Logical Expressions and Short-Circuit Evaluation in C

In C, logical expressions are used to perform decision-making using logical operators such as && (AND) or || (OR) by combining multiple conditions. These expressions return either true (non-zero) or false (0).

Meaning of Different Logical Operators

The logical expression evaluation or its meaning depends on its truth table. Let's take a look:

Here in the above table we can see that AND only evaluates to true if both of its operands are true. While OR evaluates to true even if one of the operands is true. In all the cases other than that we will receive false.

Logical AND (&&) Meaning

We can see that && AND evaluates true only when both of its operands are true. Let's see with an example :

#include <stdio.h>

int main() {
    int a = 10, b = 0;
    
    if (a != 0 && b != 0) {
        printf("a and b are non zero.");
    }
    else {
        printf("Either a or b or both are zero");
    }
}

Either a or b or both are zero

• Expression a != 0 is true.
• Expression b != 0 is false.

First operand of && is true while the second one is false. It resembles third case in the above truth table. That is why, the result of the logical expression is false. We combined two conditions (a != 0) and (b != 0) logically using && operator which finally results in false value, that is why the else body is executed.

Note: Any non-zero value can be considered true or 1. But only zero is considered false.

Logical OR (||) Meaning

We can see that || OR evaluates true if any of its operands is true. We can change the above example and replace && with || and see what happens:

#include <stdio.h>

int main() {
    int a = 10, b = 0;
    
    if (a != 0 || b != 0) {
        printf("Either a or b are non zero.");
    }
    else {
        printf("Both are zero");
    }
}

Either a or b are non zero.

The output is changed. The body of the if block is now being executed, which means that the logical expression evaluates to true. Let's break it down:

• Expression a != 0 is true.
• Expression b != 0 is false.

First operand of || is true while the second one is false. But || it only needs one of them to be true.

Example

We can combine many different conditions into a compound expression using logical operators:

#include <stdio.h>

int main() {
    int a = 10, b = 182;
    
    if (a > 0 && b > 0 || a - b == 0) {
        printf("Either a and b are equal or greater than 0 or both");
    }
    else {
        printf("else block");
    }
}

Either a and b are equal or greater than 0 or both

Keep in mind the operator precedence and associativity while writing these expressions to make sure the expression is evaluated as intended.

Short-Circuit Evaluation

From the above, we can infer that:

• For && (AND): If any operand is false, then it is false.
• For || (OR): If any operand is true, it is true.

This property can lead to an interesting optimization in the evaluation of logical expressions.

• AND (&&) can determine the result if the first operand is false without evaluating the second operand.
• OR (||) AND (&&) can determine the result if the first operand is true without evaluating the second operand.

This property is called short circuit evaluation. We can verify this property using the following code:

Logical AND Short-Circuit Example

#include <stdio.h>

int main() {
    int a = 10;
    
    if (a < 0 && (a = 22)) {
        printf("if block\n");
    }
    else {
        printf("else block\n");
    }
    
    printf("Value of a: %d", a);
    return 0;
}

else block
Value of a: 10

In the above expression, condition (a < 0) is false, so the AND expression should result in false leading to the execution of else block. This is what happens in the program.

But the second operand of the AND is an assignment expression which changes the value of the variable a. But when we print the value of a at the end, it is still not changed. This means that second operand is not even evaluated, confirming the short circuit evaluation.

Logical OR Short-Circuit Example

Just like logical AND example, we can also verify the short circuit evaluation for logical OR expressions.

#include <stdio.h>

int main() {
    int a = 10;
    
    if (a > 0 || (a = 22)) {
        printf("if block\n");
    }
    else {
        printf("else block\n");
    }
    
    printf("Value of a: %d", a);
    return 0;
}

if block
Value of a: 10

The first condition is true, so second operand is not even evaluated (shown by no value change in a variable) in the OR expression.