The selection-statement or if-statement

 

  • The if-statement:

    • The if-statement allows a computer program to select different paths of execution

  • Java has 2 forms of if-statements:

    • The one-way if-statement   or (simply as) the if-statement

    • The two-way if-statement   or the if-else-statement

  • The if-statements are also called:

    • Selection statements

The one-way if-statement

  • The one-way if-statement allows a computer program to select between:

    1. Execute some statements         and

    2. Skip the statements

  • Syntax of the one-way if-statement: 

        if ( boolean-expression )
        one-statement ; // statement executed if boolean-expr is true

 or:

    if ( boolean-expression )
    {
        statement1; // statements executed if boolean-expr is true
        statement2;
        ...
    }

  Example one-way if-statement  

  • Consider the following Java program: 

    public class Demo1
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        double  radius;
        double  area;

        System.out.print("Enter a radius: ");
        radius = input.nextDouble();
 
        if ( radius >= 0 )
        {
            // Compute and print area ONLY when radius >= 0
            area = 3.14159 * radius * radius;
            System.out.println(area);
        }
   }                       
}

    Program will only print the area if you enter a radius ≥ 0

DEMO: 03-selections/02-if-intro/Demo1.java

Flow chart

  • Flow chart = a diagram showing the pathways of program execution

  • Flow chart elements:

    Action Selection
    Meaning:
    • execute the statements (and continue)
     

    		Meaning:
    • If Condition is True, continue in the True branch
    • Otherwise, continue in the False branch

Flow chart of the one-way if-statement

  • Flow chart for the one-way if-statement:

    If-statement Flow chart 
     if ( radius >= 0 )
 {
    area = 3.14159 * radius * radius; 
    System.out.println(area);
 }

     

Flow chart of the one-way if-statement

  • Program execution pathway when the condition in one-way if-statement is true:

    If-statement Flow chart 
     if ( radius >= 0 )   // True case
 {
    area = 3.14159 * radius * radius; 
    System.out.println(area);         
 }

  • When the condition is true:

    • The statement(s) inside the one-way if-statement wil be executed

Flow chart of the one-way if-statement

  • Program execution pathway when the condition in one-way if-statement is false:

    If-statement Flow chart 
     if ( radius >= 0 )   // False case
 {
    area = 3.14159 * radius * radius; 
    System.out.println(area);
 }

 // Statements inside the
 // if-statement are
 // skipped

  • When the condition is false:

    • The statement inside the one-way if-statement will be skipped

Example one-way if-statement:    Check if a number is even

Write a Java program that reads in an int number and print the message "Even" if the number is even 

import java.util.Scanner;

public class IsEven 
{
   public static void main(String[] args) 
   {
      Scanner input = new Scanner(System.in);

      System.out.println("Enter an integer: ");

      int number = input.nextInt();    // Input number

      // Check if number entered in even
      if ( number % 2 == 0)
      {
         System.out.println("Even");
      }
   }
}

DEMO: 03-selections/02-if-intro/IsEven.java

Example one-way if-statement    Order 2 numbers

Write a program that reads in 2 int numbers into x and y and the program will re-arrange their values so that the value in x is always less (or equal to) the value in y 


Program in psuedo code:

public class ExchangeNums 
{
   public static void main(String[] args) 
   {
      read in x;       // Read in the numbers
      read in y;

      if ( x > y )     // If wrong order, exchange x and y
      {
         switch the values in x and y
      }
   }
}

Example one-way if-statement    Order 2 numbers

Write a program that reads in 2 int numbers into x and y and the program will re-arrange their values so that the value in x is always less (or equal to) the value in y 

import java.util.Scanner;

public class ExchangeNums 
{
   public static void main(String[] args) 
   {
      Scanner input = new Scanner(System.in);

      int x = input.nextInt();    // Read in the numbers
      int y = input.nextInt();

      if ( x > y )  // If wrong order, exchange x and y
      {
         int help;

         help = x;      // Program technique that we learned before
	 x = y;         // See: 
	 y = help;
      }
   }
}

DEMO: 03-selections/02-if-intro/ExchangeNums.java

The two-way if-statement

  • The two-way if-statement allows a computer program to select between:

    1. Execute one group of statements         and

    2. Execute another group of statements

  • Syntax of the two-way if-statement: 

        if ( boolean-expression )
        then-statement ; // statement executed if boolean-expr is true
    else
        else-statement ; // statement executed if boolean-expr is false

 or:

    if ( boolean-expression )
    {   // then-part
        then-statement1; // statements executed if boolean-expr is true
        ...
    }
    else
    {    // else-part
        else-statement1; // statements executed if boolean-expr is false
        ...
    }

  Example two-way if-statement  

  • Consider the following Java program: 

    public class Demo1
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner(System.in);
        double  radius;
        double  area;

        System.out.print("Enter a radius: ");
        radius = input.nextDouble();
 
        if ( radius >= 0 )
        {
            area = 3.14159 * radius * radius;
            System.out.println(area);
        }
        else
            System.out.println("Illegal radius");
   }                       
}

    Program prints area if you enter a radius ≥ 0 otherwise prints "Illegal radius"

DEMO: 03-selections/02-if-intro/Demo2.java

Flow chart of the two-way if-statement

  • Flow chart for the two-way if-statement:

    If-else-statement Flow chart 
     if ( radius >= 0 )
 {
    area = 3.14159 * radius * radius; 
    System.out.println(area);
 }
 else
 {
    System.out.println("Illegal ...");
 }

     

     

Flow chart of the two-way if-statement

  • Program execution pathway when the condition in one-way if-statement is true:

    If-else-statement Flow chart 
     if ( radius >= 0 )   // True case
 {
    area = 3.14159 * radius * radius; 
    System.out.println(area);         
 }
 else
 {
    System.out.println("Illegal ...");
 }

  • When the condition is true:

    • The statement(s) inside the "then"-part of the two-way if-statement will be executed

Flow chart of the two-way if-statement

  • Program execution pathway when the condition in two-way if-statement is false:

    If-else-statement Flow chart 
     if ( radius >= 0 )   // False case
 {
    area = 3.14159 * radius * radius; 
    System.out.println(area);
 }
 else
 {
    System.out.println("Illegal ...");
 }

  • When the condition is false:

    • The statement(s) inside the "else"-part of the two-way if-statement will be executed

Example two-way if-statement:    Print whether a number is even or odd

Write a program that reads in an int number and print "Even" if the number is even and "Odd" otherwise 

import java.util.Scanner;

public class MyProg 
{
   public static void main(String[] args) 
   {
      Scanner input = new Scanner(System.in);

      System.out.println("Enter an integer: ");
      int number = input.nextInt();

      if ( number % 2 == 0)
      {
         System.out.println("Even");
      }
      else
      {
         System.out.println("Odd");
      }
   }
}

DEMO: 03-selections/02-if-intro/IsEvenOrOdd.java

Example two-way if-statement:    Find the maximum of 2 numbers

Write a program that reads in 2 double numbers x and y and find the maximum of the numbers and assign the maximum to the variable max 

import java.util.Scanner;

public class FindMax 
{
   public static void main(String[] args) 
   {
      Scanner input = new Scanner(System.in);

      double x = input.nextDouble();
      double y = input.nextDouble();
      double max;

      if ( x >= y )
      {
          max = x;
      }
      else
      {
          max = y;
      }
   }
}

DEMO: 03-selections/02-if-intro/FindMax.java