C++ Decision Making (if, if...else and Nested-if, if...else-if...else) Statements

Decision Making Statements

• Sometimes we need to execute a block of statements only when a particular condition is met or not met. This is known as Decision Making.
• As we executing a certain program after making a decision in the program logic.
• For decision making in C++, we have four types of control statements.

There are four forms of Decision Making statements in C++.

1. if statement
2. if...else statement
3. if...else-if...else statement
4. nested statement

1. C++ if Statement

• In computer programming, we use the if statement to run a block code only when a certain condition is met.
• If statement is the most simple decision making statement.

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

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++ Program to print positive number using if Statement

// Program to print positive number entered by the user
// If the user enters a negative number, it is skipped by the if statement

#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: 2
You entered a positive number: 2
This statement is always executed.

When the user enters 2, the condition number > 0 is evaluated to true and the statement inside the body of if is executed.

Output 2

Enter a number: -2
This statement is always executed.

When the user enters -2, the condition number > 0 is evaluated to false and the statement inside the body of if is not executed.

2. C++ if...else statement

• The if statement along tells us that if a condition is true it will execute a block of statement and if the condition is false it won't.
• So, we can use the else statement with if statement to execute a block of code when condition is false.
• The if statement can have an optional else clause.

The syntax of if-else statement is:

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

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

1. 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

2. 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++ Program to check whether an integer is positive or negative using 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 statement is always printed.";
    return 0;
}

Output 1

Enter an integer: 2
You entered a positive integer: 2.
This statement 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 2. So, the condition is true. Hence, the statement inside the body of if is executed.

Output 2

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

Here, we enter -2. So, the condition is false. Hence, the statement inside the body of else is executed.

3. C++ if-else ladder statement

• if...else-if...else statement is used when we need to check multiple conditions.
• 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.
• In this control structure we have one if and one else, however we can have multiple else-if blocks.

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

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

Here,

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

Example 3: C++ check whether an integer is positive, negative or zero using if...else-if...else

// 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: 2
You entered a positive integer: 2.
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.

4. 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.

The syntax of Nested if is:

// outer if statement
if (condition1) {
    // statements

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

Example 4: C++ program to find if an integer is even or odd or neither (0) using 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(nested) if condition
        if ((num % 2) == 0) {
            cout << "The number is even." << endl;
        }
        // inner(nested) 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;
    return 0;
}

Output 1

Enter an integer: 2
The number is even.
This line is always printed.

Output 2

Enter an integer: 3
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(nested) 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(nested) 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 = 2;
                                
if (number > 0) {
cout << "The number is positive." << endl;
}
else {
cout << "The number is negative." << endl;
}

with

int number = 2;
                                
if (number > 0)
cout << "The number is positive." << endl;
else
cout << "The number is negative." << endl;

Output:

The number is positive.

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...Case.

Check out these examples to learn more about Decision making statements:

1. C++ Program to Check Whether Number is Even or Odd
2. C++ Program to Check Whether a Character is Vowel or Consonant.

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