Lecture 19: Defining new types

with typename the name we want to give to the type and decription any type of variable we have learned until now, including arrays, records and all the simple variable types. It can also be of the type pointer which we will learn in the next lesson.

Examples:

 Type float = real;
This is useful for people that are used to programming in C. After writing the line above, we can use variables of type float, just like in C.

 Type realarray = array[1..10] of real;

 Type myrecord =
   record
     name: string;
     length: real;
     width: real;
     height: real;
   end;

Note that this definition of a new type does not create a variable! It does not save space in memory and it does not assign a name to a variable. It is just a description of a type that we can use later in declaring a variable.
 

with varname the name of a new variable and the type of this variable is typename, as decribed before. After the declaration we can use the variable as if it were declared in the normal way.

Examples:

 Var f: float;
This looks already much more like C (in C it would be "float f")

 Var ra: realarray;
And in the code we can use this array:
 ra[1] := 2.68;
This is completely equivalent with
 Var ra: array[1..10] of real;
 ra[1] := 2.68;

The last example
 Type myrecord =
   record
     name: string;
     length: real;
     width: real;
     height: real;
   end;
 Var mydata: array[1..100] of myrecord;
 mydata[23].length := 3.1;

Type ra = array[1..6] of integer;
Var x: ra;
    y: array[1..7] of integer;

FUNCTION AreEqual(r: ra): boolean;
  (* Note that the definition can also be used for parameters *)
begin
  if r[1]=r[2] then
    AreEqual := TRUE;
  else
    AreEqual := FALSE;
end;

begin
  x[1] := 1;
  x[2] := 0;
  WriteLn(AreEqual(x));
  y[1] := 1;
  y[2] := 0;
   (* The following line of code is not allowed because the type of y and the type the function 'AreEqual' expects are different. Look at the size of the arrays. *)
  WriteLn(AreEqual(y));
end.

Type time =
  record
    hour, minute, second: integer;
  end;

PROCEDURE ShowTime(t: time);
  (* Will show the time in format h:m:s *)
begin
  WriteLn(t.hour, ':', t.minute, '.', t.second);
end;

Var atime: time;

begin
  atime.hour := 23;
  atime.minute := 16;
  atime.second := 9;
  ShowTime(atime);
end.

Type date =
  record
    day, month, year: integer;
  end;
Type time =
  record
    hour, minute, second: integer;
  end;
Type dateandtime =
  record
    dattime: time;
    datdate: date;
  end;
Var x: datendtime;

x.dattime.hour := 1;

x is a record containing two fields. One field is dattime which is a record of three fields. One of these fields is hour.