C++ if, if-else and Nested if…else | Examples

With the help of examples, we will learn how to use the if-else statement to write decision-making programs in this tutorial. In previous we taught for loop and while loop with example.

The if..else statement is used in computer programming to execute one block of code under specific circumstances and a different block of code under other circumstances.

You are already aware that C++ supports the standard mathematical logical conditions:

• Less than: a < b
• Less than or equal to: a <= b
• Greater than: a > b
• Greater than or equal to: a >= b
• Equal to a == b
• Not Equal to: a != b

These conditions can be used to perform various actions based on various choices.

In C++, the following conditional statements are applicable:

• If you want a section of code to run only when a certain condition is true, use the if statement.
• If the same condition is false, use else to specify that a different block of code should be run.
• When testing a new condition if the original one is false, use the else if statement.
• To specify a variety of different code blocks to be executed, use switch.

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There are three different kinds of if…else statements in C++.

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

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To specify a block of C++ code to be executed if a condition is true, use the if statement.

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

• The body of the if statement’s code is executed if the condition evaluates to true.
• If the condition evaluates to false, the code in the body of the if statement is skipped.

Note: The word “if” is written in lowercase. If or IF will produce an error if they are written in uppercase.

// Program to print positive number entered by the user
//geektocode.com

#include <iostream>
using namespace std;

int main() {
  int number;
  cout << "Enter an integer: ";
  cin >> number;
  // checks the number is positive
  if (number > 0) {
    cout << "Entered a positive integer: " << number << endl;
  }
  cout << "This statement is executed.";

  return 0;
}

OUTPUT:
Enter an integer: 44
Entered a positive integer: 44
This statement is executed.

The statement inside the body of if is executed when the user types in 44, evaluating the condition number > 0 to true.

OUTPUT:
Enter an integer: -66
This statement is executed.

The condition number > 0 is assessed to false when the user enters -66, and the statement inside the body of the if is not carried out.

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Use the else statement to define which code block should be executed if the condition is false.

if (condition) {
  // block of code to be executed if the condition is true
} else {
  // block of code to be executed if the condition is false
}

If the test results are positive,

• The body of the if statement’s code is run.
• The execution of the code included in the else body is skipped.

Suppose that the assumption is incorrect.

• The code included in else’s body is run.
• Execution of the if’s body of code is skipped.

// 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;
  }

  return 0;
}

OUTPUT:
Enter an integer: 77
You entered a positive integer: 77

OUTPUT:
Enter an integer: -99
You entered a negative integer: -99

If the first criterion is untrue, a different criterion should be stated in the otherwise if clause.

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

Here,

• The code block in question 1 is run if condition 1 evaluates to true.
• Condition 2 is assessed if condition 1 evaluates to false.
• Block 2 of the code is run if condition 2 is true.
• The code block 3 is run if condition 2 is true.

// 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;
  }

  return 0;
}

OUTPUT:
Enter an integer: 99
You entered a positive integer: 99

OUTPUT 2:
Enter an integer: -99
You entered a negative integer: -99

OUTPUT 3:
Enter an integer: 0
You entered 0.

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The use of an if statement inside of another if statement is occasionally necessary. This is referred to as a “nested if statement.”

See it as a series of if statements. A first, outer if statement is present, and an additional, inner if statement is contained within it. The syntax is:

// outer if statement
if (condition1) {

  // statements

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

• If necessary, we can supplement the inner if statement with else and else if statements.
• The outer otherwise or else if statements can likewise be put inside the inner if statement (if they exist).
• Several layers of if statements can be nested.

// C++ program to find if an integer is positive, negative or zero
// 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 > 0) {
      cout << "The number is positive." << endl;
    }
    // inner else condition
    else {
      cout << "The number is negative." << endl;
    }  
  }
  // outer else condition
  else {
    cout << "The number is 0 and it is neither positive nor negative." << endl;
  }

  return 0;
}

OUTPUT:
Enter an integer: 67
The number is positive.

OUTPUT 2:
Enter an integer: -8
The number is negative.

OUTPUT 3:
Enter an integer: 0
The number is 0 and it is neither positive nor negative.

Note: As you can see, nested if…else complicates your reasoning. Nested if…else should always be avoided, if at all possible.