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12.5 Comparision operator

The comparision operator can be overloaded to compare two different types or to compare two equal types that are not basic types. The result type of a comparision operator is always a boolean.

The comparision operators that can be overloaded are:

equal to
(=) To determine if two variables are equal.
less than
(<) To determine if one variable is less than another.
greater than
(>) To determine if one variable is greater than another.
greater than or equal to
(>=) To determine if one variable is greater than or equal to another.
less than or equal to
(<=) To determine if one variable is greater than or equal to another.

There is no separate operator for unequal to (<>). To evaluate a statement that contains the unequal to operator, the compiler uses the equal to operator (=), and negates the result.

As an example, the following operator allows to compare two complex numbers:

operator = (z1, z2 : complex) b : boolean;

the above definition allows comparisions of the following form:

Var  
  C1,C2 : Complex;  
 
begin  
  If C1=C2 then  
    Writeln(’C1 and C2 are equal’);  
end;

The comparision operator definition needs 2 parameters, with the types that the operator is meant to compare. Here also, the compiler doesn’t apply commutativity: if the two types are different, then it is necessary to define 2 comparision operators.

In the case of complex numbers, it is, for instance necessary to define 2 comparsions: one with the complex type first, and one with the real type first.

Given the definitions

operator = (z1 : complex;r : real) b : boolean;  
operator = (r : real; z1 : complex) b : boolean;

the following two comparisions are possible:

Var  
  R,S : Real;  
  C : Complex;  
 
begin  
  If (C=R) or (S=C) then  
   Writeln (’Ok’);  
end;

Note that the order of the real and complex type in the two comparisions is reversed.

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