In computer programming, we use the if statement to run a block code only when a certain condition is met.

For example, assigning grades (A, B, C) based on marks obtained by a student.

• if the percentage is above 90, assign grade A
• if the percentage is above 75, assign grade B
• if the percentage is above 65, assign grade C

There are three forms of if...else statements in C++.

1. if statement
2. if...else statement
3. if...else if...else statement

C++ if Statement

The syntax of the if statement is:

if (condition) {   // body of if statement}

The if statement evaluates the condition inside the parentheses ( ).

• If the condition evaluates to true, the code inside the body of if is executed.
• If the condition evaluates to false, the code inside the body of if is skipped.

Note: The code inside { } is the body of the if statement.

Example 1: C++ if Statement

// Program to print positive number entered by the user// If the user enters a negative number, it is skipped#include <iostream>using namespace std;int main() {    int number;    cout << "Enter an integer: ";    cin >> number;    // checks if the number is positive    if (number > 0) {        cout << "You entered a positive integer: " << number << endl;    }    cout << "This statement is always executed.";    return 0;}

Output 1

Enter an integer: 5You entered a positive number: 5This statement is always executed.

When the user enters 5, the condition number > 0 is evaluated to true and the statement inside the body of if is executed.

Output 2

Enter a number: -5This statement is always executed.

When the user enters -5, the condition number > 0 is evaluated to false and the statement inside the body of if is not executed.

C++ if...else

The if statement can have an optional else clause. Its syntax is:

if (condition) {    // block of code if condition is true}else {    // block of code if condition is false}

The if..else statement evaluates the condition inside the parenthesis.

If the condition evaluates true,

• the code inside the body of if is executed
• the code inside the body of else is skipped from execution

If the condition evaluates false,

• the code inside the body of else is executed
• the code inside the body of if is skipped from execution

Example 2: C++ if...else Statement

// Program to check whether an integer is positive or negative// This program considers 0 as a positive number#include <iostream>using namespace std;int main() {     int number;    cout << "Enter an integer: ";    cin >> number;    if (number >= 0) {        cout << "You entered a positive integer: " << number << endl;    }    else {        cout << "You entered a negative integer: " << number << endl;    }    cout << "This line is always printed.";    return 0;}

Output 1

Enter an integer: 4You entered a positive integer: 4.This line is always printed.

In the above program, we have the condition number >= 0. If we enter the number greater or equal to 0, then the condition evaluates true.

Here, we enter 4. So, the condition is true. Hence, the statement inside the body of if is executed.

Output 2

Enter an integer: -4You entered a negative integer: -4.This line is always printed.

Here, we enter -4. So, the condition is false. Hence, the statement inside the body of else is executed.

C++ if...else...else if statement

The if...else statement is used to execute a block of code among two alternatives. However, if we need to make a choice between more than two alternatives, we use the if...else if...else statement.

The syntax of the if...else if...else statement is:

if (condition1) {    // code block 1} else if (condition2){    // code block 2} else {    // code block 3}

Here,

• If condition1 evaluates to true, the code block 1 is executed.
• If condition1 evaluates to false, then condition2 is evaluated.
• If condition2 is true, the code block 2 is executed.
• If condition2 is false, the code block 3 is executed.

Example 3: C++ if...else...else if

// Program to check whether an integer is positive, negative or zero#include <iostream>using namespace std;int main() {     int number;    cout << "Enter an integer: ";    cin >> number;    if (number > 0) {        cout << "You entered a positive integer: " << number << endl;    } else if (number < 0) {      cout << "You entered a negative integer: " << number << endl;     } else {        cout << "You entered 0." << endl;    }     cout << "This line is always printed.";    return 0;}

Output 1

Enter an integer: 1You entered a positive integer: 1.This line is always printed.

Output 2

Enter an integer: -2You entered a negative integer: -2.This line is always printed.

Output 3

Enter an integer: 0You entered 0.This line is always printed.

In this program, we take a number from the user. We then use the if...else if...else ladder to check whether the number is positive, negative, or zero.

If the number is greater than 0, the code inside the if block is executed. If the number is less than 0, the code inside the else if block is executed. Otherwise, the code inside the else block is executed.

C++ Nested if...else

Sometimes, we need to use an if statement inside another if statement. This is known as nested if statement.

Think of it as multiple layers of if statements. There is a first, outer if statement, and inside it is another, inner if statement. Its syntax is:

// outer if statementif (condition1) {    // statements    // inner if statement    if (condition2) {        // statements    }}

Example 4: C++ Nested if

// C++ program to find if an integer is even or odd or neither (0)// using nested if statements#include <iostream>using namespace std;int main() {    int num;        cout << "Enter an integer: ";       cin >> num;        // outer if condition    if (num != 0) {                // inner if condition        if ((num % 2) == 0) {            cout << "The number is even." << endl;        }         // inner else condition        else {            cout << "The number is odd." << endl;        }      }    // outer else condition    else {        cout << "The number is 0 and it is neither even nor odd." << endl;    }    cout << "This line is always printed." << endl;}

Output 1

Enter an integer: 34The number is even.This line is always printed.

Output 2

Enter an integer: 35The number is odd.This line is always printed.

Output 3

Enter an integer: 0The number is 0 and it is neither even nor odd.This line is always printed.

In the above example,

• We take an integer as an input from the user and store it in the variable num.
• We then use an if...else statement to check whether num is not equal to 0.
  • If true, then the inner if...else statement is executed.
  • If false, the code inside the outer else condition is executed, which prints "The number is 0 and neither even nor odd."
• The inner if...else statement checks whether the input number is divisible by 2.
  • If true, then we print a statement saying that the number is even.
  • If false, we print that the number is odd.

Notice that 0 is also divisible by 2, but it is actually not an even number. This is why we first make sure that the input number is not 0 in the outer if condition.

Note: As you can see, nested if...else makes your logic complicated. If possible, you should always try to avoid nested if...else.

Body of if...else With Only One Statement

If the body of if...else has only one statement, you can omit { } in the program. For example, you can replace

int number = 5;    if (number > 0) {        cout << "The number is positive." << endl;    }    else {        cout << "The number is negative." << endl;    }

with

int number = 5;    if (number > 0)        cout << "The number is positive." << endl;    else        cout << "The number is negative." << endl;

The output of both programs will be the same.

Note: Although it's not necessary to use { } if the body of if...else has only one statement, using { } makes your code more readable.

More on Decision Making

In certain situations, a ternary operator can replace an if...else statement. To learn more, visit C++ Ternary Operator.

If we need to make a choice between more than one alternatives based on a given test condition, the switch statement can be used. To learn more, visit C++ switch.

Check out these examples to learn more:

C++ Program to Check Whether Number is Even or Odd

C++ Program to Check Whether a Character is Vowel or Consonant.

C++ Program to Find Largest Number Among Three Numbers