Writeing Procedures and Functions

Can some one please explain in a way that is understandable about how to write functions and procedures?  I need it for a college assignment and our teacher is not very good at his job.  If you can give some examples for me it would be a good help.  Its supposed to be for an electricity company to calculate customers bills.  I don't want it doing for me just enough info so that I can teach myself.  Thanks :0)
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dbruntonConnect With a Mentor Commented:
Different viewpoint on procedures and functions.

If you were outputting a song that had four verses and a chorus that was repeated between verses and repeated twice at end of song you could create procedures for each verse and for the chorus and assemble as follows.

{* each of these procedures is full of writeln('this is line') or similar *}

procedure verse1;

procedure verse2;

procedure verse3;

procedure verse4;

procedure chorus;


It is possilbe to pass information into the procedures - these are called parameters.

procedure verse5(no : integer);
  if no = 4 then
    writeln('Life is wonderful')
    writeln('Life is hell');

It is possible to get information out of procedures.  Note the var

procedure dble(var no : integer);
  no := no *2;

The above procedure doubles any number passed into it

number := 4;

Functions are designed to return a value.

function triple(no : integer) : integer;
  i : integer;
  i := no * 3:
  tripe := i;

The above function returns the triple of any number passed to it.

number := 4;
newnumber := triple(number);

These are basic procedures and functions.  More complicated variations exist but these are enough for a starter.

How you use and design them depends on what you are required to code.

procedures and functions use the same approach and have a little difference (explained later), basically it is a block or part of program which can be 'called' many times from different parts of 'main' program. the structure is:

procedure p1

procedure p2

{main program starts here}
x=0 {some code}
call procedure p1
y=4 {some code}
call procedure p2
x=y+3 {another main part's code}
call procedure p1
etc etc

(this is pseudocode, just for explanation ;-)

ok, here is a 'life example': imagine you have 3 friends and you want to invite them for party, your 'main program' will be
buy food & booze ;-)
invite Peter
invite Adam
invite George
begin party
end party

this means that you have to do the same actions 3 times... which actions ? - inviting ! in this example it would be:
find a phone number of X person
pick up phone
dial that number
ask for X
say 'hi'
tell him about party
return to main program ;-)

you just have replace X with different names 3 times, but actions the same, this why convenient to use procedures. Now a program example, you have to print Y values for 3 different X using formula Y = X * 2 + 1, where Xs are 1,4,6  so the program could be:

Procedure PrintY( X : Byte );
  writeln( X * 2 + 1 );

begin {main}
  PrintY( 1 );
  PrintY( 4 );
  PrintY( 6 );

this is very simple example and not very practical, but you have to understand how execution of program flows, you start in main, PrintY(1) is a call to procedure with 'parameter', so program 'jumps' inside PrintY code, executes using passed parameter, then returns back and continues to PrintY(4).
parameters maybe of any type... you can also pass some values calculated in procedure back to main program, the difference between proc. and funct. is that function always has a result, but proc. - not, so calling is different:
myprocedure(parameters);  {this is call to procedure}
x:=myfunction(parameters); {this is call to function}
honestly in Delphi you can call functions without asigning their result, as procedure, but this is another story....

got it ?

did you understand or not???
3_Armed_HumanAuthor Commented:
Thanks alot, a great help @:0)
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