if-else Statement in C

The if-else statement provides two possible paths in a program. One path executes when the condition is true, and the other executes when the condition is false.

if (condition)
{
    // true block
}
else
{
    // false block
}

if (condition)
{
    statement1;
}
else
{
    statement2;
}

Only one of the two blocks is executed for each evaluation of the if-else statement.

int age = 16;

if (age >= 18)
{
    printf("Adult");
}
else
{
    printf("Minor");
}

Since the age is less than 18, the else block executes.

The execution process is:

1. The condition is evaluated.
2. If the condition is true, the if block executes.
3. If the condition is false, the else block executes.
4. After that, program execution continues with the next statement.

int marks = 65;

if (marks >= 40)
{
    printf("Pass");
}
else
{
    printf("Fail");
}

Because the marks are 65, the condition is true and Pass is displayed.

int number = 7;

if (number % 2 == 0)
{
    printf("Even");
}
else
{
    printf("Odd");
}

The remainder of 7 divided by 2 is not zero, so the program prints Odd.

int number = -10;

if (number >= 0)
{
    printf("Positive");
}
else
{
    printf("Negative");
}

This example treats zero as non-negative.

int a = 20;
int b = 10;

if (a > b)
{
    printf("A is greater");
}
else
{
    printf("B is greater or equal");
}

The else block executes whenever a > b is false.

int a = 10;
int b = 10;

if (a == b)
{
    printf("Both are equal");
}
else
{
    printf("Both are different");
}

The equality operator == is used to compare the two values.

#include <stdio.h>

int main()
{
    int age;

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

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

    return 0;
}

The result depends on the age entered by the user.

int age;

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

if (age >= 18)
{
    printf("Eligible to vote.");
}
else
{
    printf("Not eligible to vote.");
}

The program selects one message based on the age condition.

int age = 25;
int marks = 70;

if (age >= 18 && marks >= 40)
{
    printf("Eligible");
}
else
{
    printf("Not Eligible");
}

The && operator requires both conditions to be true.

int number = 5;

if (number == 5 || number == 10)
{
    printf("Valid number");
}
else
{
    printf("Invalid number");
}

The condition is true when at least one of the two comparisons is true.

char grade = 'A';

if (grade == 'A')
{
    printf("Excellent");
}
else
{
    printf("Keep improving");
}

Characters can also be used in conditions.

float price = 150.50f;

if (price > 100.0f)
{
    printf("Expensive");
}
else
{
    printf("Affordable");
}

Floating-point values can be used in comparisons.

int number;

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

if (number > 0)
{
    printf("Positive number");
}
else
{
    printf("Zero or negative number");
}

This example has two possible outcomes.

int a = 50;
int b = 30;

if (a > b)
{
    printf("A is larger");
}
else
{
    printf("B is larger or equal");
}

The condition determines which value is larger.

int a = 20;
int b = 40;

if (a < b)
{
    printf("A is smaller");
}
else
{
    printf("B is smaller or equal");
}

The < operator is used to compare the two values.

int number = 20;

if (number % 5 == 0)
{
    printf("Divisible by 5");
}
else
{
    printf("Not divisible by 5");
}

If the remainder is zero, the number is divisible by 5.

int marks = 75;

if (marks >= 40)
{
    printf("You passed.\n");
    printf("Congratulations!\n");
}
else
{
    printf("You failed.\n");
    printf("Try again.");
}

Both blocks can contain multiple statements.

Curly braces clearly identify which statements belong to each branch. They are recommended even when a branch currently contains only one statement.

if (number > 10)
{
    printf("Greater");
}
else
{
    printf("Not greater");
}

When each branch contains exactly one statement, braces can technically be omitted.

if (number > 0)
    printf("Positive");
else
    printf("Not positive");

Using braces is usually clearer, especially when code is later modified.

Incorrect:

if (number = 10)
{
    printf("Number is 10");
}
else
{
    printf("Number is not 10");
}

The = operator performs assignment. For comparison, use ==.

Correct:

if (number == 10)
{
    printf("Number is 10");
}
else
{
    printf("Number is not 10");
}

Avoid:

if (number > 10);
{
    printf("Greater than 10");
}
else
{
    printf("10 or less");
}

A semicolon immediately after the condition ends the if statement and causes a syntax problem with the following else.

Correct:

if (number > 10)
{
    printf("Greater than 10");
}
else
{
    printf("10 or less");
}

In C, zero is treated as false and a nonzero value is treated as true.

int value = 0;

if (value)
{
    printf("True");
}
else
{
    printf("False");
}

Output:

False

An if statement can also appear inside an else block.

int number = -5;

if (number > 0)
{
    printf("Positive");
}
else
{
    if (number == 0)
    {
        printf("Zero");
    }
    else
    {
        printf("Negative");
    }
}

More complex alternatives are often easier to manage using an else-if ladder, which will be covered in the next lesson.

#include <stdio.h>

int main()
{
    int marks;

    printf("Enter your marks: ");
    scanf("%d", &marks);

    if (marks >= 40)
    {
        printf("Result: Pass");
    }
    else
    {
        printf("Result: Fail");
    }

    return 0;
}

This is a simple real-world example of using if-else for decision making.

#include <stdio.h>

int main()
{
    int number;

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

    if (number % 2 == 0)
    {
        printf("Even number");
    }
    else
    {
        printf("Odd number");
    }

    return 0;
}

The remainder operator % is used to determine whether the number is divisible by 2.

#include <stdio.h>

int main()
{
    int a, b;

    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);

    if (a > b)
    {
        printf("%d is larger", a);
    }
    else
    {
        printf("%d is larger or equal", b);
    }

    return 0;
}

This program compares two numbers and selects one of the two output messages.

Practice the following programs:

1. Check whether a number is positive or negative.
2. Check whether a number is even or odd.
3. Check whether a student has passed or failed.
4. Check whether a person is an adult or minor.
5. Find the larger of two numbers.
6. Find the smaller of two numbers.
7. Check whether a number is divisible by 5.
8. Check whether two numbers are equal.
9. Check whether a number is greater than 100.
10. Create a simple eligibility program using two conditions.

Practice writing the condition first, then create the true and false blocks.