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C Logical Operators

Logical operators in C evaluate to either True or False. Logical operators are typically used with Boolean operands.

The logical AND operator (&&) and the logical OR operator (||) are both binary in nature (require two operands). The logical NOT operator (!) is a unary operator.

Since C treats "0" as False and any non-zero number as True, any operand to a logical operand is converted to a Boolean data.

Here is a table showing the logical operators in C −

The result of a logical operator follows the principle of Boolean algebra. The logical operators follow the following truth tables.

The && operator in C acts as the logical AND operator. It has the following truth table −

The above truth table shows that the result of && is True only if both the operands are True.

C uses the double pipe symbol (||) as the logical OR operator. It has the following truth table −

The above truth table shows that the result of || operator is True when either of the operands is True, and False if both operands are false.

The logical NOT ! operator negates the value of a Boolean operand. True becomes False, and False becomes True. Here is its truth table −

Unlike the other two logical operators && and ||, the logical NOT operator ! is a unary operator.

The following example shows the usage of logical operators in C −

#include <stdio.h>

int main(){

   int a = 5;
   int b = 20;

   if (a && b){
      printf("Line 1 - Condition is true\n" );
   }
	
   if (a || b){
      printf("Line 2 - Condition is true\n" );
   }
   
   /* lets change the value of  a and b */
   a = 0;
   b = 10;
	
   if (a && b){
      printf("Line 3 - Condition is true\n" );
   } else {
      printf("Line 3 - Condition is not true\n" );
   }
	
   if (!(a && b)){
      printf("Line 4 - Condition is true\n" );
   }
   
   return 0;	
}

Run the code and check its output −

Line 1 - Condition is true
Line 2 - Condition is true
Line 3 - Condition is not true
Line 4 - Condition is true

In C, a char type is a subset of int type. Hence, logical operators can work with char type too.

#include <stdio.h>

int main(){

   char a = 'a';
   char b = '\0'; // Null character

   if (a && b){
      printf("Line 1 - Condition is true\n" );
   }

   if (a || b){
      printf("Line 2 - Condition is true\n" );
   }
   
   return 0;
}

Run the code and check its output −

Line 2 - Condition is true

Logical operators are generally used to build a compound boolean expression. Along with relational operators, logical operators too are used in decision-control and looping statements in C.

The following example shows a compound Boolean expression in a C program −

#include <stdio.h>

int main(){

   int phy = 50;
   int maths = 60;

   if (phy < 50 || maths < 50){
      printf("Result:Fail");
   }
   else {
      printf("Result:Pass");
   }
   
   return 0;
}

Result:Pass

The similar logic can also be expressed using the && operator as follows −

#include <stdio.h>

int main(){

   int phy = 50;
   int maths = 60;

   if (phy >= 50 && maths >= 50){
      printf("Result: Pass");
   }
   else {
      printf("Result: Fail");
   }
   
   return 0;
}

Run the code and check its output −

Result: Pass

The following C code employs the NOT operator in a while loop −

#include <stdio.h>

int main(){

   int i = 0;

   while (!(i > 5)){
      printf("i = %d\n", i);
      i++;
   }
   
   return 0;
}

In the above code, the while loop continues to iterate till the expression "!(i > 5)" becomes false, which will be when the value of "i" becomes more than 5.

i = 0
i = 1
i = 2
i = 3
i = 4
i = 5

C has bitwise counterparts of the logical operators such as bitwise AND (&), bitwise OR (|), and binary NOT or complement (~) operator.

c_operators.htm

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