Rose Kamal Love

Rose Kamal Love

Conditionals in C++

In computer programming, we use the if...else statement to run one block of code under certain conditions and another block of code under different conditions.

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 if statement is:

if (condition) {
  /* body of if statement
   *
   * The code we want the computer to execute
   * when this condition is true is written here
   */
}

The if statement evaluates the condition inside the parentheses ( ) then:

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

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

How if Statement works

Example: 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.";

}

Output 1:

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

Output 2:

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

C++ if...else Statement

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
}

How if-else Statement works

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

Example: C++ if...else Statement

#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.";
}

Output 1:

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

Output 2:

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

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 if...else 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.

If-elseIf-else

Note: There can be more than one else if statement but only one if and else statement

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

#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.";
}

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 Nested if

// 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;
  }

  cout << "This line is always printed." << endl;

  return 0;
}

Output 1

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

Output 2

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

Output 3

Enter an integer: 0
The number is 0 and it is neither positive nor negative.
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, we print that the number is negative.

Note: As you can see, nested if...else makes your logic complicated. I 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 ouput of both the 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.

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