C Logical Conditions

Logical operators (&&, ||, !) combine multiple individual conditions into a single expression an if statement can test, using short-circuit evaluation to skip unnecessary checks.

In this page:

Logical Operators in Conditions
Combining Conditions with &&
Combining Conditions with ||
Negating a Condition with !
Short-Circuit Evaluation

Syntax

if (condition1 && condition2) {
  // both are true
}

if (condition1 || condition2) {
  // at least one is true
}

if (!condition) {
  // condition is false
}

Logical Operators in Conditions

Logical operators combine multiple individual conditions into a single expression that an if statement can test, letting a decision depend on more than one factor at once.

Example: Logical Operators in Conditions

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Declare age and set it to 25
	int age = 25;
	// Declare hasLicense and set it to 1
	int hasLicense = 1;
	// Check whether age >= 18 && hasLicense
	if (age >= 18 && hasLicense) {
		// Print formatted text to the screen
		printf("Can drive");
	}
	// Return 0 to signal that the program finished successfully
	return 0;
}

Combining Conditions with &&

The && (logical AND) operator combines two conditions so the overall expression is true only when both individual conditions are true, commonly used to check that a value falls within a specific range.

Example: Combining Conditions with &&

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Declare score and set it to 75
	int score = 75;
	// Check whether score >= 60 && score <= 100
	if (score >= 60 && score <= 100) {
		// Print formatted text to the screen
		printf("In range");
	}
	// Return 0 to signal that the program finished successfully
	return 0;
}

Combining Conditions with ||

The || (logical OR) operator combines two conditions so the overall expression is true when at least one of them is true, useful for checking whether a value matches any of several acceptable options.

Example: Combining Conditions with ||

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Declare grade and set it to 'A'
	char grade = 'A';
	// Check whether grade == 'A' || grade == 'B'
	if (grade == 'A' || grade == 'B') {
		// Print formatted text to the screen
		printf("Good grade");
	}
	// Return 0 to signal that the program finished successfully
	return 0;
}

Negating a Condition with !

The ! (logical NOT) operator inverts a single condition, turning a true value false and a false value true, which is often used to read naturally as not in front of a condition's name.

Example: Negating a Condition with !

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Declare isBanned and set it to 0
	int isBanned = 0;
	// Check whether !isBanned
	if (!isBanned) {
		// Print formatted text to the screen
		printf("Access allowed");
	}
	// Return 0 to signal that the program finished successfully
	return 0;
}

Short-Circuit Evaluation

C evaluates && and || using short-circuit evaluation: for &&, if the first condition is false, the second is never evaluated at all, which is commonly used to safely guard against operations like division by zero.

Example: Short-Circuit Evaluation

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Declare divisor and set it to 0
	int divisor = 0;
	// Check whether divisor != 0 && (10 / divisor > 1)
	if (divisor != 0 && (10 / divisor > 1)) {
		// Print formatted text to the screen
		printf("unreachable");
	} else {
		// Print formatted text to the screen
		printf("Avoided division by zero");
	}
	// Return 0 to signal that the program finished successfully
	return 0;
}

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