1. Control Statement Examples
GC 201

2. With a block of statements –
The if Statement
With a block of statements –
Write a program that identifies the students who didn’t pass the course. The program should also print an appropriate message.
INPUT
Student’s score (variable: score, type: double)
OUTPUT
Letter grade = ‘F’ (variable: grade, type: char)
Print an appropriate message
Start
grade = ‘X’
PROCESS
if (score <60) 1) grade = ‘F’
2) Print “Failed”
Read score
FLOWCHART
True
score < 60?
NOTE THAT
grade = ‘F’
False
If score >= 60, grade will have no value to be printed. Therefore, we give an initial value to grade.
Print “Failed”
Print grade
End

3. With a block of statements - CODE
The if Statement
With a block of statements - CODE
// import necessary libraries
import java.util.*; //contains the class Scanner
public class ifStatementN {
// instantiate the object console from the class Scanner
static Scanner console = new Scanner (System.in);
public static void main (String[] args)
// Declaration section: to declare needed variables
double score;
char grade = ‘X’; //initialize grade
// Input section: to enter values of used variables
System.out.println (“Enter student’s score”); //prompt
score = console.nextDouble();
// Processing section: processing statements
if (score < 60.0) // score is double
grade = ‘F’;
System.out.println (“Failed”);
} //end if(score < 60.0)
// Output section: display program output
System.out.printf (“Student’s Grade = %3c“, grade);
} // end main
} // end class 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

4. With a block of statements - PROGRAM : CASE 1 (TRUE)
The if Statement
With a block of statements - PROGRAM : CASE 1 (TRUE)
double score;
char grade = ‘X’; //initialize grade 10 11
???
score X
grade
System.out.println (“Enter student’s score”); //prompt
score = console.nextDouble(); 13 14
Enter student’s score
50.5_ 1 2
50.5
score X
grade

5. With a block of statements - PROGRAM : CASE 1 (TRUE)
The if Statement
With a block of statements - PROGRAM : CASE 1 (TRUE)
if (score < 60.0) // score is double {
grade = ‘F’;
System.out.println (“Failed”); //output line 3
} //end if (score < 60.0) 16 17 18 19 20
Enter student’s score
50.5
Failed_ 1 2 3
50.5
score F
grade
System.out.printf (“Student’s Grade = %3c“, grade); //output line 4 22
Enter student’s score
50.5
Failed
Student’s Grade = ~~F_ 1 2 3 4
50.5
score F
grade

6. With a block of statements: PROGRAM : CASE 2 (FALSE)
The if Statement
With a block of statements: PROGRAM : CASE 2 (FALSE)
double score;
char grade = ‘X’; //initialize grade 10 11
???
score X
grade
System.out.println (“Enter student’s score”); //prompt
score = console.nextDouble(); 13 14
Enter student’s score
85_ 1 2
85.0
score X
grade

7. With a block of statements - PROGRAM : CASE 2 (FALSE)
The if Statement
With a block of statements - PROGRAM : CASE 2 (FALSE)
if (score < 60.0) // score is double {
grade = ‘F’;
System.out.println (“Failed”);
} //end if (score < 60.0) 16 17 18 19 20
Enter student’s score
85_ 1 2
85.0
score X
grade
System.out.printf (“Student’s Grade = %3c“, grade); 22
Enter student’s score 85
Student’s Grade = ~~X_ 1 2 3
85.0
score X
grade

8. With a single statement – PROGRAM : ANALYSIS
The if…else Statement
With a single statement – PROGRAM : ANALYSIS
Write a program that calculates the net salary after taxes deduction. If the salary is greater than 5000 SR, taxes are 20% of the salary; otherwise, taxes are 25% of the salary.
INPUT
Salary (variable: salary, type: double)
OUTPUT
Net Salary (variable: netSalary, type: double)
PROCESS
if (salary > 5000) netSalary=0.8*salary;
if (salary <= 5000) netSalary=0.75*salary.

9. With a single statement – PROGRAM : Flowchart
The if…else Statement
With a single statement – PROGRAM : Flowchart
Start
Read salary
salary > 5000?
False
True
netSalary = 0.8 * salary
netSalary = 0.75 * salary
Print netSalary
End

10. With a single statement - PROGRAM : CODE
The if…else Statement
With a single statement - PROGRAM : CODE
// import necessary libraries
import java.util.*; //contains the class Scanner
public class ifElseStatement1 {
// instantiate the object console from the class Scanner
static Scanner console = new Scanner (System.in);
public static void main (String[] args)
// Declaration section: to declare needed variables
double salary, netSalary;
// Input section: to enter values of used variables
System.out.println (“Enter employee’s salary”); //prompt
salary = console.nextDouble();
// Processing section: processing statements
if (salary > ) // salary is double
netSalary = 0.8 * salary;
else
netSalary = 0.75 * salary;
// Output section: display program output
System.out.printf (“Net Salary = %.2f“, netSalary); //2 digits after decimal point
} // end main
} // end class 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

11. With a single statement - PROGRAM : CASE 1 (TRUE)
The if…else Statement
With a single statement - PROGRAM : CASE 1 (TRUE)
System.out.println (“Enter employee’s salary”); //prompt
score = console.nextDouble(); 12 13
Enter employee’s salary
6000_ 1 2
if (salary > ) // salary is double
netSalary = 0.8 * salary; //condition is true
else
netSalary = 0.75 * salary; 15 16 17 18
Enter employee’s salary
6000_ 1 2
System.out.printf (“Net Salary = %.2f“, netSalary); //output line 3 20
Enter employee’s salary
6000
Net Salary = _ 1 2 3

12. With a single statement - PROGRAM : CASE 2 (FALSE)
The if…else Statement
With a single statement - PROGRAM : CASE 2 (FALSE)
System.out.println (“Enter employee’s salary”); //prompt
score = console.nextDouble(); 12 13
Enter employee’s salary
2500.0_ 1 2
if (salary > ) // salary is double
netSalary = 0.8 * salary;
else
netSalary = 0.75 * salary; //condition is false 15 16 17 18
Enter employee’s salary
2500.0_ 1 2
System.out.printf (“Net Salary = %.2f“, netSalary); //output line 3 20
Enter employee’s salary
2500.0
Net Salary = _ 1 2 3

13. The switch Statement EXAMPLE 2
Write a program that displays the following menu, then acts accordingly:
Enter your choice:
Add two positive numbers
Get the double of a positive number
Get the square of a number
The program should give an error message for invalid inputs.

14. The switch Statement EXAMPLE 2 public static void main (String[] args){
//Declaration section
Scanner read = new Scanner (System.in);
int choice, num1, num2, result = -1;
String message = “Invalid input”;
//input section
//Display menu
System.out.println (“Enter your choice:”);
System.out.println (“1. Add two positive numbers”);
System.out.println (“2. Get the double of a positive number”);
System.out.println (“3. Get the square of a number”);
choice = read.nextInt();
//processing section 1 2 3 4 5 6 7 8 9 10 11 12 13 14

15. The switch Statement EXAMPLE 2 (cont’d) //processing section
switch (choice) {
case 1: System.out.println (“Enter two positive integers”); //prompt
num1 = read.nextInt();
num2 = read.nextInt();
result = num1 + num2; // the value of result is no more equal to -1
break;
case 2: System.out.println (“Enter a positive integer”); //prompt
result = num1 * 2; // the value of result is no more equal to -1
case 3: System.out.println (“Enter an integer”); //prompt
result = num1 * num1; // the value of result is no more equal to -1
default: message =“Invalid value of choice”; //no change to “result”  result=-1
} //end switch
//output section
if (result !=-1)
System.out.println (result); //result is modified (!= -1)
else
System.out.println (message); //result is not modified (equals -1)
} //end main 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

16. Without default Statement
EXAMPLE 2
In fact, line 30 in slide 10 adds nothing to the program. Note that the variable message is already initialized to “Invalid input” in line 6.
In addition, the default is optional in the switch statement. In other words, it may be omitted from the switch statement if it is useless  Therefore, line 30 may be omitted.
The modified “Processing section” of the program is shown in the next slide:

17. Without default Statement
EXAMPLE 2
//processing section
switch (choice) {
case 1: System.out.println (“Enter two positive integers”); //prompt
num1 = read.nextInt();
num2 = read.nextInt();
result = num1 + num2; // the value of result is no more equal to -1
break;
case 2: System.out.println (“Enter a positive integer”); //prompt
result = num1 * 2; // the value of result is no more equal to -1
case 3: System.out.println (“Enter an integer”); //prompt
result = num1 * num1; // the value of result is no more equal to -1
} //end switch
//output section
if (result !=-1)
System.out.println (result);
else
System.out.println (message);
} //end main 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

18. EXAMPLE 2 – Validating user input
Programming Hint (1)
EXAMPLE 2 – Validating user input
//processing section
switch (choice) {
case 1: System.out.println (“Enter two positive integers”); //prompt
num1 = read.nextInt();
num2 = read.nextInt();
if ((num1 < 0) || (num2 < 0)) //should be made after reading num1 & num2
result = -1;
else
result = num1 + num2; // the value of result is no more equal to -1
break;
case 2: System.out.println (“Enter a positive integer”); //prompt
if (num1 < 0) //this should be made after reading num
result = num1 * 2; // the value of result is no more equal to -1
case 3: System.out.println (“Enter an integer”); //prompt
result = num1 * num1; // the value of result is always positive
} //end switch
//output section 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

19. EXAMPLE 2 – Validating user input (cont’d)
Programming Hint (1)
EXAMPLE 2 – Validating user input (cont’d)
//output section
if (result !=-1)
System.out.println (result);
else
System.out.println (message);
} //end main 37 38 39 40 41 42
Validating user input is a vital issue in programming. The programmer should take into consideration all possible values entered by the user.
In the example above, result is used to store the result of the arithmetic operation. It is also used to denote an invalid input. The sources of invalid inputs may be either the variable choice, num1 or num2.
Initially, result is initialized to -1 (line 5). This is useful in case the user entered an invalid value for the variable choice.
Now, let us reconsider the conditions in lines 20 and 27. In fact, these may be re-written in a smarter way as follows:

20. EXAMPLE 2 - Validating user input (cont’d)
Programming Hint (1)
EXAMPLE 2 - Validating user input (cont’d)
case 1: System.out.println (“Enter two positive integers”); //prompt
num1 = read.nextInt();
num2 = read.nextInt();
if ((num1 >= 0) && (num2 >= 0))
result = num1 + num2; // the value of result is no more equal to -1
break; 17 18 19 20 21 22
case 2: System.out.println (“Enter a positive integer”); //prompt
num1 = read.nextInt();
if (num1 >= 0)
result = num1 * 2; // the value of result is no more equal to -1
break; 25 26 27 28 29
We were able to omit lines 21, 22 and lines 28, 29 in slide 13 since they add nothing to the logic. However, we had to modify the logical expressions as shown above.

21. EXAMPLE 2 – Reducing Redundancy
Programming Hint (2)
EXAMPLE 2 – Reducing Redundancy
System.out.println (“Enter a positive integer”);
num1 = read.nextInt();
//processing section
switch (choice) {
case 1: System.out.println (“Enter a positive integer”); //prompt
num2 = read.nextInt();
if ((num1 < 0) || (num2 < 0)) //should be made after reading num1 & num2
result = -1;
else
result = num1 + num2; // the value of result is no more equal to -1
break;
case 2: System.out.println (“Enter a positive integer”); //prompt
if (num1 < 0) //this should be made after reading num
result = num1 * 2; // the value of result is no more equal to -1
case 3: System.out.println (“Enter an integer”); //prompt
result = num1 * num1; // the value of result is always positive
} //end switch 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

22. EXAMPLE 2 – Final version
Programming Hints
EXAMPLE 2 – Final version
public static void main (String[] args) {
//Declaration section
Scanner read = new Scanner (System.in);
int choice, num1, num2, result = -1;
String message = “Invalid input”; //this may also be declared as a constant
//input section
//Display menu
System.out.println (“Enter your choice:”);
System.out.println (“1. Add two positive numbers”);
System.out.println (“2. Get the double of a positive number”);
System.out.println (“3. Get the square of a number”);
choice = read.nextInt();
System.out.println (“Enter a positive integer”);
num1 = read.nextInt();
//processing section 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

23. EXAMPLE 2 – Final version (cont’d)
Programming Hint
EXAMPLE 2 – Final version (cont’d)
//processing section
switch (choice) {
case 1: System.out.println (“Enter a positive integer”); //prompt
num2 = read.nextInt();
if ((num1 >= 0) && (num2 >= 0))
result = num1 + num2; // the value of result is no more equal to -1
break;
case 2: if (num1 >= 0) //this should be made after reading num
result = num1 * 2; // the value of result is no more equal to -1
case 3: result = num1 * num1; // the value of result is always positive
} //end switch
//output section
if (result != -1)
System.out.println (result);
else
System.out.println (message);
} //end main 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

24. Without break Statement
EXAMPLE 4
Using a switch statement, write a program that accepts an integer value from the user ranging between -2 and 2. The program then identifies if the number is negative, zero or positive.

25. Without break Statement
EXAMPLE 4 - SOLUTION
public static void main (String[] args) {
//Declaration section
Scanner read = new Scanner (System.in);
int num;
//input section
System.out.println (“Enter an integer between -2 and +2); //prompt
num = read.nextInt();
//processing section
switch (num)
case -2:
case -1: System.out.println (“The number is negative”);
break;
case 0: System.out.println (“The number is zero”);
case 1:
case 2: System.out.println (“The number is positive”);
default: System.out.println (“Invalid input”);
} //end switch
} //end main 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25