C Programming Yellow Book

Simple Data Processing
Manipulating Data
to perform will cause data to be lost from the program. It considers every operation in
terms of "widening and narrowing" values.
Widening and Narrowing
The general principle which C# uses is that if you are "narrowing" a value it will
always ask you to explicitly tell it that this is what you want to do. If you are widening
there is no problem.
To understand what we mean by these terms we could consider suitcases. If I am
packing for a trip I will take a case. If I decide to switch to a smaller case I will have to
take everything out of the large case and put it into the smaller one. But it might not
have room, so I have to leave behind one of my shirts. This is "narrowing".
However, if I change to a bigger case there is no problem. The bigger case will take
everything that was in the smaller case and have room for more.
In C# terms the "size" of a type is the range of values (the biggest and smallest) and the
precision (the number of decimal places). This means that if I write:
int i = 1 ;
float x = i;
This works fine because the floating point type can hold all the values supported by the
integer type. However:
float x = 1;
int i = x ;
- would cause the compiler to complain (even though at the moment the variable x only
holds an integer value). The compiler is concerned that you may be discarding
information by such an assignment and treats it as an error.
Note that this applies within floating point values as well, for example:
double d = 1.5;
float f = d ;
- would cause an error as well, since the compiler knows that a double is wider than a
float.
Casting
We can force C# to regard a value as being of a certain type by the use of casting. A
cast takes the form of an additional instruction to the compiler to force it to regard a
value in a particular way. You cast a value by putting the type you want to see there in
brackets before it. For example:
double d = 1.5;
float f = (float) d ;
In the above code the message to the compiler is "I don't care that this assignment
could cause the loss of information. I, as the writer of the program, will take the
responsibility of making sure that the program works correctly". You can regard
casting as the compiler's way of washing its hands of the problem. If a program fails
because data is lost it is not because the compiler did something silly.
As we saw above, each type of variable has a particular range of possible values, and
the range of floating point values is much greater than that for integers. This means that
if you do things like this:
int i ;
i = (int) 123456781234567890.999 ;
I do not think of this as a
failing in C#. It gives you
great flexibility, at the cost of
assuming you know what you
are doing....
- the cast is doomed to fail. The value which gets placed in i will be invalid. Nothing
in C# checks for mistakes like this. It is up to you when you write your program to
make sure that you never exceed the range of the data types you are using - the
program will not notice but the user certainly will!
C# Programming © Rob Miles 2015 30