C If...Else

Types of If Statements

🔹 Simple If Statement

The if statement evaluates a condition and executes its code block only when the condition is true, providing fundamental decision-making capability. The syntax if(condition) { statements; } checks whether the condition expression evaluates to non-zero (true). For example, if(temperature > 30) { printf("Hot day"); } prints the message only when temperature exceeds 30. Single statements don't require braces but using them consistently prevents errors when adding more statements later. The condition can be any expression; relational operators (>, <, ==), logical operators (&&, ||), or function calls that return integer values all work. If statements form the foundation of program logic, enabling dynamic behavior based on runtime conditions and user input.

#include 

int main() {
    int number = 10;
    
    // Simple if statement
    if (number > 0) {
        printf("%d is a positive number\n", number);
    }
    
    // Multiple simple if statements
    if (number % 2 == 0) {
        printf("%d is even\n", number);
    }
    
    if (number >= 10) {
        printf("%d is greater than or equal to 10\n", number);
    }
    
    // If statement with multiple conditions
    if (number > 0 && number < 100) {
        printf("%d is between 1 and 99\n", number);
    }
    
    return 0;
}

🔹 If-Else Statement

The if-else statement creates a binary decision point, executing one block when the condition is true and an alternative block when false. The structure if(condition) { true_block; } else { false_block; } ensures exactly one branch executes. For example, if(age >= 18) { printf("Adult"); } else { printf("Minor"); } categorizes based on age. This construct is essential for implementing either-or logic, handling success and failure cases, and providing default behaviors. The else clause is optional but recommended when both outcomes need specific handling. If-else statements enable programs to respond appropriately to different conditions, making them crucial for input validation, error handling, state management, and implementing business logic that requires distinct actions for different scenarios.

#include 

int main() {
    int age = 16;
    
    // Basic if-else
    if (age >= 18) {
        printf("You are an adult\n");
    } else {
        printf("You are a minor\n");
    }
    
    // If-else with calculations
    int a = 15, b = 25;
    if (a > b) {
        printf("%d is greater than %d\n", a, b);
    } else {
        printf("%d is not greater than %d\n", a, b);
    }
    
    // If-else for even/odd check
    int num = 7;
    if (num % 2 == 0) {
        printf("%d is even\n", num);
    } else {
        printf("%d is odd\n", num);
    }
    
    return 0;
}

🔹 Else If Statement

The else if construct chains multiple conditions sequentially, testing each only if previous conditions were false, enabling multi-way decision logic. The pattern if(condition1) {} else if(condition2) {} else if(condition3) {} else {} evaluates conditions in order until one is true. For example, grade classification: if(score>=90) {grade='A';} else if(score>=80) {grade='B';} else if(score>=70) {grade='C';} assigns grades based on score ranges. Only the first matching condition executes, so order matters crucially—place specific conditions before general ones. This structure is more efficient than independent if statements when conditions are mutually exclusive, and it's more readable than deeply nested if-else blocks. Else if chains are perfect for range checking, state machines, and implementing priority-based logic.

#include 

int main() {
    int score = 85;
    
    // Grade calculation using else if
    if (score >= 90) {
        printf("Grade: A (Excellent!)\n");
    } else if (score >= 80) {
        printf("Grade: B (Good!)\n");
    } else if (score >= 70) {
        printf("Grade: C (Average)\n");
    } else if (score >= 60) {
        printf("Grade: D (Below Average)\n");
    } else {
        printf("Grade: F (Fail)\n");
    }
    
    // Temperature check
    int temp = 75;
    if (temp > 100) {
        printf("It's extremely hot!\n");
    } else if (temp > 80) {
        printf("It's hot\n");
    } else if (temp > 60) {
        printf("It's warm\n");
    } else if (temp > 40) {
        printf("It's cool\n");
    } else {
        printf("It's cold\n");
    }
    
    return 0;
}

🔹 Nested If Statements

Nested if statements place one if statement inside another, enabling complex decision-making based on multiple dependent conditions. While powerful for handling sophisticated logic, excessive nesting reduces readability and maintainability. For example, checking eligibility might require if(age>=18) { if(hasLicense) { if(hasInsurance) { printf("Can drive"); }}}. Best practices suggest limiting nesting depth to 2-3 levels and using logical operators (&&, ||) to combine simple conditions instead. Alternatively, early returns, guard clauses, or breaking logic into separate functions improves clarity. Deep nesting often indicates opportunities for refactoring using else-if chains, switch statements, or function decomposition that makes code more testable and maintainable while preserving the same logical behavior.

#include 

int main() {
    int age = 25;
    int has_license = 1;  // 1 = true, 0 = false
    int has_car = 0;
    
    // Nested if for driving eligibility
    if (age >= 18) {
        printf("You are old enough to drive\n");
        
        if (has_license) {
            printf("You have a license\n");
            
            if (has_car) {
                printf("You can drive your own car!\n");
            } else {
                printf("You need to get a car\n");
            }
        } else {
            printf("You need to get a license first\n");
        }
    } else {
        printf("You are too young to drive\n");
    }
    
    // Another nested example - number classification
    int num = -15;
    if (num != 0) {
        if (num > 0) {
            printf("%d is positive\n", num);
            if (num % 2 == 0) {
                printf("and it's even\n");
            } else {
                printf("and it's odd\n");
            }
        } else {
            printf("%d is negative\n", num);
        }
    } else {
        printf("Number is zero\n");
    }
    
    return 0;
}

🔹 Logical Operators in If Statements

Logical operators combine multiple conditions in if statements, enabling complex Boolean expressions without nested structures. The AND operator (&&) requires all conditions to be true, OR (||) requires at least one condition true, and NOT (!) inverts a condition's truth value. For example, if(age>=18 && hasLicense) checks both conditions simultaneously, while if(isWeekend || isHoliday) succeeds if either is true. Logical operators use short-circuit evaluation: && stops at the first false condition, and || stops at the first true condition, improving efficiency and preventing errors from evaluating undefined expressions. Understanding operator precedence (NOT before AND before OR) and using parentheses for clarity are essential for writing correct complex conditions.

#include 

int main() {
    int age = 22;
    int income = 30000;
    int credit_score = 750;
    
    // AND operator (&&) - all conditions must be true
    if (age >= 18 && income >= 25000 && credit_score >= 700) {
        printf("Loan approved!\n");
    } else {
        printf("Loan denied\n");
    }
    
    // OR operator (||) - at least one condition must be true
    int day = 6;  // Saturday
    if (day == 6 || day == 7) {  // Saturday or Sunday
        printf("It's weekend!\n");
    } else {
        printf("It's a weekday\n");
    }
    
    // NOT operator (!) - reverse the condition
    int is_raining = 0;  // 0 = false
    if (!is_raining) {
        printf("It's not raining, let's go out!\n");
    } else {
        printf("It's raining, stay inside\n");
    }
    
    // Complex condition
    int hour = 14;  // 2 PM
    if ((hour >= 9 && hour <= 17) && !is_raining) {
        printf("Good time for outdoor activities\n");
    }
    
    return 0;
}