary expressions, when to use ternary vs if-else, and practical examples for concise conditional logic.

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C Notes

Logical Operators in C Programming

Learn logical operators in C: AND (&&), OR (||), and NOT (!). Understand short-circuit evaluation, truth tables, operator precedence, and practical examples for decision-making.

The Three Logical Operators

C provides three logical operators:

Operator	Name	Description	Example
`&&`	Logical AND	True if BOTH operands are true	`(a > 0 && b > 0)`
`		`	Logical OR	True if EITHER operand is true	`(a > 0	b > 0)`
`!`	Logical NOT	Inverts the truth value	`!(a > 0)`

Just like relational operators, logical operators return 1 for true and 0 for false.

Logical AND (&&)

The AND operator returns true only when both conditions are true. If either condition is false, the entire expression is false.

Truth Table for AND:

A	B	A && B
0	0	0
0	1	0
1	0	0
1	1	1

c example

#include <stdio.h>

int main() {
    int age = 25;
    float salary = 55000;

    // Both conditions must be true
    if (age >= 21 && salary >= 50000) {
        printf("Eligible for premium credit card\n");
    }

    // Range checking
    int score = 85;
    if (score >= 80 && score <= 90) {
        printf("Score %d falls in B+ range\n", score);
    }

    // Multiple AND conditions
    int x = 15;
    if (x > 0 && x < 100 && x % 5 == 0) {
        printf("%d is positive, less than 100, and divisible by 5\n", x);
    }

    return 0;
}

Output

Eligible for premium credit card
Score 85 falls in B+ range
15 is positive, less than 100, and divisible by 5

Logical OR (||)

The OR operator returns true when at least one condition is true. It returns false only when both conditions are false.

Truth Table for OR:

A	B	A \
0	0	0
0	1	1
1	0	1
1	1	1

Example

Output

Excellent performance!
It's the weekend!
'e' is a vowel

Logical NOT (!)

The NOT operator is a unary operator that inverts the truth value of its operand. If the expression is true (non-zero), ! makes it false (0), and vice versa.

c example

#include <stdio.h>

int main() {
    int loggedIn = 0;  // 0 = false, not logged in

    if (!loggedIn) {
        printf("Please log in first\n");
    }

    // Inverting conditions
    int age = 15;
    if (!(age >= 18)) {
        printf("Not an adult (age: %d)\n", age);
    }

    // NOT with values
    printf("!0 = %d\n", !0);
    printf("!1 = %d\n", !1);
    printf("!5 = %d\n", !5);     // Any non-zero becomes 0
    printf("!(-3) = %d\n", !(-3)); // Negative non-zero also becomes 0

    return 0;
}

Output

Please log in first
Not an adult (age: 15)
!0 = 1
!1 = 0
!5 = 0
!(-3) = 0

Short-Circuit Evaluation

This is one of the most important concepts related to logical operators in C. Short-circuit evaluation means that C stops evaluating a logical expression as soon as the result is determined:

For &&: If the left operand is false, the right operand is NOT evaluated (because false AND anything = false)
For ||: If the left operand is true, the right operand is NOT evaluated (because true OR anything = true)

Example

Output

Short-circuit prevented division by zero!
x = 10 (not incremented due to short-circuit)

Demonstrating Side Effects with Short-Circuit

Example

Output

AND: b was incremented to 3
OR: c was NOT incremented, c = 3

Warning: Never rely on side effects (like ++) in short-circuited expressions. It makes code hard to read and debug.

Combining Logical Operators

You can build complex conditions by combining AND, OR, and NOT:

Example

Output

You are eligible to drive
Access granted

Operator Precedence

The precedence of logical operators relative to other operators:

Priority	Operator	Description
Higher	`!`	Logical NOT
...	Arithmetic, Relational	(evaluated first)
	`&&`	Logical AND
Lower	`		`	Logical OR

This means && has higher precedence than ||:

Example

Output

a || b && c = 1
(a || b) && c = 1
0 || 0 && 1 = 0
(0 || 0) && 1 = 0

Practical Example: Leap Year Check

A year is a leap year if it's divisible by 4, except for century years which must be divisible by 400:

Example

Output

Enter a year: 2024
2024 is a leap year

Practical Example: Triangle Validity

Example

Output

Sides 3, 4, 5 form a valid triangle
It's a scalene triangle

Difference Between Logical and Bitwise Operators

Don't confuse logical operators with bitwise operators:

Logical	Bitwise	Difference
`&&`	`&`	`&&` short-circuits; `&` evaluates both and works bit-by-bit
`		`	`	`	`	`short-circuits;`	` evaluates both and works bit-by-bit
`!`	`~`	`!` returns 0 or 1; `~` flips all bits

Example

Output

Logical: 2 && 3 = 1
Bitwise: 2 & 3 = 2
Logical: 2 || 3 = 1
Bitwise: 2 | 3 = 3

Interview Questions

Q1: What is short-circuit evaluation in C? Give an example where it prevents a crash.

Q2: What is the output of printf("%d", !0 + !0 + !0)?

The output is 3. !0 evaluates to 1 (NOT of false is true). So the expression becomes 1 + 1 + 1 = 3.

Q3: Is && the same as & in C?

Q4: What is the precedence order of !, &&, and ||?

! has the highest precedence (it's a unary operator), followed by &&, and then || has the lowest precedence among the three. So !a && b || c is evaluated as ((!a) && b) || c.

Q5: Can you use integers other than 0 and 1 with logical operators?

Yes. In C, any non-zero value is considered true, and zero is false. So 5 && 3 evaluates to 1, and 0 || -7 evaluates to 1. The NOT operator !5 gives 0, and !0 gives 1.

Summary

C has three logical operators: && (AND), || (OR), and ! (NOT)
They always return 1 (true) or 0 (false) as integers
Short-circuit evaluation is a key feature — the second operand may not be evaluated
&& has higher precedence than ||, so use parentheses for clarity
Don't confuse logical operators (&&, ||) with bitwise operators (&, |)
Any non-zero value is treated as true in logical expressions
Short-circuit evaluation can be used to guard against null pointers and division by zero

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Logical Operators in C Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, operators, logical, logical operators in c programming

Continue learning this concept

PreviousIncrement and Decrement Operators in C NextRelational Operators in C Programming