CISC 1115
Introduction to Programming Using Java
Lecture 1
Introduction and Overview

What is Programming?

• computer: a machine capable of carrying out a sequence of operations (typically arithmetic and logical)
  • arithmetic and logical because that was the motivation for calculations: census tables, ballistic trajectory tables for canon, nuclear reaction calculations, etc
    • As it turns out ALL computer-based calculations (including sound, image, video, etc) reduce to arithmetic and logical operations
  • The original use of the word computers was for humans … Computer (Occupation
• programming: the act of specifying a sequence of actions/commands to be carried out/executed by a computer
• program: a sequence of such commands
  • A program is also sometimes known as an app
• programmer: a person trained in the field of programming

An Overview of the Programming Process

We will see there are three basic types of operations required to carry out any task:

• Actions (imperatives)
• Decisions (conditionals)
• Repetition (iteration)

A decent (though not perfect) analogy of the programming process is creating and then carrying out a recipe

• In both cases one is attempting to accomplish something
• The process involves carrying out a sequence of steps
• Each step involves one or more of the above three categories
• The steps must be precisely specified and ordered

A Basic Overview of Recipes and Programs

Recipes Structure

Imagine buying a recipe box with blank recipe cards:

(Java) Program Structure

Java programs also have a basic common structure; every program will have a similar layout and common text:

Code
1.		public class <ClassName> {
2.			public static void main(String [] args) {
3. 				…
4.			}
5.			…
6.		}

A Trivial Recipe: Cereal & Milk

A Trivial Zero'th Program

We provide an equally simple first program, adding no code; all that needs to be added is the name:

public class EmptyApp {
	public static void main(String [] args) {
	}
}

A Somewhat Better First Program

Code

public class HelloWorld {
	public static void main(String [] args) {
		System.out.println("Hello world");
	}
}

Hello world

A More Substantive (but still simple) Recipe: Fried Eggs

Fried Eggs

Here is a recipe with a little more substance:

Ingredients

• 1 tablespoon extra-virgin olive oil
• 1 egg

Instructions

• Crack an egg into a small bowl and place it near the stove.
• Warm your skillet over medium-high heat until it's hot enough that a drop of water sizzles rapidly on contact.
• Reduce the heat to medium and add the olive oil to the pan.
• Gently tilt the pan around so the olive oil covers the base of the pan. The olive oil should be so warm that it shimmers on the pan (if not, give it a little more time to warm up).
• Carefully pour the egg into the skillet.
• Let the egg cook, gently tilting the pan occasionally to redistribute the oil, until the edges are crisp and golden and the yolk is cooked to your liking.
• Transfer the cooked egg to a plate.

Looking a bit more carefully at this we see the three basic categories of operations introduced above:

• (Simple) Actions, such as cracking an egg into a bowl, or tilting the pan
• Decisions, such as warming the skillet to the desired temperature, or cooking the egg until the edges are crisp
• Repetition, such as tilting the pan occasionally to redistribute the oil, or adding drops of water repeatedly until the pan is hot enough

P01.3 — A More Substantive Program

With regard to programs, we are going to proceed more slowly: we will introduce each of the above categories individually, in programs of increasing functionality.

• We will use grading as a unifying theme for the programs.

public class ExamAverager {
	public static void main(String [] args) {
		System.out.println((78+86)/2);
	}
}

82

P01.4 — Making the Output More Readable

We'd like to add some descriptive text to out output to let the user know what they're looking at:

public class ExamAverager {
	public static void main(String [] args) {
		System.out.println("Gerald Weiss received an 88 and a 92 for an average of " + (88 + 92) / 2); 
	}
}

Gerald Weiss received an 88 and a 92 for an average of 90

P01.5 — Adding a Decision (Conditional)

Let's add an example of the second category of operations: decision-making.

public class ExamAverager {
     public static void main(String [] args) {
          System.out.println("Gerald Weiss received an 88 and a 92 for an average of " + (88 + 92) / 2);
          if ((88 + 92) / 2 >= 60)
               System.out.println("Gerald Weiss passes");
          else
               System.out.println("Gerald Weiss fails");
     }
}

Gerald Weiss received an 88 and a 92 for an average of 90
Gerald Weiss passes

Introducing Variables and Types

In the above code, all the values are hardcoded they appear constantly wherever they are used. There are several issues with this approach, all of which lead to the introduction of the same concept.

Avoiding Redundant or Repetitious Values

• The above program repeated the student's name, as well as the values of the exams.
• Making a typo would result in us using one value in one place and a different one elsewhere
  • For example, misspelling 'Gerald' in one of the prints, or typing '80' in one place and '88' in another would produce inconsistent and thus incorrect output.
  • Similarly, we wrote the expression for the average twice, again inviting the possibility of making a mistake with one of them, producing inconsistent and incorrect results.

Allowing Different Names and Exam Values

The above program only provides information for a student named Gerald Weiss who received exam grades of 88 and 92.

• We typically want to be able to perform the above calculations on different exam grades as well as other-named students.
• The conditional is almost silly if it is only applied to the grade of 90; in that situation, it will never print 'fails'.

We might simply want to be able to quickly change the name of grades and run the program again.

• More importantly though, we might want to run this program once on many successive names and grades
• We can't do this if a specific name and set of grades is specified.

Making the Code More Readable

Looking at the original (88 + 92) / 2 program, it takes a bit of imagination to first realize that an average is being taken and then even more so that it is the average of two exam grades. We would like that to be clearer.

• Associated with this is the notion of avoiding putting 'magic numbers' into our code, i.e., numbers that seem to come from nowhere. We will have more to say about this later, when we discuss proper program design and style.

P01.6 — The Exam Averager Program Using Variables, Types, and Declarations

public class ExamAverager {
	public static void main(String [] args) {
		String name = "Gerald Weiss";
		int midterm = 88;
		int finalExam = 92;
		int average = (midterm + finalExam) / 2;
		System.out.println(name + " received an " + midterm + " and a " + finalExam + " for an average of " + average);
		if (average >= 60)
			System.out.println(name + " passes");
		else
			System.out.println(name + " fails");
	}
}

Gerald Weiss received an 88 and a 92 for an average of 90
Gerald Weiss passes

Making Use of the 'Variable' In Variable

• We can now use variables to apply our logic to different values; the question remains however as to populate the variables with those values.
• Even with variables and initialization, the programs — as currently written — have the values hardcoded into the code
  • While its true the values appear in the initialization of variables, it is still the case that to change the value requires editing and recompiling the program
• We need some way of providing different sets of values without having to constantly be changing our code
• We do this by having the user supply data from the keyboard

Reading Values (Input) From the Keyboard

• One would guess that if there is a System.out.println to print things out, there should also be a System.in.readln (or something like that) to read things in.
• It's close but not quite … a bit more work need to be done to read in values from the keyboard.
• For the moment, we will simply present the code as boilerplate again, but we will explain it in a bit

import java.util.Scanner;

public class <ClassName> {
	public static void main(String [] args) {
		Scanner scanner = new Scanner(System.in);
		…
		String name = scanner.next();		// next reads in text (until the next blank or 'return' key) typed at the keyboard
		int midterm = scanner.nextInt();		// nextInt reads in an integer typed at the keyboard
		…
	}
}
		

User Prompts

• next and nextInt simply wait for the user to enter a value (and press the 'return' (↵) key)
• Without anything further, the user would:
  • be presented with a blinking cursor, but no other inidicatio the system is waiting and thus …
  • … would not realize the system is expecting them to input anything
  • even if they realized the system is waiting, they would have no idea WHAT they are supposed to input (i.,e., a name?, an exam grade?, which exam?)
• To solve this problem, we introduce the notion of a user prompt, i.e., a message displayed to the screen telling (prompting) the user the both input is expected and furthermore, what sort of input.
  • For example, if we wish the user to type in a name, we would code:

System.out.print("Name? ");
String name = scanner.next();

Notice that no new Java was introduced to solve this problem, merely understanding the issue and applying a clever / common-sense approach. We'll call such an approach a technique; we will encounter many such techniques over the course of the semester.

P01.7 — The Exam Grader With Input From the Keyboard

We can now present a program that prompts the user and accepts data typed in from the keyboard.

import java.util.Scanner;

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

		System.out.print("Name? ");
		String name = scanner.next();
		System.out.print("Midterm? ");
		int midterm = scanner.nextInt();
		System.out.print("Final? ");
		int finalExam = scanner.nextInt();

		int average = (midterm + finalExam) / 2;
		System.out.println(name + " received an " + midterm + " and a " + finalExam + " for an average of " + average);
		if (average >= 60)
			System.out.println(name + " passes");
		else
			System.out.println(name + " fails");
	}
}

Weiss 
88
92

Name? Midterm? Final? Weiss received an 88 and a 92 for an average of 90
Weiss passes

Adding Repetition

• Now that we have variables that can hold different values, and are able to populate them using input from the keyboard (rather than hardcoded values), we can now introduce the facility to repeat our logic on different values and apply our grading logic to each set of data input

Repeating Sections of Code — the for Loop

• Probably the most powerful feature pf programming language is the ability to repeat a section of code an arbitrary number of times: once, twice, ten times, a million times … all with essentially the same (or no) effort on the part of the programmer.
• For the moment we will limit ourselves to a repetition mechanism known as a for loop:

  for (int i = 1; i <= 10; i = i + 1) {
  	…
  }
  		

  • The above causes the code between the { and } to be repeated 10 times.
  • We will again treat this as boilerplate to some degree; i.e., if we want to repeat something 4 times, we will write:.

    for (int i = 1; i <= 4; i = i + 1) {
    	…
    }	
    				

    • you may be able to make sense of the various parts of the loop header (i.e., int i = 1, etc); if not, don't worry, and again, just treat it as boilerplate with a simple modification to change the number of times to repeat the code.
  • Don't forget to:
    • include the closing brace (})
    • indent the lines of code between the { and }
      • this indicates the section of code to be repeated
      • this section of code is called the loop body

P01.8 — The Grading Program for Three Students

We can now present a program that repeats our grading logic for several students. A for loop will provide the repetition and we will use a Scanner to read the sets of data from the keyboard.

import java.util.Scanner;

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

		for (int i = 1; i <= 3; i = i + 1) {
			System.out.print("Name? ");
			String name = scanner.next();
			System.out.print("Midterm? ");
			int midterm = scanner.nextInt();
			System.out.print("Final? ");
			int finalExam = scanner.nextInt();

			int average = (midterm + finalExam) / 2;
			System.out.println(name + " received an " + midterm + " and a " + finalExam + " for an average of " + average);
			if (average >= 60)
				System.out.println(name + " passes");
			else
				System.out.println(name + " fails");
		}
	}
}

Weiss 
88
92
Arnow
75
95
Cox
80
100

Name? 
Midterm? 
Final? 
Weiss received an 88 and a 92 for an average of 90
Weiss passes
Name? 
Midterm? 
Final? 
Arnow received an 75 and a 95 for an average of 85
Arnow passes
Name? 
Midterm? 
Final? 
Cox received an 80 and a 100 for an average of 90
Cox passes

P01.9 — The Grading Program: Letting the User Decide How Many Students to Process

• The above for loop program finally illustrates the power of the computer, with a trivial modification we can have the program perform the grading computation for one million students.
• However, a similar problem arises to the one that motivated us to introduce variables: regardless of three, ten, or a million students, our program is again limited; this time to the number of students processed.
• A better approach would be to allow the user to tell us in advance how many students they want to process.
• We will do this by prompting the user to enter how many students and then using that value as the terminating value of our for loop.
• The logic of the loop body does not change whatsoever

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

		System.out.print("How many students do you wish to process? ");
		int howMany = scanner.nextInt();

		for (int i = 1; i <= howMany; i = i + 1) {
			System.out.print("Name? ");
			String name = scanner.next();
			System.out.print("Midterm? ");
			int midterm = scanner.nextInt();
			System.out.print("Final? ");
			int finalExam = scanner.nextInt();

			int average = (midterm + finalExam) / 2;
			System.out.println(name + " received an " + midterm + " and a " + finalExam + " for an average of " + average);
			if (average >= 60)
				System.out.println(name + " passes");
			else
				System.out.println(name + " fails");
		}
	}
}

3
Weiss 
88
92
Arnow
75
95
Cox
80
100

How many students do you wish to process? 
Name? 
Midterm? 
Final? 
Weiss received an 88 and a 92 for an average of 90
Weiss passes
Name? 
Midterm? 
Final? 
Arnow received an 75 and a 95 for an average of 85
Arnow passes
Name? 
Midterm? 
Final? 
Cox received an 80 and a 100 for an average of 90
Cox passes

The Next Level: Organization and Responsibility

• We've just illustrated the operations necessary for any task: basic actions, decisions, and repetition.
• While anything that can be written using a computer can be achieved using those three relatively simple categories, additional work needs to be done to be able to deal with tasks involving a large number of operations, many of them quite complex in and of themselves.

Let's revisit the recipe world for a moment and look at a more complex egg recipe:

A More Complex Recipe: Huevos Rancheros

Ingredients

• 1 tablespoon olive oil, divided
• 2 cups salsa (see recipe below)
• 16-ounce can Bush's Cocina Latina Frijoles Negros Machacados
• 4 round corn tortillas
• 1 tablespoon butter
• 4 large eggs
• 1/2 cup crumbled cotija (or feta) cheese, or shredded monterey jack

Instructions

• Heat a skillet over medium heat; add 1 tablespoon of olive oil. When olive oil ripples, add salsa and cook over medium heat, stirring occasionally for about 10 minutes.
• Meanwhile, begin warming beans in a separate pan.
• Transfer cooked salsa to bowl; cover to keep warm.
• Wipe out skillet; heat skillet over medium heat. Add tortillas in single layer to dry skillet (working in batches as needed), cook until warm and slightly toasted, turning as needed. Remove and cover to keep warm.
• Heat skillet over medium heat. Coat pan with a bit of butter. Fry eggs to preference; season to taste with salt and pepper. Add additional butter as needed.
• Serve fried eggs on warm tortillas, topped with cooked salsa, warmed black beans, and cheese. Add any optional chopped fresh cilantro, sliced/diced avocado or avocado, sliced jalapeno or fresh corn as desired.

Salsa
…
a salsa recipe would go here
…

• There are two things happening here:
  • There is a subtask you are asked to perform (the salsa)
  • There is something else here that you are asked to get from elsewhere (the Bush's beans) either because you don't have the time or knowledge to prepare them yourself, or because they are better than what you could prepare.
• Conceivably, if this is an extremely complicated recipe (for example some sort of fancy cake), the recipe might suggest you ask someone with experience in cake decoration to do the final frosting
• Basically, this all boils down to controlling the complexity of the recipe: break it into subtasks and use outside resources when necessary, possible, and/or desirable.

Methods and Classes

Similarly with software development; which is typically much more complicated than most recipes. To control such complexity we will organize our code into various levels.

Methods

Methods are logical units of code that represent subtasks.

• just as the salsa was a self-contained 'subrecipe', we will design our methods to be logically self-contained, in the sense of accomplishing a specific, well-defined subtak

We will cover methods shortly; deciding what should be turned into a method, and then designing the method proper takes practice.

Classes

Classes are just collections of resources — methods and data (variables).

• The code we have been writing is part of a class
• we also can use outside resources, i.e., code written outside of the code we are currently working on
  • We might use these resources (to refer back to our recipe analogy) either because
    • it's quicker to do so
    • we don't know how to code the necessary task
    • they might simply do a better job than we could
• these classes may be
  • supplied by Java
  • written by a third-party
  • written by us

Deciding what should be a class and how to design it — as with methods — requires practice (even more so); and this is deferred to the next course in the sequence.

• We will however be using classes to some extent in our work.
  • Two examples are the classes System and Scanner
    • As we said above, although we're treating them in this introductory lecture as boilerplate, more detailed explanations will be forthcoming.

But, remember, at the end of the day, all that is needed to accomplish any programmable task are actions, decisions, and repetition; methods and classes are 'merely' organizational elements.

One Last Thing to Think About

Turtles All the Way Down …

• Consider the effect of scanner.next:
  • it reads characters typed in from the keyboard
  • it stops when a blank is encountered
• The first of these is a repetition, the second a decision; yet we think of next as a 'simple', 'basic' action.
• This is similar to the recipe action 'stir the pot'; while this is indeed an action, it contains within it a repetition
• Similarly, "preheat oven to 350 degrees" contains a decision
• Other examples:
  • Adding numbers; e.g. 135 + 457; the machine performs the operation in a manner similar to what we do, i.e., summing each pair of digits and dealing with carry — a repetition of simple, single-digit additions to achieve full addition
  • println need to display each individual character on the screen; it does this by repeating the action of displaying a single character for each character in the text it is displaying
• The basic idea is that actions are themselves tasks composed of the operations of simpler actions, decisions, and repetition
• Where does this stop? … one could think when one reaches the hardware of the machine; but even then one could then think of the electrical circuits, the electrons flowing in the wires, etc
  • In practice, we stop when we have an action that suits us … println does what we need; it might be interesting to look into it to see how it works, but not necessary

… and All the Way Up

• The same idea works in the other direction: sequences of action, decision, and repetition operations combine to form new actions
  • In the recipe world, combining the recipes of salsa fried eggs, and mashed black beans forms the huevos rancheros recipe.
  • In our grading example, calculation of the average could be thought of as an 'averaging' action, while the decision logic could be thought of as forming a pass/fail test action
• In fact, that's exactly the idea behind a method, i.w., a sequence of operations (code) that are form a cohesive logical unit.
  • … And collections of methods and other stuff) are then grouped together to form classes.

Where Do We Go From Here?

• As mentioned, the three basic operations: actions, decisions, and repetition form the foundation of performing any task; whether done by a human or computer.
• This introductory lecture has illustrated each of those categories.
  • We've also introduced the notions of methods and classes
• There is much more work to do though, in order to master programming:
  • In addition to the basic boilerplate presented in the empty app, quite a bit of other code has been handed to you as boilerplate as well (e.g. System.out.print, Scanner scanner = new Scannner(System.in)); we need to explain and explore these in more detail
  • We need to investigate expressions in more detail; arithmetic as well as logical expressions — expressions that form the basis of conditions
  • the decisions in our sample programs above were trivial; real problems typically require conditions of far greater complexity; we need to get comfortable with these as well, learn how to manipulate and evaluate them
  • Similarly, although the for loop provides a powerful mechanism for repetition, it is nowhere sufficient to address many situations; loops will form a major part of our curriculum.
  • As the programs get more complex, we need to get a handle on how to test them to make sure they do what they are supposed to be doing
  • While looping allows us to repeatedly execute the same logic on different data, we need some way of supplying and working with that data; typing millions of values in from the keyboard is clearly not a efficient, productive, nor reasonable approach. We need to learn how to work with files on disk, as well as large amounts of data in memory.
• And all along, we need to learn how to think clearly, precisely and algorithmically, i.e., learn how to provide step-by-step solutions to our programming tasks.

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Labs for this Lecture