Conditions in C If Statements: Logic Operators & Nesting

1. Introduction

Example: “If variable a is 10 or greater, and b is less than 5, then execute the process”

2. Basics of C Language if Statements

2.1. Basic Structure of if Statements

if (condition) {
    // Code to execute when the condition is true
}

Example: Display a message if variable x is 10 or greater

#include <.h>

int main() {
    int x = 15;

    if (x >= 10) {
        printf("x is 10 or greater.\n");
    }

    return 0;
}

2.2. Basic Structure of if-else Statements

if (condition) {
    // Code to execute when the condition is true
} else {
    // Code to execute when the condition is false
}

Example: Determine whether the number entered by the user is positive

#include <stdio.h>

int main() {
    int num;
    printf("Please enter an integer: ");
    scanf("%d", &num);

    if (num > 0) {
        printf("The entered number is positive.\n");
    } else {
        printf("The entered number is not positive.\n");
    }

    return 0;
}

2.3. Basic Structure of if-else if-else Statements

if (condition1) {
    // Code to execute when condition 1 is true
} else if (condition2) {
    // Code to execute when condition 2 is true
} else {
    // Code to execute when all conditions are false
}

Example: Display a grade based on the entered score

#include <stdio.h>

int main() {
    int score;
    printf("Please enter the score: ");
    scanf("%d", &score);

    if (score >= 80) {
        printf("Grade: A\n");
    } else if (score >= 60) {
        printf("Grade: B\n");
    } else {
        printf("Grade: C\n");
    }

    return 0;
}

3. How to handle multiple conditions with if statements

3.1. Using logical operators (&&, ||)

• Executed when all conditions are met

#include <stdio.h>

int main() {
    int a = 15, b = 3;

    if (a > 10 && b < 5) {
        printf("a is greater than 10 and b is less than 5.
");
    }

    return 0;
}

• Executed if either condition is met

#include <stdio.h>

int main() {
    int a = 8, b = 3;

    if (a > 10 || b < 5) {
        printf("a is greater than 10 or b is less than 5.
");
    }

    return 0;
}

• Inverts the condition

#include <stdio.h>

int main() {
    int a = 8;

    if (!(a > 10)) {
        printf("a is 10 or less.
");
    }

    return 0;
}

3.2. Nesting if statements

• By placing an if statement inside another if, you can create finer-grained branching

Example: Checking both age and height

#include <stdio.h>

int main() {
    int age = 20, height = 170;

    if (age >= 18) {
        if (height >= 160) {
            printf("You are an adult and at least 160 cm tall.
");
        } else {
            printf("You are an adult but under 160 cm tall.
");
        }
    } else {
        printf("You are a minor.
");
    }

    return 0;
}

4. Best Practices for Handling Multiple Conditions

4.1. Improving Readability

4.1.1. Splitting Conditional Expressions

int isEligibleForDiscount = (score >= 80);
int isAdult = (age >= 18);
int isMemberValid = (isMember == 1);

if (isEligibleForDiscount && isAdult && isMemberValid) {
    printf("You are eligible for the discount.\n");
}

4.1.2. Encapsulating Conditional Logic in a Function

#include <stdio.h>

int isEligible(int score, int age, int isMember) {
    return (score >= 80 && age >= 18 && isMember == 1);
}

int main() {
    int score = 85, age = 20, isMember = 1;

    if (isEligible(score, age, isMember)) {
        printf("You are eligible for the discount.\n");
    } else {
        printf("You are not eligible for the discount.\n");
    }

    return 0;
}

4.2. Performance Considerations

4.2.1. Leveraging Short-Circuit Evaluation

if (a != 0 && b / a > 1) {
    printf("Result: %d\n", b / a);
}

4.2.2. Optimizing Evaluation Order

if (a > 0 && isHeavyCalculation()) {
    printf("Executing process\n");
}

4.3. Improving Maintainability

4.3.1. Emphasizing Clarity of Conditional Expressions

if (user.age >= 18 && user.score >= 50) {
    printf("Conditions met.\n");
}

4.3.2. Avoiding Magic Numbers

#define ADULT_AGE 18

if (age >= ADULT_AGE) {
    printf("Adult.\n");
}

5. Practical Example: Program Using if Statements

5.1. Display Message Based on User’s Age

#include <stdio.h>

int main() {
    int age;

    printf("Please enter your age: ");
    scanf("%d", &age);

    if (age >= 18 && age < 65) {
        printf("You are an adult.
");
    } else if (age >= 65) {
        printf("You are a senior.
");
    } else {
        printf("You are a minor.
");
    }

    return 0;
}

5.2. Simple Login Authentication Program

#include <stdio.h>
#include <string.h>

int main() {
    char username[20];
    char password[20];

    printf("Username: ");
    scanf("%s", username);

    printf("Password: ");
    scanf("%s", password);

    if (strcmp(username, "admin") == 0 && strcmp(password, "1234") == 0) {
        printf("Login successful!
");
    } else {
        printf("Login failed...
");
    }

    return 0;
}

5.3. Triangle Classification Program

#include <stdio.h>

int main() {
    int a, b, c;

    printf("Please enter the three sides of the triangle: ");
    scanf("%d %d %d", &a, &b, &c);

    if (a == b && b == c) {
        printf("It is an equilateral triangle.
");
    } else if (a == b || b == c || a == c) {
        printf("It is an isosceles triangle.
");
    } else {
        printf("It is a scalene triangle.
");
    }

    return 0;
}

5.4. Numeric Range Check

#include <stdio.h>

int main() {
    int number;

    printf("Please enter a number: ");
    scanf("%d", &number);

    if (number >= 50 && number <= 100) {
        printf("The number is appropriate.
");
    } else {
        printf("Out of range.
");
    }

    return 0;
}

6. FAQ (Frequently Asked Questions)

6.1. Which should be used, if statements or switch statements?

When to use if statements

• When a range (greater/lesser comparison) is needed
• When using complex logical expressions

if (score >= 90) {
    printf("Grade: A
");
} else if (score >= 80) {
    printf("Grade: B
");
} else {
    printf("Grade: C
");
}

When to use switch statements

• When evaluating specific values (integers or characters)

switch (grade) {
    case 'A':
        printf("Excellent.
");
        break;
    case 'B':
        printf("Good grade.
");
        break;
    default:
        printf("Invalid rating.
");
}

6.2. Should you use nested if statements or logical operators?

When using nesting

if (age >= 18) {
    if (hasID) {
        printf("You may enter.
");
    } else {
        printf("Identification is required.
");
    }
}

When using logical operators

if (age >= 18 && hasID) {
    printf("You may enter.
");
}

6.3. How to write if conditions simply

Using the ternary operator

int result = (a > 10) ? 1 : 0;

Encapsulating the condition in a function

int isAdult(int age) {
    return age >= 18;
}

if (isAdult(age)) {
    printf("You are an adult.
");
}

6.4. Causes when an if condition is wrong and doesn’t work

= (assignment operator) and == (comparison operator) mistake

if (x == 5) {
    printf("x is 5.
");
}

Explicitly specify condition precedence

if ((a > 10 && b < 5) || c == 3) {
    printf("Condition met
");
}

Floating-point comparison

#include <math.h>

if (fabs(a - b) < 0.0001) {
    printf("a and b are almost equal.
");
}

7. Summary

7.1. Basic Structure of if Statements

if (condition) {
    // code to execute when condition is true
} else {
    // code to execute when condition is false
}

7.2. How to Handle Multiple Conditions in if Statements

Using Logical Operators

if (age >= 18 && hasID) {
    printf("You may enter.
");
}

Using Nesting

if (age >= 18) {
    if (hasID) {
        printf("You may enter.
");
    } else {
        printf("ID is required.
");
    }
}

7.3. Writing Style Considering Readability and Performance

Encapsulating Conditions in Functions

int isEligible(int score, int age) {
    return (score >= 80 && age >= 18);
}

if (isEligible(85, 20)) {
    printf("Eligible for benefits.
");
}

Using Short-Circuit Evaluation

if (a != 0 && b / a > 1) {
    printf("Result: %d
", b / a);
}

7.4. Practical Usage Examples

if (age >= 18 && age < 65) {
    printf("You are an adult.
");
}

if (strcmp(username, "admin") == 0 && strcmp(password, "1234") == 0) {
    printf("Login successful!
");
}

if (number >= 50 && number <= 100) {
    printf("The number is appropriate.
");
}

7.5. Common Mistakes and Their Countermeasures

if (x == 5) {
    printf("x is 5.
");
}

if ((a > 10 && b < 5) || c == 3) {
    printf("Condition met
");
}

#include <math.h>

if (fabs(a - b) < 0.0001) {
    printf("a and b are almost equal.
");
}

7.6. Summary of This Article

7.7. Looking Ahead to Future Learning