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Logical operators

Learn boolean logic with &&, || and !.

Logical operators

There are three logical operators: && (AND), || (OR) and ! (NOT). They combine true/false values.

int a = 1, b = 0;
int r = a && b;  /* 0: not both true */
int s = a || b;  /* 1: at least one true */
int t = !b;      /* 1: b is false, opposite is true */

The result is again an int: 1 (true) or 0 (false). Logical operators are often used in if conditions.

Short-circuit evaluation

The && and || operators short-circuit. In && if the left side is false the right side is not checked; the result is certainly false. In || if the left side is true the right side is not checked; the result is certainly true.

int x = 0;
if (x != 0 && 10 / x > 1) { /* safe */ }

This is useful to guard against dangerous cases like division by zero. The left condition protects the right side.

Practice Print logical operator results

Define int a = 5, b = 0;. With four printf calls print a && b, a || b, !b and !a on separate lines. Output should be 0, 1, 1, 0.

Starter code

main.c
#include <stdio.h>

int main(void) {
    int a = 5, b = 0;
    /* print a && b, a || b, !b, !a */
    return 0;
}

Sample solution

main.c
#include <stdio.h>

int main(void) {
    int a = 5, b = 0;
    printf("%d\n", a && b);
    printf("%d\n", a || b);
    printf("%d\n", !b);
    printf("%d\n", !a);
    return 0;
}

Expected output

0
1
1
0

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which expression gives 1 (true)?

  • 1 && 0
  • 0 || 0
  • !1
  • 1 && 1 Correct answer

Explanation: 1 && 1 gives 1 because both sides are true.

2 Match logical operators

Term Meaning
&& logical AND
|| logical OR
! not
1 && 1 true

3 Logical AND in a condition

Question 
int a = 5, b = 3;
if (a > 0 ____ b > 0) { /* both positive */ }
Answer 
int a = 5, b = 3;
if (a > 0 && b > 0) { /* both positive */ }

Explanation: && requires both sides to be true.

All lessons in Beginner

1 C basics

  1. What is C?
  2. Your first program
  3. Compile and run
  4. Comments
  5. printf and escapes
  6. Summary - C basics

2 Variables

  1. Declaring variables
  2. Variable assignment
  3. const
  4. Variable scope
  5. typedef
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. char
  4. sizeof
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Menu loop
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Bitwise operators
  6. Summary - Operators

6 I/O

  1. printf formatting
  2. scanf
  3. Multiple inputs
  4. getchar and putchar
  5. puts and fputs
  6. Summary - I/O

7 Conditionals

  1. The if statement
  2. else and else if
  3. The switch statement
  4. The ternary operator
  5. Nested conditionals
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. break and continue
  5. Nested loops
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding products
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Array size
  4. Multidimensional arrays
  5. Arrays and loops
  6. Summary - Arrays

11 Strings

  1. String basics
  2. String literals
  3. strlen
  4. strcpy and strcat
  5. strcmp
  6. Summary - Strings

12 Functions

  1. Defining functions
  2. Return values
  3. Parameters
  4. Function prototypes
  5. Recursion
  6. Summary - Functions

13 Pointer basics

  1. What is a pointer?
  2. The & address operator
  3. Pointer dereference
  4. NULL
  5. Pointer arithmetic
  6. Summary - Pointer basics

14 Arrays and pointers

  1. Array name as pointer
  2. Pointer to array
  3. Passing arrays to functions
  4. Pointer loop
  5. Multidim array pointer
  6. Summary - Arrays and pointers

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String functions

  1. strchr
  2. strstr
  3. strtok
  4. sprintf
  5. sscanf
  6. Summary - String functions

17 Structs

  1. Defining structs
  2. Using structs
  3. typedef struct
  4. Array of structs
  5. Nested structs
  6. Summary - Structs

18 Memory and stack

  1. Stack and heap
  2. Automatic storage
  3. malloc
  4. free
  5. calloc and realloc
  6. Summary - Memory and stack

19 Preprocessor

  1. #define
  2. #include
  3. Conditional compilation
  4. Macro functions
  5. Header files
  6. Summary - Preprocessor

20 Beginner final

  1. Project design
  2. Data model
  3. Functions
  4. Memory management
  5. Putting it together