C If…Else

Welcome to The Coding College! In this tutorial, we’ll explore the if…else statement in C, one of the most powerful tools for decision-making in programming. With practical examples and step-by-step explanations, you’ll master how to control the flow of your programs based on conditions.

What is an If…Else Statement?

The if…else statement in C is used to execute a block of code only when a specified condition is true. If the condition is false, the program can execute an alternate block of code using the else clause.

Basic Syntax:

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

• condition: An expression that evaluates to either true (non-zero) or false (0).
• The if block is executed when the condition is true.
• The else block is executed when the condition is false.

Simple If…Else Example

#include <stdio.h>

int main() {
    int age = 20;

    if (age >= 18) {
        printf("You are an adult.\n");
    } else {
        printf("You are not an adult.\n");
    }

    return 0;
}

Output:

You are an adult.

Explanation:

• The condition age >= 18 is true, so the if block is executed.
• If age were less than 18, the else block would execute.

Nested If…Else

You can nest if...else statements for more complex decision-making.

Example:

#include <stdio.h>

int main() {
    int marks = 85;

    if (marks >= 90) {
        printf("Grade: A\n");
    } else if (marks >= 75) {
        printf("Grade: B\n");
    } else if (marks >= 50) {
        printf("Grade: C\n");
    } else {
        printf("Grade: F\n");
    }

    return 0;
}

Output:

Grade: B

Explanation:

The program checks multiple conditions in sequence. When it finds the first true condition (marks >= 75), it executes the corresponding block and skips the rest.

Using Logical Operators with If…Else

You can combine multiple conditions using logical operators:

• &&: Logical AND
• ||: Logical OR
• !: Logical NOT

Example:

#include <stdio.h>

int main() {
    int age = 25;
    int hasLicense = 1; // 1 = true, 0 = false

    if (age >= 18 && hasLicense) {
        printf("You can drive.\n");
    } else {
        printf("You cannot drive.\n");
    }

    return 0;
}

Output:

You can drive.

Inline If…Else with the Ternary Operator

C also supports a shorthand version of if...else called the ternary operator.

Syntax:

(condition) ? expression_if_true : expression_if_false;

Example:

#include <stdio.h>

int main() {
    int age = 16;

    (age >= 18) ? printf("You are an adult.\n") : printf("You are not an adult.\n");

    return 0;
}

Output:

You are not an adult.

Common Mistakes in If…Else

1. Missing Braces {}

If there’s only one statement in the block, braces are optional, but omitting them in multi-statement blocks causes logical errors.

Incorrect:

if (1 > 0)
    printf("True\n");
    printf("False\n"); // This always runs!

Correct:

if (1 > 0) {
    printf("True\n");
    printf("False\n"); // Part of the if block
}

2. Using Assignment Instead of Equality

Incorrect:

if (x = 10) {  // Single = assigns, doesn't compare
    printf("This is always true.\n");
}

Correct:

if (x == 10) {  // Double == compares values
    printf("This is correct.\n");
}

Practical Applications

Example: Check Even or Odd Number

#include <stdio.h>

int main() {
    int num = 4;

    if (num % 2 == 0) {
        printf("%d is even.\n", num);
    } else {
        printf("%d is odd.\n", num);
    }

    return 0;
}

Output:

4 is even.

Example: Find the Largest Number

#include <stdio.h>

int main() {
    int a = 10, b = 20;

    if (a > b) {
        printf("%d is larger.\n", a);
    } else {
        printf("%d is larger.\n", b);
    }

    return 0;
}

Output:

20 is larger.

Key Takeaways

1. Basic Usage: Use if...else to control program flow based on conditions.
2. Logical Operators: Combine conditions with &&, ||, and !.
3. Nested Conditions: Use nested if...else for complex decisions.
4. Ternary Operator: Use (condition) ? true : false for concise conditions.
5. Error Avoidance: Always use == for comparison and add braces {} for clarity.