Lesson 4: Functions
Now that you have learned all about variables, loops,
and if statements it is time to learn
the next thing in programming: Functions. Obviously, you
should have a good idea about
what a function is, as you have used ones like cout
before. However, this lesson will be
more in detail about not only functions that are already
made, but about making your own,
or maybe I will continue this later...
A good way to describe a function is to show its
prototype. That means, what it should return,
and what it should take as an argument. For example, the
prototype of getch() is... int
getch(void); The int means that it returns an integer,
the void means that it does not take an
argument. Now, you may know that getch returns a
character! However, that does not mean it
must return a character. The fact that the return type
is an integer makes no difference,
because the ASCII character set does not care what type
of number it is, as long as it is a
number...don't worry if you don't understand, it is not
too important right now.
What is important is that you understand prototypes.
Another prototype is... int kbhit(void);
It returns an integer, and it takes no value. Now that I
hope you understand this then you
will be able to use the help files much more easily, and
I can go into more about functions.
First, what functions are useful.
There are many useful functions, and often they are hard
to find. For Turbo C++ Lite some
useful functions include, but are no limited to:
cout<< iostream.h output
cin>> iostream.h input
int getch(void) conio.h get characters
void clrscr(void) conio.h clear screen
Okay, you might be thinking that this is nothing! Four
measly functions, and you are
right! If there were only a few functions then C/C++
would not be useful. However, a lot
of programs do not need all that many functions. Of
course, I suggest that you know ever
function that you can, or at least the name. For this
purpose, I will be posting an entire
listing of ever function that I can find out of either
help or books I have read. However,
for now, these are probably the most useful functions.
After all, if you can clear the
screen, get input and output, and get keypresses, which
are useful for stopping the program
from immediately going back to the IDE you can do quite
a bit! Believe me, there are a few
specialized, but very useful functions, the thing is,
you don't really need to use them all
the time! If you have a problem with a function that you
need, and I have not put up my
list yet, then email me at lallain@concentric.net, and I
will find you what you need!
Anyway, after that long spiel on not needing a lot of
functions, I am going to show you how
to make your own functions! Wow, I can do that? Of
course, otherwise C/C++ would not be
useful! So, prepare to learn how to make functions.
First let me give you an entire example program. Then we
will look at it, and learn how to
make our own programs.
#include <iostream.h>
#include <conio.h>
int mult(int x, int y);
void main()
{
int x, y;
cout<<"Input a number, and what number to multiply it
by";
cin>>x>>y;
cout<<endl<<mult(x, y);
getch();
}
int mult(int x, int y)
{
return x*y;
}
How is this useful...Well, in this program the function
is totally useless, and it can only
multiply integers! But this is just to show you how to
make functions, after you
understand the basics I hope you will be able to do
anything yourself.
What my example does: #includes...basic includes What is
int mult(int x, int y); ? That is
a prototype of the function, without it you would not be
able to use mult! What
is void main()? You should know that. What is cout<<endl<<mult(x,y);
Well, basically it
puts us down a line, and then it outputs what mult
returns. More on this later. What is
int mult(int x, int y)
{
return x*y;
}
Well, it is a function! It says that the return type is
an integer. When the keywork
return is used it says that the function mult has a
value of whatever x*y would be...as it
says return x*y. The two ints that it takes are x and y.
So it multiplies them and returns
that value. Then outputs mult(x, y); It is perfectly
legal. Perhaps it would help if you
think of it as saying that the function mult has a value
of whatever x*y is. This is not
true really, it just returns that value, and you can do
whatever you want with it, but I
hope it helps.
What can you do? You can make void functions that do
anything...
#include <iostream.h>
void function(void);
void main()
{
function();
}
void function(void)
{
cout<<"This is a useless and totally wasteful function";
}
What is does is declare that there is going to be a
function, by prototyping, and then at
the bottom the function is defined, and it only does one
thing...outputs "This is a useless
and totally wasteful function" However, what if you
wanted to do something that took 3
lines four hundred times in different places? Say,
#include <iostream.h>
void function(void);
void main()
{
LOTS OF CODE THAT NEEDS TO OUTPUT Line 1Line 2Line 3
}
void function(void)
{
cout<<"Line 1";
cout<<"Line 2";
cout<<"Line 3";
}
That, aside from the fact that it is a bad example, is
where you would use it. When you
need to call something a lot of times, but don't want to
cut and paste. Functions are very
useful, and I hope I explained them well enough for you
to understand.
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