If,If...Else & Nested If...Else

 

 

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

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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

 

Example: 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: 5
You entered a positive number: 5
This 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: -5
This 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

 

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

Output 2

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

Output 3

Enter an integer: 0
You 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 statement
if (condition1) {
    // statements

    // inner if statement
    if (condition2) {
        // statements
    }
}

Notes:

• We can add else and else if statements to the inner if statement as required.
• The inner if statement can also be inserted inside the outer else or else if statements (if they exist).
• We can nest multiple layers of if statements.

 

Example: 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: 34
The number is even.
This line is always printed.

Output 2

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

Output 3

Enter an integer: 0
The 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.