A BASIC introduction to programming

Submitted By: AngelFish Date: August 11, 2011, 04:31:34 am Views: 1859
Summary: A language independent introduction to programming for beginning programmers.

Before we begin, I tried to make this as language independent as possible because I have no idea what language(s) you want to learn. Also, I don't claim to even be a particularly good programmer. This is just what I've figured out is a good basis for programming in certain areas.

A BASIC introduction to programming:

One of the fundamental principles of programming is that you are giving a series of instructions to a machine to be executed.1 In most languages, these instructions are executed serially; the order in which they are written is the order in which they are executed. As an example, let us say that you are getting bored of doing addition by hand. It just takes too long. If you were to write a program to automate this, you would write a series of steps that the computer can understand to produce the result. With addition, this might look something like:

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a = 5
b = 6
Answer = a + b

This is a program. What the program is doing is storing the value "5" to the variable "a" and the value "6" to the variable "b." After executing those pieces of code, the computer looks at the next line in the program and sees that it needs to perform the "+" operation on the values of the variables "a" and "b," then set the variable "Answer" equal to the result. As it happens, the "+" operation is exactly the same as the addition operation we wanted to automate, so the computer just adds the variables together and stores the result. We've now automated addition.

At this point, you may be asking "what is a variable?" The answer is actually quite simple, especially if you've ever taken a course on Algebra that had equations like y=x^2. Variables here are almost exactly the same. To put it succinctly, a variable is an area of memory that can hold any piece of data you place in it. The variable will [ideally] "remember" that data until you store something else to it.

Now that we've written our first program, the question arises "How does the computer know that I wanted it to add two numbers and not say, subtract them?" This too is an important question and the answer is surprisingly complex. However, I'll try to explain in the simplest way possible. The first thing is what we call computers are actually collections of different components that work together to collect and process information. A computer such as a laptop contains many individual pieces such as a Central Processing Unit (commonly called a CPU or processor) that performs most of the computations in a computer and controls the rest of the components, a screen to communicate information to the user, a screen controller translate the commands of the CPU into pixels on the screen, different types of memory (RAM, the many types of ROM, a Hard drive) to store data for the CPU to access, a "Bus" to transfer information between the CPU and memory, a keyboard and mouse to get input from the user, etc... All of these parts work together in a system called a computer. It needs to be reiterated that the CPU is the main component of the computer. All other parts of a computer in some way work to allow the CPU to perform computations for the user.

The second thing is that, somewhat surprisingly, computers can't actually understand most of what are termed computer languages. Computers really only understand one language,2 called machine code. Machine code is composed of a series of 1's and 0's.3 As one might expect, Machine code is difficult to read or write for humans. Here is the same program we wrote earlier, but in machine code for a Renesas SuperH 3 processor. It looks fun to program in, doesn't it?

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Because even the simplest things are very difficult to do in machine code, programmers invented something called Assembly Language (often called ASM or Assembly) to make programming easier. To create Assembly, each instruction the processor was capable of executing was given a short mnemonic to help programmers remember them. These mnemonics were written down as if they were the machine code and run through another program called an Assembler. The Assembler looked at the Assembly and for each mnemonic outputted the corresponding machine code.

The machine code from above in Assembly looks like this:

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mov #5,r0
mov #6,r1
add r1,r0

While Assembly is definitely an improvement over machine code, it's still very difficult to do a lot of "simple" things in it. For example, the Assembly necessary to print out the words "Hello World" on the screen involves a lot of advanced code and tricks that are difficult to program.

After getting frustrated with spending hours doing even the simplest things in Assembly, people started looking for other ways to write code. The first way developed was something called an "Compiler" by a women named Grace Hopper in the early 1950's. The Compiler was a lot like an Assembler. It took a file containing code as an input and produced code in another language as output. However, Hopper's Compiler didn't read Assembly. Instead, it read a much higher level language called Fortran. When I say "higher level," I mean that the programmer didn't have to worry about the individual instructions on the computer. Instead, if someone put in the code
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print *, "Hello World", the screen would display "Hello World". The concept of a Compiler revolutionized programming and a lot of extremely popular languages use Compilers to produce code for the computer. Examples include Axe, C, C++, Fortran, etc...

There were a few problems with compilers, though. For one thing, compilers took a long time to operate. You could make a change to one line of code among thousands, but it may take several minutes or hours of compilation before you can test your code. Secondly, the machine code produced wasn't portable between different types of computers. These problems were solved by what are probably the most common types of languages today, interpreted languages. These used a program called an interpreter written in Assembly or a compiled language to translate or "interpret" a language written in a universal language into machine code. To put it in simpler terms, the Interpreter translates code from a language recognized by all interpreters for that language into CPU specific machine code. To run interpreted code on a new system, all you had to have was an interpreter for the language the code was written in. Also, since Interpreted code was not [originally] compiled, you could test changes as soon as you made them. Example languages include TI- and Casio- BASIC, BBC BASIC, Python, Java, Groovy, Lua, etc...4

The relationships between the different types of languages are illustrated by the following diagram. Note that the compiler produces "Compiled Assembly." Almost all modern compilers produce Assembly as an intermediate output for portability and optimization reasons, not raw machine code. This "Compiled Assembly" is then run through through a system specific Assembler.

To answer the question that provoked this long discussion, the computer knows what to do with your code because there's another program that converts your program to the machine instructions that the computer knows how to execute. The details of how this all is done are numerous enough to fill several college courses and a library's worth of books.

To get back to a more relevant subject, the meat of programming isn't the programming paradigm or language syntax, it's algorithms. The syntax of any language is generally well documented by its designers. Similarly, the point of any programming paradigm is to make some set of tasks easier. These are merely aesthetic aspects of programming. However, underlying all non-trivial programs are algorithms. Indeed, as one may learn, the whole art of programming is solving problems. The logical procedures used to solve these problems are what we call algorithms. More simply, an algorithm is a series of steps that can be taken to solve a particular problem. Earlier, we wrote a program to perform addition for us. That program can easily be translated into an algorithm by observing the steps we took:

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Step 1: Load Value1 into variable "a"
Step 2: Load Value2 into variable "b"
Step 3: Add variable "a" to variable "b"
Step 4: Load answer from Step 3 into variable "Answer"

While this may seem obvious, other more complex pieces of code are most certainly easier to comphrehend when stated as an algorithm. Here is some perfectly valid Axe code:



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