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Decision Making in C++


August 17, 2022, Learn eTutorial
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Decision-making is the process of determining the order in which statements should be executed based on certain conditions or of repeating a group of statements until certain specified conditions are met.

Programmers must specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is true, and optionally, additional statements to be executed if the condition is false, in order for decision making structures to work.

The general form of a typical decision-making structure found in most programming languages is shown below.

The following statements are supported by C++ for handling decision-making:

  1. if statement
  2. switch statement
  3. conditional operator statement
  4. goto statement

C++ if statement

How decision making is done with the if statement?

In an if statement, a boolean expression is followed by one or more statements. Depending on the complexity of the conditions to be tested, the if statement can be implemented in a variety of ways. 

The various forms are as follows:

  • Simple if statement
  • if....else statement
  • Nested if....else statement
  • else if statement

Simple if statements

The simplest statement for making a decision is the if statement. It is used to determine whether a specific statement or block of statements will be executed or not, i.e., whether a block of statements will be executed if a specific condition is true otherwise the block of statements is not executed.

if control flowchart

syntax for simple if statements


if(expression)
{
    statement-inside;
}
    statement-outside;

 

If the expression is true, then 'statement-inside' is executed; otherwise, it is skipped and only 'statement-outside'  is executed.

if control flowchart

Working of If Statement

An example: Program to print a positive number entered by the user and suppose if the user enters a negative number, it will be skipped.


#include <iostream>
using namespace std;

int main() {

  int number;

  cout << " Please enter an integer: ";
  cin >> number;

  // checks whether if the given number is positive
  if (number > 0) {
    cout << " yes you entered a positive integer: " << number << endl;
  }

  cout << "This statement is always executed.";

  return 0;
}

Output1:

Please enter an integer: 8
yes you entered a positive integer: 8
This statement is always executed.

When the user enters 8, the condition number > 0 is evaluated to true, and the statement contained within the body of if is executed successfully.

Output2:

Please enter an integer: -9
This statement is always executed.

When the user enters -9, the condition number > 0 is evaluated to be false, and the statement contained within the body of if is not executed.

C++ if....else statement


if(expression)
{
    statement-block1;
}
else
{
    statement-block2;
}
 

If the 'expression' is true or returns true, the'statement-block1' is executed; otherwise, the'statement-block1' is skipped and'statement-block2' is executed.

if control flowchart

How if….else statement works

if control flowchart

If the condition is true, The code within the body of if is executed and the code within the body of else is skipped without executing.

Suppose If the condition is discovered to be false, The code contained within the body of else is executed and the code contained within the body of if is not executed.

For example: let us write a Program to check whether an integer is positive or negative and as per this program, 0 is considered a positive number.


#include <iostream>
using namespace std;

int main() {

  int number;

  cout << " Please enter an integer: ";
  cin >> number;

  if (number >= 0) {
    cout << "You  successfully entered a positive integer: " << number << endl;
  }
  else {
    cout << "You successfully entered a negative integer: " << number << endl;
  }

  cout << "This is always printed..";

  return 0;
}

Output1:

Please enter an integer: 2
You  successfully entered a positive integer: 2
This is always printed..

The condition number >= 0 is present in the preceding program. If we enter a number that is greater than or equal to zero, then the condition is true.

suppose we entered an integer 2,  so the condition has been met. As a result, the statement within the body of if is successfully executed.

Output2:

Please enter an integer: -1
You successfully entered a negative integer: -1
This is always printed..

suppose we entered an integer -1,  so the condition has not been met. As a result, the statement within the body of else is successfully executed.

C++ Else if statements


if(expression 1)
{
    statement-block1;
}
else if(expression 2) 
{
    statement-block2;
}
else if(expression 3 ) 
{
    statement-block3;
}
else 
    default-statement;

 

The expression is put to the test from the top to down. The statement associated with the true condition is executed as soon as it is discovered.

Nested if control flowchart

For an example,

let us write a C++ program to find if an integer is positive, negative, or zero using else if statements.


#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 is always printed..";

    return 0;
}

Output:

Enter an integer: 2
You entered a positive integer: 2
This is always printed..

Enter an integer: -1
You entered a negative integer: -1
This is always printed..
 
Enter an integer: 0
You entered 0.
This is always printed.

If the value is greater than zero, the code contained within the if block is executed. If the value is less than zero, the code contained within the else if block is executed. Otherwise, the code contained within the else block is executed.

C++ Nested if....else statement

We may need to use an if statement inside another if statement at some times. This is referred to as a nested if statement.

Nested if control flowchart

syntax for Nested if statements


if (expression) {
    if (expression1) {
        statement - block1;
    } else {
        statement - block2;
    }
} else {
    statement - block3;
}
 

If 'expression' is false or returns false, the ‘statement-block3' is executed; otherwise, execution enters the if condition and checks for 'expression 1'. Then, if 'expression 1' is true or returns true,'statement-block1' is executed; otherwise,'statement-block2' is executed.

For an example

let us write a 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 << " Please enter an integer: ";
    cin >> num;

    // outer if condition
    if (num != 0) {

        // inner if condition
        if (num > 0) {
            cout << "yes the number is positive." << endl;
        }
        // inner else condition
        else {
            cout << "yes the number is negative." << endl;
        }
    }
    // outer else condition
    else {
        cout << "The number is 0 and it is neither positive nor negative." << endl;
    }

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

    return 0;
}
 

Output:


Please enter an integer: 4
yes the number is positive.
This is always printed..

Please enter an integer: -9
yes the number is negative.
This is always printed..

Please enter an integer: 0
The number is 0 and it is neither positive nor negative.
This is always printed..

In the above program; We accept an integer from the user and store it in the variable num. Then, we use an if...else statement to see if the num is not equal to zero.

  • When the condition is true, the inner if...else statement is carried out.
  • If false, the code contained within the outer else condition is executed, and  printing "The number is 0 and it is neither positive nor negative."

The inner if...else statement determines whether the input number is positive, i.e. whether the num is greater than zero.

  • If this is true, we print a statement stating that the number is positive.
  • If this is false, we print a statement stating that the number is negative.

The switch statements, conditional operator statements, and goto statements which are supported by C++ for handling decision-making will be discussed in the next tutorial.