Constants !

Hello ! We meet again ! Get ready for this short one ! In real world, we meet many constants, such as pi = 3.1415926513... or e = 2.7182818284529... Many of these are impractical number so that it is quite hard for us to remember, at least for some of us. Pascal has a good facility to use the constants. Even it has predefined ones, like pi.

How to define our own constant ? Easy, just like a variable definition, we just give const above our constants AND start tayloring our needs.

const
  :
  myconst = 1234;
  :
  :
  (Like variable definition)

We just omit the constant type, Pascal will understand it. We can use it just like a normal variable. Example :

const
  myconst = 1234;
var
  i : word;
begin
  i:=40;
  writeln(i*myconst);
end.

Easy, no ? But heed my word, constants is unchangeable. If you try to change it, Pascal will say error. Wait a minute ! I heard that Pascal's constants can be changed somewhere in the program. You are absolutely right ! Now, the declaration becomes :

const
  myconst : mytype = thevalue;

example:

const
  abc : word = 1234;

You may not declare constants with values exceeding the type, like these :

const
  aaa : word = 100000;  { Illegal because word is integer ranging 0-65535 }
  bbb : byte = 123.44;  { Illegal because of the fractional part }
  ccc : string = 123;   { Illegal because incompatible type }

Clear ? How can we modify them inside our program ?

const
  myconst : word = 1234;
var
  i : word;
begin
  i:=40; myconst:=1000;
  writeln(i*myconst);
end.

Easy, just like a normal variable, doesn't it ? That's all for this chapter !

Where to go ?

By : Roby Joehanes, © December 1996