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Introduction to Computer Programming

A few points I want to expose you to are the general kinds of things being done.


In the observation step, you think hard about what is happening versus what you expected to be happening. An example is a program that is drawing something, but the jumble of lines you see on the display does not look right. Or, your program has a button that doesnt do anything when you click on it. As a programmer, you approach these bugs like the legendary Sherlock Holmes approached his cases. Review the program youve written and ask questions like: How could these instructions produce what is happening?

Hopefully this gives you feel for how primitive computer instruction sets are. Im not going to go into the details of every instruction. If you want to go through it in detail on your own,the PDP-10 Machine Language is detailed here.

Why red, green, and blue? Why these colors?

Your computer successfully creates the illusion that it contains photographs, letters, songs, and movies. All it really contains is bits, lots of them, patterned in ways you cant see. Your computer was designed to store just bits – all the files and folders and different kinds of data are illusions created by computer programmers. (


Mark Guzdialhas a blog post,Why should we teach programming, that elaborates on many reasons learning computer programming is important.

Look closely at the binary representations of uppercase and lowercase letters. You can convert uppercase to lowercase by turning on a single bit. Or clearing the bit converts lowercase to uppercase.

Programs are constructed by connecting blocks, each representing some functionality available in the system. Figure1.1 shows a simple program that asks the person running it for their name and then says hello. Color is used for categories that the blocks belong to. Notice that theaskblock and the correspondinganswerblock are the same shade of blue. The shapes of the blocks determine where they can be placed to form an acceptable program. The rounded greenjoinblock fits into a rounded hole in the violetsayblock.

One of Scratchs strengths is the ease with which students can construct games and animated simulations and stories. Another strength is theScratch websiteitself which provides access to many tutorials and a community of users with programs theyve written. To learn more about Scratch, visit theScratch Wiki.

Still confusing?Dont worry, we will get into the details of Logo operators inlesson 8.

Table1.1 is an example of DEC PDP-10 assembler language, a function that returns the largest integer in a group of them, named NUMARY. The group contains NUMNUM members.

But there is a problem with assembler language – it is unique for every computer architecture. Although most deskside and notebook computers these days use the Intel architecture, this is only recently the case. And… a variety of computer architectures are commonly used in game systems, smart phones, tablets, automobiles, appliances, etc…

Uppercase A = decimal 65 = binary 01000001 = 01 000 001 = octal 101 Uppercase Z = decimal 90 = binary 01011010 = 01 011 010 = octal 132 The digit 1 = decimal 49 = binary 00110001 = 00 110 001 = octal 061

Well, with only two symbols, we would write the same sequential numbers as above: 0, 1, 10, 11, 100, 101, 110, 111, 1000, 1001, 1010, 1011. The decimal number 1234 in binary is .

Debugging is a process. The good news for us is that mistakes in introductory level programs are not that hard to diagnose and fix. Its basically narrowing in on the instruction, or two, that are not doing what you intended. Steps you take are like solvingSudokuorMastermindpuzzles.

Heres a little trivia, from days long past when computers were so slow (compared with today).

So, all a computer has in it is bits. Youve seen how they are used to represent stuff, pixels, numbers and characters. Ive mentioned that computers perform operations on the bits, like move them around, add pairs of them together, etc… One final obvious question is: how are instructions that a computer performs represented?

Spanish Translation(provided by theWindowsHelper Team)

Since even reasonable numbers that we use all the time make for very long binary numbers, the bits are grouped in 3s and 4s which are simple to convert into numbers in the octal and hexadecimal number systems. For octal, we group three bits together. Take the binary equivilent of decimal 1234, , and put spaces in between each group of three bits – starting at the right and going left.

Table 1.2 compares the decimal, binary, octal, and hexadecimal number systems.

In this introduction, you will work with two computer programming languages: Logo and Java. Logo comes from Bolt, Beranek & Newman (BBN) and Massachusetts Institute of Technology (MIT).Seymour Papert, a scientist at MITs Artificial Intelligence Laboratory, helpedWally Feurzeigat BBN design Logo in the 1970s. More research of its use in educational settings exists than for any other programming language.

A microprocessor, which is the heart of a computer, is very primitive but very fast. It takes groups of bits and moves around their contents, adds pairs of groups of bits together, subtracts one group of bits from another, compares a pair of groups, etc… – that sort of stuff.

transfering control to an instruction thats not in the standard sequential order – down the page.

Ok… Ive exposed you to a variety of objects that you commonly see when you are using a computer, things that can be manipulated with instructions in a computer programming language. Now lets move on to the computers instructions, one more thing that is just a bunch of bits!

Blocks-based programming is a great way to get started. But, as the size of the programs you want to write grows or you need a feature not available in Scratch, its time to switch to text-based programming.

Table 1.4 is a small slice from the full ASCII character set, just enough to give you a flavor of its organization.

Again, do not worry if you do not understand exactly how this procedure works. It will be a while before you will be writing anything like this. But, I want you to see that the words that make up the programs instructions and the instructions themselves are similar to English sentences, e.g., the first line and a half in the procedure are similar to the sentences:

As an example, to display a thin vertical line, the color values of a column of pixels are set to the desired color of the line. If you want a thicker vertical line, you set the color values of the pixels of a group of consecutive columns to the desired color. Figure 1.5 shows a red line thats a single pixel wide and an orange line thats three pixels wide. The orange line is actually a very thin rectangle.

A deep understanding of programming, in particular the notions of successive decomposition as a mode of analysis and debugging of trial solutions, results in significant educational benefits in many domains of discourse, including those unrelated to computers and information technology per se. (

Well, if you instructed a computer in its native language (machine language), you would have to write instructions in the form of (yes, once again)binary numbers. This is very, VERY hard to do. Although the pioneers of computer science did this, no one does this these days.

Well… a pixels color is also specified as numbers, three of them, called RGB (Red, Green, Blue) values. Play with the following JavaScript program which lets you see what number values generate which colors. What color do you get if you set red to 170, green to 85, and blue to 255? Whats the RGB value for your favorite color?

When you group four bits together and use sixteen symbols (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, A, B, C, D, E, and F) as their abbreviations, you have a hexadecimal representation.

No other computer programming language has the depth of research as Logo, based on its use in educational settings. Its roots are in the development of interactive learning environments. Wally Feurzeig was researching the use of a timeshared computer to improve teaching mathematical concepts while at BBN (Bolt, Beranek, and Newman). The question he wanted to answer was whether kids would embrace the new technology and learn using it. With some success demonstrated using an existing programming language, Wally contracted Seymour Papert to help with Logos design. Seymour wrote the functional specification for Logo.Daniel Bobrowthen wrote the first Logo interpreter. Since these early days, hundreds of books and research papers have been written about its use in the classroom.Cynthia Solomon, who started MITs Logo Group with Dr.Papert, has put together a comprehensive website on .

When this exercise is directed by an experienced teacher or mentor it is excellent for demonstrating how careful you need to be, how detailed you need to be, when writing a computer program.A demonstration of this exercise is available on YouTube.

The lesson walks you through programming a very simple robot computer. And, I wrote a simulator for the Robot Computer in jLogo that you can play with HERE.

In this source code, thePRINTcommands input is the output of thePRODUCToperator.PRODUCTmultiplies whatever follows it by whatever follows that and outputs the result. So,PRODUCTneeds two inputs.RANDOMis an operator that outputs a number that is greater than or equal to zero (0) and less than the number following it. So,PRODUCTgets its second input from the output ofRANDOM.

decrementing the value in a register,


Objective: Students will write specific and sequential steps on how to make a peanut butter and jelly sandwich. Procedure: Students will write a very detailed and step-by-step paragraph on how to make a peanut butter and jelly sandwich for homework. The next day, the students will then input (read) their instructions to the computer (teacher). The teacher will then make the programs, being sure to do exactly what the students said…

One final reason, it can be lots of fun! A co-worker once said: I cant believe Im paid so well for something I love to do.

moving objects (numbers) into the computers registers – very fast temporary storage,

Finally, heres a snipet of advanced Logo source code, just to give you a feeling for what it looks like. This is a procedure definition for selecting the maximum number from a list of numbers.

Follow this link to an applet which lets you experiment with painting magnified pixels.

Ok… since you are reading this, accessing this web page on the net, you have Google and it takes a fraction of a second to answer this question.

No introduction to computer programming would be complete without at least mentioningdebugging. The term refers to the discovery and correction of mistakes in computer programs. The computer is doing what you instructed it to do, not what you meant it to do. If you enjoy puzzles, theres a good chance you will find the process of debugging an interesting challenge.

Basically, writingsoftware(computer programs) involves describingprocesses,procedures; it involves the authoring ofalgorithms. Computer programming involves developing lists of instructions – thesource coderepresentation of software The stuff that these instructions manipulate are different types of objects, e.g., numbers, words, images, sounds, etc… Creating a computer program can be like composing music, like designing a house, like creating lots of stuff. It has been argued that in its current state it is anart, not engineering.

It should come as no surprise that symbols that make up alphabets are just numbers, groups of bits, too. But how do we know which numbers are used to represent which symbols, or characters as Im going to call them from this point on?

Just to give you a feel for what programming is like in a high-level language, heres a program that greets us, pretending to know English.

A fair question you may have isWhy should I learn how to program a computer?

I like using the Logo language to teach introductory programming because it is very easy to learn. The faster you get to write interesting computer programs the more fun you will have. And… having fun is important! But do not let Logos simplicity fool you into thinking it is just atoyprogramming language. Logo is a derivative of the Lisp programming language, a very powerful language still used today to tackle some of the most advanced research being performed.Brian Harveyshows the power of Logo in hisComputer Science Logo Styleseries of books.Volume 3: Beyond Programmingcovers six college-level computer science topics with Logo.

Testing, even if it does not provide an answer, gets you additional information that can be used to repeat the process. You narrow your exploration until you find your mistake.

Ok, so numbers are simply groups of bits. What other objects will the computers instructions manipulate? How about the symbols that make up an alphabet?

In addition to commands, Logo hasoperatorsthat output some sort of result. Although its a bit contrived, here is a program that displays the product of a constant number (ten) and a random number in the range of zero through fourteen.

It has often been said that a person does not really understand something until he teaches it to someone else. Actually a person does not really understand something until after teaching it to a computer, i.e., express it as an algorithm. (

Computers have proven immensely effective as aids to clear thinking. Muddled and half-baked ideas have sometimes survived for centuries because luminaries have deluded themselves as much as their followers or because lesser lights, fearing ridicule, couldnt summon up the nerve to admit that they didnt know what the Master was talking about. A test as near foolproof as one could get of whether you understand something as well as you think is to express it as a computer program and then see if the program does what it is supposed to. Computers are not sycophants and wont make enthusiastic noises to ensure their promotion or camouflage what they dont know. What you get is what you said. (

Finnish translationthanks to Oskari Laine, Helsinki, Finland.

If you really want more details now,here is a side lesson from one of my favorite introductory computer science books: The Computer Continuum.

/ Noun: A series of actions conducted in a certain order.

Fromthe Wikipedia entry for AdaLovelace:

/ Noun: A series of actions or steps taken to achieve an end.

The origin of the term came from a bug (a moth) found in a relay of a computer in 1947, byAdmiral Grace Murray Hopper. She found why her program was not working.

But wait… these numbers are all positive (Whole Numbers). If we are going to allow for subtraction operations on numbers, which can result in negative numbers, we need Integers. Modern computers use one bit in each of the groups to represent the sign (positive or negative) when the groups are used to represent integers.Table1.3shows the range of numbers that can be represented with groups of 8, 16, 32, and 64 bits.

HERE is a jLogo program that converts an 8-bit byte into numbers and ASCII characters. Try it out!.

Almost all of the computer programming these days is done withhigh-levelprogramming languages. There are lots of them and some are quite old. COBOL, FORTRAN, and Lisp were devised in the 1950s!!! As you will see, high-level languages make it easier to describe the pieces of the program you are creating. They help by letting you concentrate on what you are trying to do rather than on how you represent it in a specific computer architecture. Theyabstractaway the specifics of the microprocessor in your computer. And, all high-level languages come with large sets of common stuff you need to do, called libraries.

Thats about as deep as I want to get into the representation of numbers in computers and the binary, octal and hexadecimal number systems. Yes, computers have division operators but I am not going to cover numbers that include fractional parts, i.e., therationalandirrationalnumbers due to the complexity of their implementations. If you want to read more, I googled and found what looks like a good place for you to read more. Start atAll About Circuits – Systems of numerationand read through it and continue on for a few more web pages in the series.The link to binary numberat the start of this section points to a Wikipedia page that also will give you additional depth including the history of the binary number system.

Remember what I said in the Introduction to this lesson?

Programming in a natural language, say the full scope of the English language, seems like a very difficult task. But, before moving on to languages we can write programs in today, I want to leave on a high note.Click here to read about how Stephen Wolfram sees programming in a natural language happening.

The octal representations of the binary patterns are certainly easier to read, write, and remember than the binary counterparts. An even more compact representation can be achieved by grouping the bits in chunks of four and converting these to hexNumerals.

Both Logo and Java have the same sort of stuff needed to write computer programs. Each has the ability to manipulate objects (for example, arithmetic functions for working with numbers). Each lets you compare objects and do a variety of things depending on the outcome of the comparison. Most importantly, they let you definenamedprocedures. Named procedures are lists of built-in instructions and other named procedures. The abstraction of naming stuff lets you write programs in a language you yourself define. This is the stuff that programming is really all about, as you will see.

As you continue to explore how computers work, youll hear more about numbers expressed in octal and hex; these are just more manageable representations of binary information -the digital world.

Before we see what a computer programming language looks like, lets use the English language to describe how to do something as a series of steps. A common exercise that really gets you thinking about what computer programming can be like is to describe a process you are familiar with.

Given a hypothesis, the next step is to figure out how to test it by predicting what should be happen if the hypothesis it correct. Continuing with the example of the misbehaving drawing program, lets say that you notice some source code that might not produce proper results if what it is given is not in the bounds expected. This code could produce strange results. This is a prediction.

Basically, computer instructions perform operations on groups of bits. A bit is either on or off, like a lightbulb. Figure1.3_a shows an open switch and a lightbulb that is off – just like a transistor in a computer represents a bit with the value: zero. Figure1.3_b shows the switch in the closed position and the lightbulb is on, again just like a transistor in a computer representing a bit with the value: one.

So, just as groups of bits represent numbers and symbols, they are used to form pixels.

A detailed explanation of any computers instruction set is beyond what can be presented here. I just wanted you to see how the symbolic information in assembler language programs needs to be converted to numbers (bits) before a computer can perform it.

Just to give you something to look at, just to compare, Table1.5 shows what the assembler language program inTable1.1could look like assuming that the machine instructions are loaded into memory at addresses 100 through 107. Also, the group of numbers starts at memory address 111 and the size of the group is in memory address 110.

There are many good explanations on the Internet. If you are interested, search the net or checkout the Wikipedia entry forcolor vision.

Clearing two bits will convert an ASCII digit to its numeric value. Setting the same bits converts a number in the range 0…9 into its ASCII character representation.

Lets walk through a couple of examples, entries in the table. Here are some characters, their decimal value and their binary value which is then transformed into an octal number.

Figure1.2 shows aplumbing diagram, a graphical representation of how all these procedures fit together.

One abstract layer above a computers native language is assembler language. In assembler language, everything is given human-friendly symbolic names. The programmer works with operations that the microprocessor knows how to do, with each given a symbolic name. Objects in the microprocessor and addresses of stuff in the computers memory can also be given meaningful names. This is actually a very big step over what a computer understands, but still tedious for writing a large program. Assembler language instructions still have a place for little snipits of software that need to interact directly with the microprocessor and/or those that are executed many, many, many times.

Its all about standards. In these lessons, we will use the American Standard Code for Information Interchange (ASCII) standard. It is so ubiquious that it even has its own web page,

Today, most people dont need to know how a computer works. Most people can simply turn on a computer or a mobile phone and point at some little graphical object on the display, click a button or swipe a finger or two, and the computer does something. An example would be to get weather information from the net and display it. How to interact with a computer program is all the average person needs to know.

Observation should lead you to the point where you can make a hypothesis about the bad behavior. For the drawing not coming out right, a hypothesis might be something like: If computing the metrics (orientation, length, …) of the first line is off, that might produce what Im seeing. For a button not working, a hypothesis could be: If the mouses location when it is clicked is computed wrong, the program will ignore the click. The hypotheses made are often based on intuition. This is because usually the person debugging either wrote the program or at least modified it.

So, if the most common groupings of bits in a computer are 8, 16, 32, and 64, what kinds of numbers can these groups represent?

So, as youve seen, higher-level programming languages provide similar functionality and in a form that is closer to the English language.

Java is a fairly recent programming language. It appeared in 1995 just as the Internet was starting to get lots of attention. Java was invented by James Gosling, working at Sun Microsystems. Its sort-of a medium-level language. One of the big advantages of learning Java is that there is a lot of software already written (see: Java Class Library) which will help you write programs with elaborate graphical user interfaces that communicate over the the Internet. You get to take advantage of software that thousands of programmers have already written. Java is used in a variety of applications, from mobile phones to massive Internet data manipulation. You get to work with window objects, Internet connection objects, database access objects and thousands of others. Java is the language used to writeAndroidapps.

Inside a microprocessor, at a very low level, everything is simply a bunch of switches, also known as bits – things that are either on or off! Time to expand on how this is done; first lets explore how groups of bits can be used to form numbers.

Computers are full of zillions of bits that are either on or off. The way we talk about the value of a bit in the electical engineering and computer science communities is first as a logical value (trueif on,falseif off) and secondly as abinary number(1 if the bit is on and 0 if its off). Most bits in a computer are manipulated in groups, so we humans need a way to describe groups of bits, things/objects a computer manipulates. Today, bits are most often grouped in quantities of 8, 16, 32, and 64.

Ok, we are almost at a point where I can show you machine language, the *native* language of a computer. But for you to understand it, Im going to have to explain how everything is represented in a computer.

But, since you are going to learn how to write computer programs, you need to know a little bit about how a computer works. Your job will be to instruct the computer to do things.

If the list of numbers to process is empty then output the maximum number processed. If the first number in the list is greater than the maximum number processed so far then …

So, how does thebinary number systemused inside computers work?

Take a moment to see if you understand the explanation of binary numbers.

So, a high-level programming language is *sort-of* like English, just one step closer to what the language a computer really understands looks like. Now lets move on to what a computers native language looks like when it is given a symbolic representation.

An important reason to consider learning is that the concepts underlying this will be valuable to you, regardless of whether or not you go on to make a career out of it. One thing that you will learn quickly is that a computer is very dumb, but obedient. It does exactly what you tell it to do, which is not necessarily what you wanted. Programming will help you learn the importance of clarity of expression.

/ Noun: An ordered set of steps to solve a problem.

Donald Knuth, in American Mathematical Monthly, 81

So, why do these lessons start with the Logo programming language?

, was an English mathematician and writer chiefly know

Now use the symbols 0, 1, 2, 3, 4, 5, 6, and 7 (eight symbols, so OCTAL) to replace each group.

A little less than fifteen years ago,Mitchel Resnickand friends atMITintroduced a programming environment calledScratch. It provides a new approach to teaching computer programming through a graphical user interface that eliminates the possibilty of making certain mistakes common in text-based programming.

So… just what do instructions a computer understands look like? And, what kinds of objects do the instructions manipulate? By the end of this lesson you will be able to answer these questions. But first lets try to write a program in the English language.

Finally, either the program is modified or a debugging feature in the programming environment is used to test the prediction. Modification of the program can be the addition of instructions that print stuff on the display. Most programming environments include debugging features, likeor setting breakpoints to suspend a program to examine its internal state. Either way, the programmer gathers more information about what the program is actually doing, which is, in the case of bugs, not what you expect.

Debugging a program can be done in steps that match theScientific Method.

The image on your computers display (actually all digital stuff) consists of a bunch of colored points called pixels. A pixel is an object. It has a color and a position (its coordinates) which consists of the row and column it is at. Figure1.4 shows an artists rendition, a magnification of a display with a circle drawn in yellow. The tiny black dots are the pixels and the big yellow dots are the pixels that have been colored.

Think about how you write down sequential numbers starting with zero: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, etc… Our decimal number system has ten symbols. In this sequential series, when we ran out of symbols, we combined them. You learned how to do this so long ago, in grade school, that today you just naturally think in terms of single digit numbers, then tens, hundreds, thousands, etc… The decimal number 1234 is one thousand, two hundreds, three tens, and four units.

So, the location of each pixel is obviously specified by a pair of numbers; what about the pixels color?

Rather than write my own version of this exercise, I searched the Internet for the words computer programming sandwich usingGoogle. One of the hits returned was At the link, Deb Sweeney (Tamaqua Area Middle School, Tamaqua, PA) described the problem as:

comparing the contents of a register to some value in the computers memory, and

This is one of the simplest programs that can be written in most high-level languages.PRINTis acommandin Logo When it is performed, it takes whatever follows it and displays it. The Hello world program is famous;checkout its description on Wikipedia by clicking here.

A group of eight bits has binary values 00000000 through 11111111, or expressed in decimal 0 through 255. A group of sixteen bits has binary values 0 through 1, or decimal 0 through 65535. Im not going to type in binary representations for groups of 32 and 64 bits. The range of decimal values for a group of 32 bits is 0 through 4,294,967,295. The range of decimal values for a group of 64 bits is 0 through 18,446,744,073,709,551,615 -or almost eighteen and a half quintillion.

Hal Abelson, Ken Ledeen, Harry Lewis, in Blown to Bits

There are only 10 different kinds of people in the world: those who know binary and those who dont. – Anonymous

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