I am trying to define a const in Delphi (Delphi 2005 that is) that is based on other constants, but Delphi complains about it not being a constant expression. This is what my code looks like:
program myProgram;
const
Xpoints = 20;
Ypoints = 30;
ArraySize = trunc(sqrt(Xpoints*Ypoints));
var
myArray : array[1..ArraySize] of integer;
I could do ArraySize = Xpoints*Ypoints but the sqrt causes problems. The idea is that I would like the array to be sized by the constants Xpoints and Ypoints. I could do something like this:
program myProgram;
const
sqrtXpoints = 4.472135956;
sqrtYpoints = 5.47722557506;
Xpoints = trunc(sqrtXpoints*sqrtXpoints);
Ypoints = trunc(sqrtYpoints*sqrtYpoints);
ArraySize = trunc(sqrtXpoints*sqrtYpoints);
var
myArray : array[1..ArraySize] of integer;
taking care to slightly overestimate the square root values for the trunc. Everything will update correctly if I change sqrtXpoints or sqrtYpoints, but this approach just seems so... stupid.
As a temporary fix I can evaluate the constant myself like this:
program myProgram;
const
Xpoints = 20;
Ypoints = 30;
ArraySize = 24;
var
myArray : array[1..ArraySize] of integer;
but I don't like this because ArraySize does not automatically update if I change Xpoints or Ypoints.
It seems like the compiler should know how to evaluate a constant defined as a mathematical function of another constant at compile time for things like the example above and for even simpler things like this:
const
pi = 4.0*arctan(1.0);
but I can't get it to accept it. Any suggestions? Thanks in advance for your help!
Delphi do not allow to use most of functions in definition of constants.
But you can easy solve it, just use dynamic arrays and you will be able to calculate all what you want from your code:
const
Xpoints = 20;
Ypoints = 30;
var
myArray : array of integer;
procedure TForm9.FormCreate(Sender: TObject);
begin
setlength(myArray, trunc(sqrt(Xpoints*Ypoints)));
end;
Related
I'm modifying some existing code which uses OpenPrinter to instead use OpenPrinter2. It contains one more parameter PPRINTER_OPTIONS which in Delphi is encapsulated as a TPrinterOptions, defined like so:
_PRINTER_OPTIONSW = record
cbSize: Cardinal;
dwFlags: DWORD;
end;
I'm having trouble understanding how to use this struct to encapsulate the flag PRINTER_OPTION_NO_CACHE, and I can't even find that constant in any of the existing VCL/RTL.
var
PD: TPrinterDefaults;
PO: TPrinterOptions;
begin
PO.dwFlags:= ???
if OpenPrinter2(ADevice, #FPrinterHandle, #PD, #PO) then begin
...
I'm not having much luck searching for Delphi implementation of either OpenPrinter2 or TPrinterOptions. How do I make PRINTER_OPTION_NO_CACHE work?
EDIT
Is this correct?
const
PRINTER_OPTION_NO_CACHE = 0;
PRINTER_OPTION_CACHE = 1;
PRINTER_OPTION_CLIENT_CHANGE = 2;
...
PO.dwFlags:= PRINTER_OPTION_NO_CACHE;
The MSDN docs say this:
typedef enum tagPRINTER_OPTION_FLAGS {
PRINTER_OPTION_NO_CACHE,
PRINTER_OPTION_CACHE,
PRINTER_OPTION_CLIENT_CHANGE
} PRINTER_OPTION_FLAGS;
A C enum, as declared here, is really just an int. The first enum value is 0, the next 1, and so on.
But the header file tells a different story, contradicting the documentation. The header file says:
typedef enum _PRINTER_OPTION_FLAGS
{
PRINTER_OPTION_NO_CACHE = 1 << 0,
PRINTER_OPTION_CACHE = 1 << 1,
PRINTER_OPTION_CLIENT_CHANGE = 1 << 2,
PRINTER_OPTION_NO_CLIENT_DATA = 1 << 3,
} PRINTER_OPTION_FLAGS;
So translate to Pascal like this.
const
PRINTER_OPTION_NO_CACHE = 1;
PRINTER_OPTION_CACHE = 2;
PRINTER_OPTION_CLIENT_CHANGE = 4;
PRINTER_OPTION_NO_CLIENT_DATA = 8;
Populate the record like this:
var
Options: TPrinterOptions;
....
Options.cbSize := SizeOf(Options);
Options.dwFlags := PRINTER_OPTION_NO_CACHE;
This code gives me error
ex.Cells [3, 2]. HorizontalAlignment: = xlright;
ex.Cells [row, 1]. NumberFormat: = 'dd / mm / yy, #';
how can I fix this?
thanks
Based on your other questions, you're using late binding (using a Variant) instead of early binding (using the VCL TExcelApplication and other components). This means that the specific types like xlRight are not defined, because you haven't used the type library import that provides them. You'll have to define them yourself:
var
...
NewAlign: OleVariant;
const
xlBottom = $FFFFEFF5;
xlLeft = $FFFFEFDD;
xlRight = $FFFFEFC8;
xlTop = $FFFFEFC0;
xlHAlignCenter = $FFFFEFF4;
xlHAlignCenterAcrossSelection = $00000007;
xlHAlignDistributed = $FFFFEFEB;
xlHAlignFill = $00000005;
xlHAlignGeneral = $00000001;
xlHAlignJustify = $FFFFEFDE;
xlHAlignLeft = $FFFFEFDD;
xlHAlignRight = $FFFFEFC8;
begin
// XLApp is the instance of Excel you set up from your previous
// questions
XLApp.Workbooks[1].Worksheets[1].Range['C1:C5'].NumberFormat := 'dd/mm/yy';
NewAlign := xlHAlignRight;
XLApp.Workbooks[1].Worksheets[1].Range['C1:C5'].HorizontalAlignment := NewAlign;
end;
Assigning dd/mm/yy # makes no sense, because it's either a date format dd/mm/yy or text #, but not both.
Given an enumerated type declaration in Delphi such as:
TMyType = (Item1, Item2, Item3);
is there any way to add a fourth item, say Item4, to the enumerate type at runtime so that
at some point during the application's execution I have:
TMyType = (Item1, Item2, Item3, Item4);
Or are types fixed in Delphi?
You can create a set.
The following example initializes the set with items item1 and item4.
After that item5 is added.
It shows whether item5 is in the set, before and after adding, so you'll get this output:
FALSE
TRUE
Example:
program Project1;
{$APPTYPE CONSOLE}
uses SysUtils;
type
TMyType = (Item1, Item2, Item3, Item4, Item5,Item6);
const
cItems1And4 : set of TMyType = [Item1,Item4];
var
MyItems:set of TMyType;
begin
MyItems := cItems1And4;
WriteLn(Item5 in MyItems);
Include(MyItems,Item5);
WriteLn(Item5 in MyItems);
ReadLn;
end.
...
I wanted to type the following as a comment to Andreases reply, but the comment system doesn't let me properly format stuff..
If you're not stuck with an ancient Delphi, this is probably a better idea:
TComputerType = record
const
Desktop = 0;
Server = 1;
Netbook = 2;
Tabled = 3;
end;
That makes sure you don't pollute your namespace, and you'll get to use it as if it's a scoped enum:
TComputerType.Netbook
I usually do it like that nowadays.. You can even create some handy helper-functions or properties in there, for example to convert from and to strings.
No, you 'can't' do this. It is against the way Delphi works. (Recall that Delphi checks your types already at compile time.)
If I were you, I'd not do
TComputerType = (ctDesktop, ctServer, ctLaptop, ctNetbook, ctTablet)
Instead, I'd do
TComputerType = integer;
const
COMPUTER_TYPE_DESKTOP = 0;
COMPUTER_TYPE_SERVER = 1;
COMPUTER_TYPE_LAPTOP = 2;
COMPUTER_TYPE_NETBOOK = 3;
COMPUTER_TYPE_TABLET = 4;
I am sure you get why.
Types are fixed at compile time in Delphi—it is, after all, a statically typed language.
You can, at compile time, define subranges of an enumeration:
type
TEnum = (item1, item2, item3, item4);
TSubRangeEnum = item1..item3;
Since I cannot yet comment, I will add one addendum to what Andreas suggested that might help if you have many such lists to maintain. Adding a "base" constant for each grouping might help keep them better organized within your code and help with debugging the constants later (assuming each group has a unique base, of course).
TComputerType = integer;
const
COMPUTER_TYPE_BASE = 100;
COMPUTER_TYPE_DESKTOP = COMPUTER_TYPE_BASE + 1;
COMPUTER_TYPE_SERVER = COMPUTER_TYPE_BASE + 2;
COMPUTER_TYPE_LAPTOP = COMPUTER_TYPE_BASE + 3;
COMPUTER_TYPE_NETBOOK = COMPUTER_TYPE_BASE + 4;
COMPUTER_TYPE_TABLET = COMPUTER_TYPE_BASE + 5;
I want to create light object data-package to pass between client and server applications.
It is a so simple task, that I can control with only 1 byte, so
each bit in a byte will have a different meaning,
Using only the bit
0 = False
1 = True
Itens I need now:
1 - Loaded from database
2 - Persisted
3 - Changed
4 - Marked to Delete
5 -
6 -
7 - Null Value
8 - Read Only
1) How do I use bit operators in Delphi to check each bit value?
2) How do I set the bit Values?
Solution
After all help, Ill use the next Set
TStateType = (
stLoaded = 0, // loaded from persistance
stNative = 2, // value loaded and converted to native type
stPersisted = 3, // saved
stChanged = 4, // object or member changed
stToDelete = 5, // marked to delete
stReadOnly = 6, // read only object, will not allow changes
stNull = 7 // value is null
);
TState = Set of TStateType;
And for stream -> persistance, this will be the record to be used:
TDataPackage = record
Data: TBytes;
TypeInfo: TMetaInfo;
State: Byte;
Instance: TBuffer;
end;
Thank you guys, for all the answers and comments.
I'd really use a set for this. However, I see you really want a byte. Use sets everywhere then typecast to a byte in the end.
This solution will require much less typing, has support for standard delphi operators and really carries no performance penalty as Barry Kelly has pointed out.
procedure Test;
type
TSetValues = (
TSetValue1 = 0,
TSetValue2 = 1,
TSetValue4 = 2,
TSetValue8 = 3,
TSetValue16 = 4,
TSetValue32 = 5,
TSetValue64 = 6,
TSetValue128 = 7
);
TMySet = set of TSetValues;
var
myValue: byte;
mySet: TMySet;
begin
mySet := [TSetValue2, TSetValue16, TSetValue128];
myValue := byte(mySet);
ShowMessage(IntToStr(myValue)); // <-- shows 146
end;
I would use a set for this:
type
TMyDatum = (mdLoaded, mdPersisted, mdChanged, mdMarkedToDelete, ...);
TMyData = set of TMyDatum;
var
Foo: TMyData;
begin
Foo := [mdLoaded, mdChanged];
if (mdPersisted in Foo) then ...
These are implemented as integers, so you can pass them easily. And I find the code much, much more readable than bitwise operators.
This page describes Delphi operators, including bitwise operators.
It sounds like you need to use the and operator. For example:
const
LOADED_FROM_DATABASE = 1;
PERSISTED = 2;
CHANGED = 4;
// etc...
//...
if (bitFlags and LOADED_FROM_DATABASE) <> 0 then
begin
// handle LOADED FROM DATABASE
end;
if (bitFlags and PERSISTED) <> 0 then
begin
// handle PERSISTED
end;
// etc...
In order to set the flags, you can use OR:
bitFlags := LOADED_FROM_DATABASE or PERSISTED or CHANGED;
I have an integer field in a ClientDataSet and I need to compare to some values, something like this:
I can use const
const
mvValue1 = 1;
mvValue2 = 2;
if ClientDataSet_Field.AsInteger = mvValue1 then
or enums
TMyValues = (mvValue1 = 1, mvValue2 = 2);
if ClientDataSet_Field.AsInteger = Integer(mvValue1) then
or class const
TMyValue = class
const
Value1 = 1;
Value2 = 2;
end;
if ClientDataSet_Field.AsInteger = TMyValues.Value1 then
I like the class const approach but it seems that is not the delphi way, So I want to know what do you think
Declaration:
type
TMyValues = class
type TMyEnum = (myValue1, myValue2, myValue3, myValue4);
const MyStrVals: array [TMyEnum] of string =
('One', 'Two', 'Three', 'Four');
const MyIntVals: array [TMyEnum] of integer =
(1, 2, 3, 4);
end;
Usage:
if ClientDataSet_Field.AsInteger = TMyValues.MyIntVals[myValue1] then
A cast would generally be my last choice.
I wouldn't say that class consts are not the Delphi way. It's just they have been introduced to Delphi quite recently, and a lot of books and articles you'll find on the internet were written before their introduction, and thus you won't see them widely used. Many Delphi developers (I'd say the majority) will have started using Delphi before they were made available, and thus they're not the first thing that one thinks about.
One thing to consider is backwards compatibility - class constants are relatively new to Delphi so if your code has to be sharable with previous versions than they are out.
I typically use enumerated types, with the difference from yours is that my first enumeration is usually an 'undefined' item to represent NULL or 0 in an int field.
TmyValues = (myvUndefined, myvDescription1, myvDescription2)
if ClientDataSet_Field.AsInteger = Ord(myvDescription1) then...
To use a little bit of Jim McKeeth's answer - if you need to display to the user a text viewable version, or if you need to convert their selected text into the enumerated type, then an array comes in handy in conjuction with the type:
const MYVALS: array [TmyValues ] of string = ('', 'Description1', 'Description2');
You can then have utility functions to set/get the enumerated type to/from a string:
Function MyValString(const pMyVal:TmyValues):string;
begin
result := MYVALS[Ord(pMyVal)];
end;
Function StringToMyVal(const pMyVal:String):TMyValues;
var i:Integer;
begin
result := myvUndefined;
for i := Low(MYVALS) to High(MYVALS) do
begin
if SameText(pMyVal, MYVALS[i]) then
begin
result := TMyValues(i);
break;
end;
end;
end;
Continuing on... you can have scatter routine to set a combo/list box:
Procedure SetList(const DestList:TStrings);
begin
DestList.Clear;
for i := Low(MYVALS) to High(MYVALS) do
begin
DestList.Insert(MYVALS[i]);
end;
end;
In code: SetList(Combo1.Items) or SetList(ListBox1.Items)..
Then if you are seeing the pattern here... useful utility functions surrounding your enumeration, then you add everything to it's own class and put this class into it's own unit named MyValueEnumeration or whaterver. You end up with all the code surrounding this enumeration in one place and keep adding the utility functions as you need them. If you keep the unit clean - don't mix in other unrelated functionality then it will stay very handy for all projects related to that enumeration.
You'll see more patterns as time goes and you use the same functionality over and over again and you'll build a better mousetrap again.
When using constants I recommend assigning the type when the data type is a numeric float.
Delphi and other languages will not always evaluate values correctly if the types do not match...
TMyValue = class
const
// will not compare correctly to float values.
Value1 = 1; // true constant can be used to supply any data type value
Value2 = 2; // but should only be compared to similar data type
// will not compare correctly to a single or double.
Value3 = 3.3; // default is extended in debugger
// will not compare correctly to a single or extended.
Value1d : double = Value1; // 1.0
Value2d : double = Value2; // 2.0
end;
Compared float values in if () and while () statements should be compared to values of the same data type, so it is best to define a temporary or global variable of the float type used for any comparison statements (=<>).
When compared to the same float data type this format is more reliable for comparison operators in any programming language, not just in Delphi, but in any programming language where the defined float types vary from variable to constant.
Once you assign a type, Delphi will not allow you to use the variable to feed another constant, so true constants are good to feed any related data type, but not for comparison in loops and if statements, unless they are assigned and compared to integer values.
***Note: Casting a value from one float type to another may alter the stored value from what you entered for comparison purposes, so verify with a unit test that loops when doing this.
It is unfortunate that Delphi doesn't allow an enumeration format like...
TController : Integer = (NoController = 0, ncpod = 1, nextwave = 2);
or enforce the type name for access to the enumeration values.
or allow a class constant to be used as a parameter default in a call like...
function getControllerName( Controller : TController = TController.NoController) : string;
However, a more guarded approach that provides both types of access would be to place the enumeration inside a class.
TController = class
//const
//NoController : Integer = 1;
//ncpod : Integer = 2;
//nextwave : Integer = 3;
type
Option = (NoController = 0, ncpod = 1, nextwave = 2);
public
Class function Name( Controller : Option = NoController) : string; static;
end;
implementation
class function TController.Name( Controller : Option = NoController) : string;
begin
Result := 'CNC';
if (Controller = Option.nextwave) then
Result := Result + ' Piranha'
else if (Controller = Option.ncpod) then
Result := Result + ' Shark';
Result := Result + ' Control Panel';
end;
This approach will effectively isolate the values, provide the static approach and allow access to the values using a for () loop.
The access to the values from a floating function would be like this...
using TControllerUnit;
function getName( Controller : TController.Option = TController.Option.NoController) : string;
implementation
function getName( Controller : TController.Option = TController.Option.NoController) : string;
begin
Result := 'CNC';
if (Controller = TController.Option.nextwave) then
Result := Result + ' Piranha'
else if (Controller = TController.Option.ncpod) then
Result := Result + ' Shark';
Result := Result + ' Control Panel';
end;
so many options! :-) i prefer enums and routinely use them as you describe. one of the parts i like is that i can use them with a "for" loop. i do use class constants as well but prefer enums (even private enums) depending on what i'm trying to achieve.
TMyType=class
private const // d2007 & later i think
iMaxItems=1; // d2007 & later i think
private type // d2007 & later i think
TMyValues = (mvValue1 = 1, mvValue2 = 2); // d2007 & later i think
private
public
end;
An option you haven't thought of is to use a lookup table in the database and then you can check against the string in the database.
eg.
Select value, Description from tbl_values inner join tbl_lookup_values where tbl_values.Value = tbl_lookup_values.value
if ClientDataSet_Field.AsString = 'ValueIwant' then