Control Structures

Overview

Allow program to choose whether or not to perform the a command.
Allow the program to perform the same command more than once

Conditional structures

The conditional statement we seen:

Syntax of the 'if' Statement

Example

Explanation

if ( condition ) 
   statement

OR

if ( condition ) {
   statement1;
   statement2;
   ...
}

int x;
int max;


...


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

A condition is a true/false expression.

If the condition is true, then the body of the if-statement is executed. Otherwise, it is not executed.

if(condition){         
    statement(s); 
 } 
 else{        
     statement(s);  
}

int x;
int y;
int max;


...


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

else{
   max = y;
}

A condition is a true/false expression.

If the condition is true, then the body of the if-statement is executed. Otherwise, the statements in the body of the if-statement is executed.

Exercise 1 (To be submitted on BlackBoard)

Assume that all the statement are valid and that all variables have been declared and assigned a value.
Discuss the type of each expression and subexpression.
For each variable, can we infer (deduce) the type, or not?
Turn a document called Exercise1_username.doc and complete the answers to questions 2 and 3. Change username for filename your Penn username. E.g. Exercise1_palsetia.doc

No.	Statements	Description
1	if (value < 10) { value = 0; }	The type of the expression in parentheses is boolean. If value is less than 10 then the condition is true and the body of the if-statement, which is one statement (an assignment statement) will be executed. If the condition is false the assignment statement won't be executed. In this case, we don't know the value of value before the if statement is executed, so we can't tell if the body of the if statement will be executed. The assignment statement assigns the value 0 to the variable value. We can infer that value is an integer (e.g. int) or a floating point number (e.g. double). If its type were boolean, the Java compiler would complain.
2	if ((choice == 'Y') \|\| (choice == 'y')) { gameOver = true; } else{ gameOver = false; }
3	if (cost < amountInPocket) { System.out.println("Spending = $" + cost); amountInPocket = amountInPocket - cost; }

Loop Structures

Allows us to perform a statement over and over
Usually set up to repeat an action until some condition is satisfied
There are two kinds we have looked at: while and for loop

Code	Explanation
int x = 1; while (x <= 10) { System.out.println(x); x = x + 1; }	*An example of* *a while loop* *that has this* *pattern*

for (int x = 1; x <= 10; x = x + 1){
  System.out.println(x);
}

A for loop that does the same thing

Exercise 2

The factorial n! is defined for a positive integer n as

n! = n* (n-1)..2*1

So, for example, 4! = 4 * 3 * 2 *1 = 24

Implement the factorial function in Dr Java interactions pane. Follow the steps below:

Declare variable called n of type and initialize with 4
You need declare and intializae a variable called "factorial", which stores the factorial value and will be use to print the final answer.
- What will its type be and what should its intial value be?
Write a factorial function using while statement
You may need to declare and intialize another variable(s)
Complete the condition statement and add statements within the while block to complete the assignment

>int n = 4;
>//Declare and intialize factorial 
> 
>//Declare and intialize any other variables
>
>while(condition){

factorial = ?? 
//Any other statements

} 
>//Print the value of the factorial
>System.out.println(factorial);

Now translate while loop into for loop to implement factorial

Exercise 3 (To submitted on BlackBoard)

Complete the Java program Exercise3.java to compute the factorial of first 10 numbers. Your program must compile and run/execute. Upon execution, it should print to screen the following:

1!= 1
2!= 2
3!= 6
4!= 24
5!= 120
6!= 720
7!= 5040
8!= 40320
9!= 362880
10!= 3628800

Exercise 4 (To be submitted on BlackBoard)

In this exercise we will utilize both conditionals and loops to solve problems at at hand. We are intersted in determining even numbers in the range 1 to 25. Complete the Java program Exercise4.java. Your program must compile and run/execute. Upon execution, it should print to screen the following:

Even Numbers between 1 and 25 are:
2
4
6
8
10
12
14
16
18
20
22
24