Pascal for Visual Basic users
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.
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; begin WriteLn('Print something'); MyVar2: 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
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 three kind 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
3. 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.
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);