Or
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The reserved word or
is a binary operator.
Originally it stood for the logical disjunction of two boolean values only, but with the advent of operator overloading FPC allows everything else, too.
FPC also defines the or
operator accepting two ordinal types while performing calculations on their internal binary representation.
boolean operation
The expression A or B
represents the term [math]A \lor B[/math] as it is familiar from classical logic.
In electrical engineering writing [math]A + B[/math] is common, too, but in programming the meaning of the plus sign differs.
A
and B
are both boolean values.
The expression evaluates to either true
or false
.
It is only false
if both operands are false
:
A

B

A or B


false

false

false

false

true

true

true

false

true

true

true

true

bitwise operation
Since virtually all instruction sets have an or
instruction, it is no surprise some highlevel languages, especially those which aim to be suitable for hardware programming, provide some comparable functionality by itself.
In FPC the or
operator is defined appropriately.
Such an expression, also known as bitwise or, requires two ordinal operands.
The operation virtually performs a logical or taking each corresponding bit from both operands.
0101'1010 or 0000'1011 ―――――――――――― 0101'1011
For a typical usage example of or
confer program messageNo
's passing remark.
setting a bit
A common task is to set a specific bit.
To achieve this utilizing the or
operator elicits a smart implementation:
type
integerBitIndex = 0..bitSizeOf(integer)1;
{$push}
{$rangeChecks on} // instead of raising an exception, generate an RTE
function maskOn(const x: integer; const i: integerBitIndex): integer;
begin
maskOn := x or (%1 shl i);
end;
{$pop}
For example calling maskOn(%1000, 1)
will result in %1010
(%1000
equals decimal eight, and %1010
is ten).
comparative remarks
Note: The concept of sets is an integral part of Pascal. Whilst in other programming languages considering operations on the bit level is not unusual, Pascal provides you with a powerful notion that relieves you from the burden of thinking about bits. Take it under advisement whether your programming task can be modeled with sets even better.
When using sets a plus sign +
virtually does the same as the bitwise or
does (depending on the compiler's implementation of the data type set
).
This notation is, as already mentioned above, familiar from electrical engineering.
However, when handling sets the bitwise or
is called a union (of sets).
operators 


see also 
