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CHAP01.TXT
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1989-01-18
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Chapter 1
WHAT IS A COMPUTER PROGRAM?
THIS IS FOR THE COMPUTER NOVICE
______________________________________________________________
If you are a complete novice to computers, you will find the
information in this chapter useful. If you have some
experience in computer use, and especially programming, you
can completely ignore this chapter. It will deal with a few
of the most fundamental topics of computers and will have
nothing to do with the Modula-2 programming language.
WHAT IS A COMPUTER PROGRAM?
______________________________________________________________
A computer is nothing more than a very dumb machine that has
the ability to perform mathematical operations very rapidly
and very accurately, but it can do nothing without the aid of
a program written by a human being. Moreover, if the human
being writes a program that turns good data into garbage, the
computer will very obediently, and very rapidly, turn good
data into garbage. It is possible to write a large program
with one small error that will do just that. In some cases,
the error will be obvious, but if the error is subtle, the
answers may appear to be right, and the error will go
unnoticed. It is up to you, the human programmer, to write
a correct program to tell the computer what to do. You can
think of the computer as your very obedient slave ready to do
your every whim. It is up to you to tell your slave what you
want it to do.
A computer program is a recipe which the computer will use on
the input data to derive the desired output data. It is
similar to the recipe for baking a cake. The input data is
comparable to the ingredients, including the heat supplied by
the oven. The program is comparable to the recipe
instructions to mix, stir, wait, heat, cool, and all other
possible operations on the ingredients. The output of the
computer program can be compared to the final cake sitting on
the counter ready to be cut and served. A computer then is
composed of two parts, the data upon which the program
operates, and the program. The data and program are
inseparable as implied by the last sentence.
WHAT ARE CONSTANTS?
______________________________________________________________
Nearly any computer program requires some numbers that never
change throughout the program. They can be defined once and
used as often as needed during the operation of the program.
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Chapter 1 - What is a Computer Program?
To return to the recipe analogy, once you have defined how big
a tablespoon is, you can use the same tablespoon without
regard to what you are measuring with it. When writing a
computer program, you can define the value of PI = 3.141592,
and continue to use it wherever it makes sense knowing that
it is available, and correct.
WHAT ARE VARIABLES?
______________________________________________________________
In addition to constants, nearly any computer program uses
some numbers that change in value throughout the program.
They can be defined as variables, then changed to any values
that make sense to the proper operation of the program. An
example would be the number of eggs in the above recipe. If
a single layer of cake required 2 eggs, then a triple layer
cake would require 6 eggs. The number of eggs would therefore
be a variable.
HOW DO WE DEFINE CONSTANTS OR VARIABLES?
______________________________________________________________
All constants and variables have a name and a value. In the
last example, the name of the variable was "eggs", and the
value was either 2 or 6 depending on when we looked at the
value. In a computer program, the constants and variables
are given names in much the same manner, after which they can
store any value within the defined range. Any computer
language has a means by which constants and variables can be
first named, then assigned a value. The means of doing this
in Modula-2 will be given throughout the remainder of this
tutorial.
WHAT IS SO GOOD ABOUT MODULA-2?
______________________________________________________________
Some computer languages allow the programmer to define
constants and variables in a very haphazard manner and then
combine data in an even more haphazard manner. For example,
if you added the number of eggs, in the above recipe, to the
number of cups of flour, you would arrive at a valid
mathematical addition, but a totally meaningless number. Some
programming languages would allow you to do just such an
addition and obediently print out the meaningless answer.
Since Modula-2 requires you to set up your constants and
variables in a very precise manner, the possibility of such
a meaningless answer in minimized. A well written Modula-2
program has many cross checks to minimize the possibility of
a completely scrambled and meaningless output.
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Chapter 1 - What is a Computer Program?
Notice however, in the last statement, that a "well written"
Modula-2 program was under discussion. It is still up to the
programmer to define the data structure in such a way that the
program can prevent garbage generation. In the end, the
program will be no better than the analysis that went into the
program design.
If you are a novice programmer, do not be intimidated by any
of the above statements. Modula-2 is a well designed tool
that has been used successfully by many computer novices and
professionals. With these few warnings, you are ready to
begin.
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