Programming in Java I

Lecture Notes, class 5 for csc211.

Prepared by: Anthony Larrain

These notes serve as an outline to the lecture, they are not intended to be complete. You should be reading the assigned sections. During class I may include or omit certain topics.

The while Statement

Recall three ways to alter flow of control

• invoking a method
• decisions( if - else )
• looping

while( condition )
{
   // Execute this block while the condition is true.
}

• Used to execute a statement or statements multiple times.
• Uses a boolean expression as the condition.
• If the condition is true and the statement is executed control jumps back to the condition.
• The curly braces are needed if the loop block has more than one statement.

Examples using while loop.

// Prints the numbers 1 to and including 10
int i = 1;
while( i <= 10 )
{
   System.out.println( i );
   i += 1;        // Same as i = i + 1;
 }
 
 // Count down from 100, stop at 1

int i = 100;
while( i > 0 )
{
   System.out.println( i );
   i -= 1;                      // Same as i = i - 1;
}

// Print the numbers 1 to 100 except 50;

int i = 1;
while( i <= 100 )
{
   if ( i == 50 )
     ++i;
   System.out.println(i);
   ++i;
}

• In a count-controlled loop, the loop body is executed a fixed number of times.
• In a sentinel-controlled loop, the loop body is executed repeatedly until a designated value, called a sentinel, is encountered

See DiceGame2.java

More on if - else

if( condition )
{
   execute this block only if the condition is true.
}
else
{
   execute this block if the condition is false.
}

• If any of the blocks contain only one statement then the braces are optional.
• Good for beginners to always put in the braces.

int score = 78;
if(score > 59)
   System.out.println("You passed");
else
   System.out.println("You failed");
System.out.println("See ya);

// output is  .. You passed and See ya

• Can get confusing.

int score = 78;
if(score > 59)
   System.out.println("You passed");
else
   System.out.println("You failed");
   System.out.println("See ya);

// output STILL  is  .. You passed and See ya

• An if-else is used when data falls into two mutually exclusive categories.
  • Pass or Fail
  • Roll a seven or Not.
  • Heads or Tails
  • Valid data or invalid data
• Can combine if-else together with flexibility in using spaces and braces to handle data that is more than two mutually exclusive categories.
• To deal with data that falls into more than two mutually exclusive categories, we construct an if - else - if ladder.

if(condition 1)
{
   execute this block if condition 1 is true
}
else if(condition 2)
{
   execute this block if condition 2 is true
}
else if(condition 3)
{
   execute this block if condition 3 is true
}
else
{
   execute this block if all the above conditions are false.
}

Example 1

int score = 85;

if(score >= 90)
{
   System.out.println("Excellent");
}
else if (score >= 60) {
   System.out.println("You passed");
}
else {
   System.out.println("You failed");
}
System.out.println("See ya");

// Output is .. You passed and See ya

Example 2

int score = 95;

if(score >= 90)
{
   System.out.println("Excellent");
}
else if (score >= 60) {
   System.out.println("You passed");
}
else {
   System.out.println("You failed");
}
System.out.println("See ya");

// Output is .. Excellent and See ya

Example 3

int score = 55;

if(score >= 90)
{
   System.out.println("Excellent");
}
else if (score >= 60) {
   System.out.println("You passed");
}
else {
   System.out.println("You failed");
}
System.out.println("See ya");

// Output is .. You failed and See ya

Example 4

int score = 85;

if(score >= 90)
{
   System.out.println("Excellent");
}
else if (score >= 75) {
   System.out.println("Good");
}
else if (score >= 60) {
   System.out.println("You passed");
}
else {
   System.out.println("You failed");
   System.out.println("Retake");
}
System.out.println("See ya");

// Output is .. Good and See ya

Example 4

• Noticed the braces are removed for the ones that have only one statement

int score = 85;

if(score >= 90)
   System.out.println("Excellent");
else if (score >= 75) 
   System.out.println("Good");
else if (score >= 60)
   System.out.println("You passed");
else {
   System.out.println("You failed");
   System.out.println("Retake");
}
System.out.println("See ya");

// Output is .. Good and See ya

Logical Operators

Simple boolean expressions can be joined together using the relational operators. A simple boolean expression consists of two operands and one relational or equality operator.

!    -- NOT
&&   -- AND
||   -- OR

NOT has the highest precedence of the Logical operators, followed by AND then by OR. The logical operators have lower precedence than the relationaloperators.

if( age > 12 && age < 20)
    Your a teen

if( result == 7 || result == 11 )
    you won

boolean value = false;
if( !value )
   value is false

if( score >= 0 && score <= 100)
  valid score

Increment and Decrement Operators

• Unary operators that operate on variables of integer and floating point types.
• Easy way to add or subtract the number one from a variable.

  The Increment operator   ++

   int days = 20;
   ++days;         // changes days to 21
   
   int weeks = 23;
   ++weeks;        // changes weeks to 24
   ++weeks;        // changes weeks to 25
  

  The Decrement operator   --

  int hrs = 12;
  --hrs;         // changes hrs to 11
  

More Operators

Shorthand Assignment Operators

• The Assignment Operator ( = ) , gives a variable a value.

• +=

                int sum = 30;
                sum += 5;      now sum is 35
                Equivalent to  sum = sum + 5;
          

• -=

                int a = 5;
                a -= 2;         a is 3
                Equivalent to   a = a - 2;
          

• *=

                int x = 20;
                x *= 3;         x is now 60
                Equivalent to   x = x * 3;
          

• /=

                int z = 40;
                z /= 2;         z is now 20
                Equivalent to   z = z / 2;
          

Characters

• In Java, single characters are represented using the data type char.
• Character constants are written as symbols enclosed in single quotes:

  char ch1 = ‘X’;
  
  String street = "western";
  char firstChar = street.charAt(0);
  

Question

What is the difference between ..

'F'  AND  "F"  

Example 1

This program will assign a letter grade to an average

import java.util.Scanner;
class Grade {
	public static void main(String [] args){
		Scanner readScore = new Scanner(System.in):
		
		System.out.println("Enter you final average");
		int avg = readScore.nextInt();
		
		char grade;
		if(avg >= 90){
			grade = 'A';
		}
		else if(avg >= 80){
			grade = 'B';
		}
		else if(avg >= 70){
			grade = 'C';
		}
		else if(avg >= 60){
			grade = 'D';
		}
		else{
			grade = 'F';
		}
		
		System.out.println("Your grade is " + grade);
	}
}

Example 2

Same as example 1, except braces are removed.

import java.util.Scanner;
class Grade {
	public static void main(String [] args){
		Scanner readScore = new Scanner(System.in):
		
		System.out.println("Enter you final average");
		int avg = readScore.nextInt();
		
		char grade;
		if(avg >= 90)
			grade = 'A';
		else if(avg >= 80)
			grade = 'B';		
		else if(avg >= 70)
			grade = 'C';		
		else if(avg >= 60)
			grade = 'D';		
		else
			grade = 'F';
			
		System.out.println("Your grade is " + grade);
	}
}

Increment and Decrement Operators

• Unary operators that operate on variables of integer and floating point types.
• Easy way to add or subtract the number one from a variable.

  The Increment operator   ++

   int days = 20;
   ++days;         // changes days to 21
   
   int weeks = 23;
   ++weeks;        // changes weeks to 24
   ++weeks;        // changes weeks to 25
  

  The Decrement operator   --

  int hrs = 12;
  --hrs;         // changes hrs to 11