Pascal basic syntax (I)

preface

Sometimes I'm bored. I look at pascal, but I forget it after reading it So record it
pascal heard that it would be disabled by NOIP, but I didn't verify it Whatever, he was just curious

environment

Using the free compiler fpc[ https://www.freepascal.org/]

The editor can just find one you like, and the IDE can choose lazarus[ https://www.lazarus-ide.org/ ], cross platform

pascal version has been changed and supports many dialects. Based on the current best practice, ${mode objfpc} {$H +} {$J -} {$I +} {$R +} is used in all codes
{$H +} enable AnsiString

{$J-} const constant cannot be modified

{$I+} i/o error detection

{$R +} open out of bounds check
Note: {$mode objfpc} can also be changed to {$mode delphi}
This compilation instruction needs to be written before uses
pascal is not case sensitive. As a result of custom, it adopts a coding specification similar to C#

notes

There are many options for pascal annotations. The first two support multiple lines, and the last line is a single line annotation. pascal annotations allow nesting, but tp and delphi do not support it. In order to be portable, it is recommended not to nest annotations

(* some comments *)
{ some comments }
// some comments

Keywords (reserved words)

Language built-in identifier with special meaning. The keyword list is as follows

absolute  and  array  asm  begin  case  const  constructor  destructor  div  do  downto  else  end  file  for  function  goto  if  implementation  in  inherited  inline  interface  label  mod  nil  not  object  of  operator  or  packed  procedure  program  record  reintroduce  repeat  self  set  shl  shr  string  then  to  type  unit  until  uses  var  while  with  xor  as  class  dispinterface  except  exports  finalization  finally  initialization  inline  is  library  on  out  packed  property  raise  resourcestring  threadvar  try  

Under normal circumstances, write cannot define the same identifier as the keyword Sometimes it must be (for ex amp le, calling the DLL developed by c language) can be preceded by the & symbol

var    
  &var : integer;    
   
begin    
  &var:=1;    
  Writeln(&var);    
end.

identifier

Identifiers can use letters, numbers, underscores (), Cannot start with a number, length 1-127

Overview of data types

The following categories refer to freepascal's documentation

Basic type

Integer: byte,shortint,smallint,word,integer,longint,int64
The default value is decimal. If hexadecimal uses $prefix, octal uses & prefix and binary uses% prefix TP(Turbo Pascal) and delphi do not support octal and binary prefix forms

Floating point type: real,single,double,extended,comp,currency
boolean: boolean

String type

String: shortstring, AnsiString, widering, Unicode string, PChar
The string uses single quotation marks and does not support escape in the form of \ r\n\t similar to C language Instead, use a method similar to the following
The LINEENDING constant can be used to break lines across platforms

var a:string;
begin
a := 'contain tab '#9' example ';
a:='contain new line '#13#10' example';
a:='contain new line '+LINEENDING+' example';
end.

structure type

Subbound, enumeration, array, collection, class, file
See the following for specific use

Pointer

var i:integer;p: ^integer; // definition

p:=@i;i=p^; // Take address

It seems that the pointer operation of c language is not as smooth as that of c language. The use of pointers should be discouraged

Basic structure

program {Program name}
uses {Comma separated Unit}
const {Constant definition}
var {Global variable definition}

//Function definition
function foo();boolean;
{ Local variable definition }
begin
...
end;

//process definition
procedure bar();
{ Local variable definition }
begin
...
end;

begin { Main program start}
...
end. { End of main program }

See the following for the structure of this executable program and the creation of dll
The following is the structure of Unit, which is similar

unit {Unit name};
interface

//Function declaration
function foo();boolean;
//Process declaration
procedure bar();

implementation
//Function implementation
function foo();boolean
{ Local variable definition }
begin
end;


//Process implementation
procedure bar();
{ Local variable definition }
begin
end;

end.

var a:integer; // Variable definition method

a:=10;// Assignment method

Functions and procedures

Function has a return value, procedure does not Everything else is the same

function Function name(Variable name 1:Variable type;Variable name 2:Variable type...):Return type;
var
    Variable name 3:Variable type;
    Variable name 4:Variable type;
begin
    Function definition

    result := Return value     
end;    

The return value can also use the function name: = return value. I like to use result: = return value. In addition, result can be used for recursive calls

function Process name(Variable name 1:Variable type;Variable name 2:Variable type...);
var
    Variable name 3:Variable type;
    Variable name 4:Variable type;
begin
    process definition
end;    

Cyclic structure

While do loop

while (condition) do S;
//for example
while i<10 do 
begin
i := i-1;
writeln(i);
end;

for loop

for < variable-name > := < initial_value > to [down to] < final_value > do 
   S;
//for example

for i:=1 to 10 do writeln(i);

for i:=10 downto 1 do writeln(i);

until loop

repeat S until condition;
// for example
repeat
    sum := sum+ i;
    i := i-1
until i=0;

Break and continue are used to jump out of the loop, which is the same as C language
Of course, there are goto statements, which are hardly used by individuals

Branching structure

if condition then S;
if condition then S1 else S2;

Note that there is no else if

case (expression) of
   L1 : S1;
   L2: S2;
   ...
   ...
   Ln: Sn;
end;

case (expression) of
   L1 : S1;
   L2,L3 : S2;
   ...
   ...
   Ln: Sn;
else
   Sm;
end;

Code example

//Only for grammar demonstration, no specific requirements
demo01.pas

Program demo01;
${mode objfpc } {$H+}{$J-}{$I+}{$R+}

Uses SysUtils,Common;

Const PI = 3.14;

Function FactTail(n,a:integer): longint;
Begin
  If n < 0 Then result := 0
  Else
    Begin
      If n<=2 Then result := a
      Else
        result := FactTail(n-1,n*a);
    End
End;

Function Sum(n:integer): integer;

Var 
  s: integer;
  i: integer;
Begin
  s := 0;
  For i:=1 To n Do
    s := s+i;
  result := s;
End;

Function Mean(n:integer): real;

Var 
  s: integer;
  i:integer;
Begin
  s := 0;
  i := n;
  Repeat
    s := s + n;
    i := i-1;
  Until i=0;
  result := s*1.0/n;
End;

Var a,b,c: integer;
Begin
  a := 10;
  b := 20;
  c := 10;

  Swap(a,b);

  writeln(FactTail(5,1), ', ',Sum(c),', ',Mean(c));

End.

common.pas

Unit common;
${mode objfpc } {$H+}{$J-}{$I+}{$R+}

Interface
Function RectangleArea(l,w :real ): real;
Function CircleArea(r :real ): real;
Function TriangleArea(a,b,c :real ): real;
Procedure Swap(Var a:integer;Var b:integer);

Implementation
Procedure Swap(Var a:integer;Var b:integer);

Var 
  temp: integer;
Begin
  temp := a;
  a := b;
  b := temp;
End;

Function RectangleArea(l,w :real ): real;
Begin
  result := l*w;
End;

Function CircleArea(r :real ): real;

Const PI =  3.14;
Begin
  result := PI*r*r;
End;

Function TriangleArea(a,b,c :real ): real;

Var s : real;
Begin
  s := (a+b+c)/2.0;
  result := sqrt(s*(s-a)*(s-b)*(s-c));
End;

End.