Difference between revisions of "Pascal for Visual Basic users"
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Revision as of 15:00, 4 May 2018
This wiki page has been created to help you if you are switching from VisualBasic to FreePascal (the Pascal dialect and compiler used by Lazarus). It is not meant to convince you that Lazarus is better than VB (or vice versa). Although VB and Pascal have superficial similarities these two languages differ greatly. Since VB programmers beginning to use Pascal face immediate difficulties in trying to do things which in VB have nice syntax that Pascal does not provide, we focus here on those differences.
Comparison and assignment of values
Values in Pascal are compared using the = operator.
Values in Pascal are assigned using the := operator (there is a : added).
if a = 1 then b := 2;
Beginning and ending statements
Blocks of Pascal code have no distinguishing end statements for functions, procedures, loops, etc. Pascal needs only begin and end. You can easily add a comment manually following end statements to clarify which named routine corresponds to this particular end;.
for i := 0 to 100 do begin ... end; // for i do
In Pascal all variables have to be declared before use, and you have to make the declarations in a special var section which must precede the code that makes use of the variable. You cannot declare a new variable in the middle of code.
procedure VarDecl; var MyVar1: string; Myvar2, MyVar3: integer; //Multiple variables can be declared on the same line. begin WriteLn('Print something'); var MyVar4: string = 'else'; // This declaration is not allowed here - it must be moved three lines above to the var section WriteLn('Print something ', MyVar2); end; //VarDecl
Local variables do not get initialized upon declaration. They get random value (whatever value happen to be on the stack where they landed). Global variables are initialized to zero.
To initialize a local variable upon declaration the following syntax can be used in procedures and functions only.
var MyVar1: string = ''; MyVar2: integer = 0;
Types of variables
Besides the variables known in VB, FreePascal supports unsigned integers. You should pay special attention not to mix them with signed integers.
FreePascal has four kinds of loops.
This is quite similar to the For.. Next loop in VB, but more limited in syntax. The following restrictions apply in Pascal:
1. A for loop counter must be an integer (it cannot be a floating point type).
2. There is no Step property. To emulate the VB Step syntax you have to introduce a second counter variable yourself.
3. To decrement the value of the loop counter from a positive value, you use the keyword DOWNTO in place of TO.
procedure ForLoop; var i: integer; begin for i := 50 downto 0 do begin ... end; // for i end. // procedure ForLoop
4. In Lazarus it is possible to loop down when the index is a byte, which is not possible in VB6.
procedure ForLoop; var i: byte; begin for i := 200 downto 195 do begin ShowMessage (IntToStr (i)); end; end;
5. The value of the counter (i in the example above) cannot be changed programmatically inside the loop. Attempting to do so will throw a compiler error. Instead one could use Continue to skip execution of a loop:
procedure ForLoop; var i: integer; begin for i := 0 to 5 do begin if i=2 then Continue; //Following part of the loop will not be xecuted ShowMessage (IntToStr (i)); end; // for i end. // procedure ForLoop
6. In order to interrupt execution of a loop one could use Break.
Functions and procedures
Output of functions is supplied as
Note that Result:= does not act as Return. Function will not exit after assigning it.
One should use
In FPC a function or a procedure cannot see another one, which is written after it.
procedure Proc1; begin Proc2; //This is not possible end; procedure Proc2; begin Proc1; //This is possible end;
To enable visibility, disregarding writing order, functions and procedures have to be declared.
unit MyUnit; ... procedure Proc1; procedure Proc2; ... implementation procedure Proc1; begin Proc2; //This is possible now end; procedure Proc2; begin Proc1; //This is possible end;
Using default parameters in functions and procedures
In Pascal functions and procedures it is possible for parameters to automatically take default values. However, in the parameter list the parameters that might take default values must be declared as a contiguous list at the end of the parameter declaration part. Also, when a routine with default parameter(s) is subsequently called in your code, you can only specify non-default values beginning at the first parameter which would otherwise take a default value. You cannot specify a non-default value for a later parameter, if there are preceding parameters you have to specify values for which have not also been specified explicitly in the function or procedure call.
For example: If a procedure is declared (correctly) as follows with two default parameters,
procedure SampleProc(parm1: integer; parm2: string= 'something'; parm3:Boolean= True); begin end. //proc
it is not possible to call SampleProc as in VB like this:
This will produce a compiler error.
When calling a procedure (or function), if you specify a parameter which has a default value, you must also specify all parameters that have a default value that are in front of it. So, in the example above if you want the third parameter to be False, then you must also specify the value of second parameter.
These are valid calls:
SampleProc(5,'something',False); SampleProc(5,'nothing'); SampleProc(5,''); SampleProc(5);
In Pascal the minimum index of a dynamic array is 0, i.e. MyArray (1) will return the second elements of an array named MyArray.
Arrays are resized using
setlength (MyArray, NewDimension);
SetLength does not clear the array, i.e. it acts like ReDim Preserve, not like ReDim.
Translation of some commonly sought commands
DoEvents = TApplication.ProcessMessages
var app: TApplication; i: integer; begin app := TApplication.Create(nil); for i := 0 to 1000000 do begin app.ProcessMessages; // Handling process messages. OutputDebugString(PChar(IntToStr(i))); end end;
While the OutputDebugString printing the number to Event Log, you may do any events with Form without Freezing except when you close the form, you must wait the loop until finished.
VB commands, which are not present in fpc/Lazarus
Split - fpc and Lazarus have no native function to split a string into a string array. TStrings could be used instead of a string array, in case that the string separator is a character.
Join - fpc and Lazarus have no native function to join string arrays.
Lazarus IDE has no Instant panel.
In order to debug in Windows the dos prompt windows shall be enabled the following way: Lazarus IDE-> Main menu -> Project -> Project options... -> Compiler Options -> Config and Target -> uncheck Win32 gui application (-WG)
In Linux the console invokes itself.
Then the write and writeln commands can be used to print debug messages.
Compiler might give some more information if the following items are checked in the IDE main menu -> Project -> Project Option -> Compiler Option -> Debugging -> -Ci, -Cr, -Co, -St, -Cr, -Sa.
|>>||Shr||bit shift right|
|<<||Shl||bit shift left|
|New line||;||In Pascal lines end with the ; character not by a new line.|
|_||No symbol is required for wrapping a line and new line symbols are acceptable within it.|
|'||//||End of line Comment (only one line comment)|
|Exit For||Break||Interrupt execution of a loop|
|VB type||Pascal type||Size (bits)||Range||Remark|
|SByte||Shortint||8-bit||-128 .. 127|
|Byte||Byte||8-bit||0 .. 255|
|Short||SmallInt||16-bit||-32768 .. 32767|
|Char||Word||16-bit||0 .. 65535|
|UInteger||Cardinal||32-bit||0 .. 4294967295|
|Single||Single||32-bit||1.5E-45 .. 3.4E+38||Decimal delimiter in both languages, applied in source code is dot on the line.|
|Double||Double||64-bit||5.0E-324 .. 1.7E+308|
|Format(item, formatstring)||format(formatstring, items)|
|left(some_string,n)||leftstr(some_string, n) or copy (some_string,1,n)|
|Mid(some_string, startpos, numChars)||copy(some_string, startpos, numChars)|
There is a list of component prefixes for Delphi, which can be used in Lazarus. Using them might provide better code readability.
It is also usual to start the name of a type with the capital letter T.
Arrays of controls
Unlike in VB6 and like in VB.NET arrays of controls cannot be created statically (trough the IDE), and they can be created only dynamically.
Example, creating 6 buttons above each other:
procedure TForm1.FormCreate(Sender: TObject); var i: Integer; btnArray: array of TButton; begin SetLength(btnArray, 6); for i := 0 to 5 do begin btnArray[i] := TButton.Create(Self); btnArray[i].Left := 0; btnArray[i].Top := i * 30; btnArray[i].Parent := Self; end; end;