C Programming Yellow Book

Simple Data Processing
Manipulating Data
2.2 Manipulating Data
2.2.1 Variables and Data
2.2.2 Storing Numbers
When you are writing a
specification you should
worry about the precision to
which values are to be held.
Too much accuracy may slow
the machine down - too little
may result in the wrong
values being used.
In this section we are going to take a look at how we can write programs that
manipulate data, how values can be stored, retrieved and generally fiddled with. This
provides us with the ability to perform the data processing part of programs.
In the glazing program above we decided to hold the width and the height of the
windows that we are working on in variables that we described as double. Before we
can go much further in our programming career we need to consider just what this
means, and what other types of data we can store in programs that we write.
Programs operate on data. A programming language must give you a way of storing the
data you are processing, otherwise it is useless. What the data actually means is
something that you as a programmer decide (see the above digression on data).
A variable is a named location where you can store something. You can think of it as a
box of a particular size with a name painted on the box. You chose the name to reflect
what is going to be stored there (we used sensible names like woodLength in the
above program). You also need to choose the type of the variable (particular size and
shape of box) from the range of storage types which C# provides. The type of the
variable is part of the metadata about that variable.
Programs also contain literal values. A literal value is just a value in your program
which you use for some purpose. For each type of variable the C# language has a way
in which literal values of that type are expressed.
When considering numeric values there are two kinds of data:
• Nice chunky individual values, for example the number of sheep in a
field, teeth on a cog, apples in a basket. These are referred to as integers.
• Nasty real world type things, for example the current temperature, the
length of a piece of string, the speed of a car. These are referred to as
reals.
In the first case we can hold the value exactly; you always have an exact number of
these items, they are integral.
In the second case we can never hold what we are looking at exactly. Even if you
measure a piece of string to 100 decimal places it is still not going to give you its
exact length - you could always get the value more accurately. These are real. A
computer is digital, i.e. it operates entirely on patterns of bits which can be regarded
as numbers. Because we know that it works in terms of ons and offs it has problems
holding real values. To handle real values the computer actually stores them to a
limited accuracy, which we hope is adequate (and usually is).
This means that when we want to store something we have to tell the computer
whether it is an integer or a real. We also need to consider the range of possible
values that we need to hold so that we can choose the appropriate type to store the
data.
You tell C# about a variable you want to create by declaring it. The declaration also
identifies the type of the thing we want to store. Think of this as C# creating a box of
a particular size, specifically designed to hold items of the given type. The box is
tagged with some metadata (there is that word again) so that the system knows what
can be put into it and how that box can be used.
C# Programming © Rob Miles 2015 22