Set
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Introduction
A Set encodes many values from an enumeration into an Ordinal type.
For example let's consider this enumeration:
TSpeed = (spVerySlow,spSlow,spAverage,spFast,spVeryFast);
And this set:
TPossibleSpeeds = set of TSpeed
Constant instances of TPossibleSpeeds can be defined using brackets to hold set elements:
const
RatherSlow = [spVerySlow,spSlow];
RatherFast = [spFast,spVeryFast];
RatherSlow and RatherFast are some Set of TSpeed.
Manipulating sets
You can assign a set's contents directly:
var
SomeSpeeds: TPossibleSpeeds = [spAverage];
begin
SomeSpeeds := [spSlow,spVerySlow];
end;
Two functions defined in the RTL System unit are used to manipulate set elements individually: Include(ASet,AValue) and Exclude(ASet,AValue).
var
SomeSpeeds: TPossibleSpeeds;
begin
SomeSpeeds := [];
Include(SomeSpeeds,spVerySlow);
Include(SomeSpeeds,spVeryFast);
end;
Sets cannot be directly manipulated if they are published. You usually have to make a local copy, change the local copy and then to call the setter.
procedure TSomething.DoSomething(Sender: TFarObject);
var
LocalCopy: TPossibleSpeeds;
begin
LocalCopy := Sender.PossibleSpeeds; // getter to local
Include(LocalCopy,spVerySlow);
Sender.PossibleSpeeds := LocalCopy; // local to setter.
end;
The reserved word in
is also used to test if a value is in a set. It's usually used in this fashion:
var
CanBeSlow: Boolean;
const
SomeSpeeds = [Low(TSpeed)..High(TSpeed)];
begin
CanBeSlow := (spVerySlow in SomeSpeeds) or (spSlow in SomeSpeeds);
end;
Empty square brackets represent an empty set. You can test if a set currently contains no values at all by comparing against an empty set:
var
SomeSpeeds: TPossibleSpeeds;
begin
if SomeSpeeds = [] then SomeSpeeds := [spAverage];
end;
Bitmasks
Sets can be used to create bitmasks as shown in the example.
(*
The set [FLAG_A, FLAG_C] will be stored like this:
TFlags : 0000'0101
│││
FLAG_C ──────────┘││
FLAG_B ───────────┘│
FLAG_A ────────────┘
*)
type
TFlag = (FLAG_A, FLAG_B, FLAG_C);
TFlags = set of TFlag;
var
Flags: TFlags;
[..]
Flags:= [FLAG_A, FLAG_C];
if FLAG_A in Flags then .. // check FLAG_A is set in flags variable
simple data types |
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complex data types |