CISC 1115 Basics II

CISC 1115
Introduction to Programming Using Java
Conditionals

Reading from the Text

Chapter 3

Sections 3.7-3.9
You can also try reading 3.5-3.6 — it's the setup to the example in 3.7-3.9. The first section deals with an alternative to println we will be covering later
- alternatively you can simply substitute println for printf in sections 3.7-3.9 — that should help you get through it

Chapter 5

Sections 5.1-5.4

Conditionals-- the `if` statement

Decision-making element of a programming language

consists of a condition (true/false expression), a statement to be executed if the condition is true, and an optional statement to be executed if the condition is false

true statement only

if (condition) true statement

if (temperature > 98.6) System.out.println("You have a fever");

true and false statements

if (condition) 
	true statement
else
	false statement

if (x > y) 
	System.out.println(x);
else
	System.out.println(y);

Relational Operators

Operators forming conditions (true/false statements)
Used as the condition expression of a conditional (if statement) and for loop

Java's Relational Operators

== — Equality
!= — Inequality
<
>
<=
>=

Note: to compare Strings, use the equals method (of the String class), rather than ==. (There are other methods for greater, less, etc — we'll cover them later):

String s1, s2;
…
s1 = …
…
s2 = …
…
if (s1.equals(s2))	// NOT s1 == s2
	…
…

P03.1 — A Simple `if`

P03.1 SimpleIf

Write a program that accepts a name and midterm grade and issues a warning if the grade is below 70.

import java.util.*;

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

	System.out.print("Enter last name: ");
	String lastName = scanner.next();
	System.out.print("Enter first name: ");
	String firstName = scanner.next();
	System.out.print("Enter midterm: ");
	int midterm = scanner.nextInt();

	if (midterm < 70) 
		System.out.println(lastName + ", " + firstName + " better shape up!");
	}
}

Notes

Simplest type of conditional: if without else

Sample Runs

Here is a sample execution of the program.
User input is in bold. Your program should replicate the prompts and output:

Enter last name: Weiss
Enter first name: Gerald
Enter midterm: 67
Weiss, Gerald better shape up!

stdout (System.out)

Enter last name: Enter first name: Enter midterm: Weiss, Gerald better shape up!

Here is a sample execution of the program.
User input is in bold. Your program should replicate the prompts and output:

Enter last name: Arnow
Enter first name: David
Enter midterm: 75

stdout (System.out)

Enter last name: Enter first name: Enter midterm:

P03.2 — A Simple `if/else`

P03.2 SimpleIfElse

Write a program that accepts a name, midterm, and final exam grades and prints out a pass/fail grade.

import java.util.*;

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

		System.out.print("Enter last name: ");
		String lastName = scanner.next();
		System.out.print("Enter first name: ");
		String firstName = scanner.next();
		System.out.print("Enter midterm: ");
		int midterm = scanner.nextInt();
		System.out.print("Enter final: ");
		int finalExam = scanner.nextInt();

		double average = (midterm + finalExam) / 2.0;

		if (average >= 60) 
			System.out.println(lastName + ", " + firstName + " " + "passes the course");
		else
			System.out.println(lastName + ", " + firstName + " " + "fails the course");
	}
}

Notes

Simple if/else

Sample Runs

Here is a sample execution of the program.
User input is in bold. Your program should replicate the prompts and output:

Enter last name: Weiss
Enter first name: Gerald
Enter midterm: 55
Enter final: 61
Weiss, Gerald fails the course

Here is a sample execution of the program.
User input is in bold. Your program should replicate the prompts and output:

Enter last name: Arnow
Enter first name: David
Enter midterm: 65
Enter final: 56
Arnow, David passes the course

A Sidebar: Divisibility

Operators, operands, evaluation (result/value of an expression)

Remainder (Modulo) %

Write a program that prints out those numbers between 1 and 100 that are evenly divided by 7

public class Divisibility {
	public static void main(String [] args) {
	    for (int i = 1; i <= 100; i = i + 1) {
		   if (i % 7 == 0)
			  System.out.println(i + " is divided evenly by 7");
	    }
	}
}

stdout

7 is divided evenly by 7
14 is divided evenly by 7
21 is divided evenly by 7
28 is divided evenly by 7
35 is divided evenly by 7
42 is divided evenly by 7
49 is divided evenly by 7
56 is divided evenly by 7
63 is divided evenly by 7
70 is divided evenly by 7
77 is divided evenly by 7
84 is divided evenly by 7
91 is divided evenly by 7
98 is divided evenly by 7

Notes

What about even/odd?

More on Conditionals

Remember — loose rule of thumb — only statements that do not contain other statements terminate with a semicolon (;)
- So the statement(s) in the true and false portions (for example, the print or assignment) take ;'s because they do not contain any statements within themselves, but the if itself, and the else don't take a ; because the contain the true and false statement respectively.
- Effect of putting a semicolon (;) after the ')' of the condition
```
if (temperature > 98.6); System.out.println("You have a fever");
				
```
  - The above is not an error, as the language permits an empty statement, i.e. a statement containing nothing.
  - This statement — like all statemenets — is followed by a semicolon (;);
- Effect of putting semicolon after else
```
if (temperature > 98.6)
	System.out.println("You have a fever");
else; 
	System.out.println("You're fine … get to school");
				
```
  - Same thing here

Another Sidebar:The Empty Statement

It's legal
Useful in empty loops
```
for (…) 
	;
		
```
i.e., all the action is happening in the loop header (we'll have to wait for an example)

Now that We Mentioned Loops … More about the `for` Loop

The three components of the for loop header
```
for (int i = 1; i <= 3; i = i + 1)		// loop header	
	…					// loop body
```
- initialization: int i = 1
  - declares and initializes the loop-counting variable (i in this case)
- test for termination: i <= 3
  - specifies how far the counting variable should go
- progress: i = i + 1
  - causes the counting variable to increase by 1 (increment) each time

Variations and common errors

variable for termination limit

// User controls number of times through loop
System.out.println("How many times do you want to print\"I will not talk in class?\"");
int n = scanner.nextInt();
for (int i = 1; i <= n; i = i + 1)
	System.out.println("I will not talk in class!");

progress by a value other than 1

for (int i = 1; i <= 100; i = i + 2)		// odd numbers
	System.out.println(i);

initialization by a value other than 1

for (int i = 2; i <= 100; i = i + 2)		// even numbers
	System.out.println(i);

leaving out progress

for (int i = 1; i <= n; )
	System.out.println("Hello");

results in an infinite loop

leaving out terminating condition

for (int i = 1; ; i = i + 1)
	System.out.println("Hello");

what about

for (int i = 1; i <= 10; i = i - 1)
	System.out.println("Hello");

???

Effect of putting a semicolon (;) after the ')' of the header

for (int i = 1; i <= 3; i = i + 1);
	System.out.println("Hello");

Conditionals Within Conditionals

We often need multiple or more sophisticated conditions in our decision-making.

One way to handle this is to have the true and/or false part of a conditional itself be a conditional
- these are called nested conditionals
Later, we will lean other ways of expressing multiple conditions without having to nest conditionals.

Nesting Only Within the `if` Part

1. if (<condition₁>)
2. 	if (<condition₂>>)
3. 		statement

the above code the <statement> on line 3 is executed when <condition₁> and <condition₂> are both true
Here's the general case:

The True-Stmt is executed only when all the conditions are true; i.e., the conditions act as a 'gatekeeper' for the True-Stmt

In order to drive, one must be a resident of New York, and be 18 or older. Write a conditional that prints "Can drive" to the screen under the proper conditions.

if (stateOfResidence.equals("NY"))
	if (age >= 18)
		System.out.println("Can drive);

Suppose we also want to print 'Can't drive'

Nesting Only Within the `else` Part

1. if (<condition₁>)
2.	…
3. else
4.	if (<condition₂>)
5. 		statement

the above code the <statement> on line 5 is executed when <condition₁> is not true and <condition₂> is true
Here's the general case:
- Notice this is somewhat different than the nested if's:
  - The nested if's 'led' to a single True-Stmt which is executed only if all conditions are true.
  - Here, the semantics are almost like an or, e.g., if (<condition₁> is true execute <True-Stmt₁>; otherwise (i.e, or else) if <condition₂> is true, execute <True-Stmt₂>; otherwise … if <condition_n> is true, execute <True-Stmt_n>; otherwise (i.e., the final else) execute <False-Stmt>.
Due to the 'try one, then the next one, then the next one …' nature of this conditional, it is called a cascaded conditional (see below).

Multiple if/else: Assigning letter grades

P03.3 LetterGrader

Write a program that prompts for a midterm and final, calculates the average and assigns a letter grade based upon the following table:

Average Grade
90-100 A
80-89 B
70-79 C
60-69 D
59 and below F

import java.util.*;

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

		System.out.print("Midterm: ");
		int midterm = scanner.nextInt();
		System.out.print("Final: ");
		int finalExam = scanner.nextInt();

		double average = (midterm + finalExam) / 2.0;

		String grade;
		if (average >= 90)
			grade = "A";
		else if (average >= 80)
			grade = "B";
		else if (average >= 70)
			grade = "C";
		else if (average >= 60)
			grade = "D";
		else
			grade = "F";

		System.out.println("average: " + average + " grade: " + grade);
	}
}

What about in the other order? i.e., starting with F
What about leaving out the elses?
What about having
```
else if (average < 60)
	grade = "F";
		
```

Nested, Cascaded, Waterfall Versions of if Statements

Nested

if (…)
	if (…)
		…
	else
		…
else
	if (…)
		…
	else
		…

Most general form of conditional
of course, some of the else's may be omitted
- What about
```
if (…) 
		if (…)
			…
   else
 	 	…
				
```

Cascaded

if (…)
	…
else if (…)
	…
else if (…)
	…
else
	…

A special case of the nested form
Testing conditions until one is true (skips the rest)
Usually the conditions are testing the same variable (or expression)

Waterfall

if (…) …
if (…) …
if (…) …

Each condition is tested — one after the other
often used in conjunction with a candidate variable

Maximum, Minimum of 2 Values; of 3 Values...

Illustrated using Nested and Waterfall

Program 3.3

Using a sequence of nested if/else's, find the maximum of three integers

Program 3.4

Using a waterfall approach, find the maximum of three integers

The `switch` Statement

There are times you are comparing the same variable against several values

Program

Write a program that reads in book data from the keyboard. The information for each book is:

Title (string)
Author (string)
Category (char)
Number of copies available

The legal values for category are:

F - Fiction
B - Biography
N - Non-fiction
A - Anthology
S - Short Stories
J - Juvenile

Print out the information for each book together with its expanded category

Notice the logic of expanding from the single character to the expanded string:

if (category == 'F')
	 expanded = "Fiction";
else if (category == 'B')
	 expanded = "Biography";
else if (category == 'N')
	 expanded = "Non-fiction";
else if (category == 'A')
	 expanded = "Anthology";
else if (category == 'S')
	 expanded = "Short Stories";
else if (category == 'J')
	 expanded = "Juvenile";
else 
	 expanded = "Unknown category";

It is often the case that we have such a form of conditional, i.e.,:

if (var == some value)
   ...
else if (var == a second valuevalue)
   ...
else if (var == a third value)
   ...
...
else
   ...

This construct may be replaced with a switch statement:

switch (var) {
case some value:
   ...
	 break;
	 
case a second value:
   ...
	 break;

case a third value:
   ...
	 break;
...
default:
   ...	 break;
}

How it works:

Upon entry to the switch, the value of var is compared against the various cases.
If the value of var matches one of the cases, the code for that case is executed, up to the break statement.
If none of the case values matches the value in var, the code in the default block is executed.

Using a switch, the book category code then becomes:

switch (category) {
case 'F':
	 expanded = "Fiction";
	 break;
case 'B':
	 expanded = "Biography";
	 break;
case 'N':
	 expanded = "Non-fiction";
	 break;
case 'A':
	 expanded = "Anthology";
	 break;
case 'S':
	 expanded = "Short Stories";
	 break;
case 'J':
	 expanded = "Juvenile";
	 break;
default: 
	 expanded = "Unknown category";
	 break;
}

Note: The expression of the switch statement is limited to certain types, int, char, and string. Expressions of type double are not legal for 'switch'.

CISC 1115
Introduction to Programming Using Java
Conditionals

Reading from the Text

Chapter 3

Chapter 5

Conditionals-- the `if` statement

Relational Operators

Java's Relational Operators

P03.1 — A Simple `if`

P03.2 — A Simple `if/else`

A Sidebar: Divisibility

More on Conditionals

Another Sidebar:The Empty Statement

Now that We Mentioned Loops … More about the `for` Loop

Conditionals Within Conditionals

Nesting Only Within the `if` Part

Nesting Only Within the `else` Part

Nested, Cascaded, Waterfall Versions of if Statements

Nested

Cascaded

Waterfall

Maximum, Minimum of 2 Values; of 3 Values...

The `switch` Statement

Program

Files used in Lecture 3

Lab for Lecture 3

CISC 1115 Introduction to Programming Using Java Conditionals

Reading from the Text

Chapter 3

Chapter 5

Conditionals-- the if statement

Relational Operators

Java's Relational Operators

P03.1 — A Simple if

P03.2 — A Simple if/else

A Sidebar: Divisibility

More on Conditionals

Another Sidebar:The Empty Statement

Now that We Mentioned Loops … More about the for Loop

Conditionals Within Conditionals

Nesting Only Within the if Part

Nesting Only Within the else Part

Nested, Cascaded, Waterfall Versions of if Statements

Nested

Cascaded

Waterfall

Maximum, Minimum of 2 Values; of 3 Values...

The switch Statement

Program

Files used in Lecture 3

Lab for Lecture 3

CISC 1115
Introduction to Programming Using Java
Conditionals

Conditionals-- the `if` statement

P03.1 — A Simple `if`

P03.2 — A Simple `if/else`

Now that We Mentioned Loops … More about the `for` Loop

Nesting Only Within the `if` Part

Nesting Only Within the `else` Part

The `switch` Statement