MyBasic - A Custom-BASIC language interpreter written in C++

• Download demo project - 30 Kb
• Download source - 8.43 Kb

Introduction

In my recent project I needed a smart and light application language , so I wrote MyBasic a custom-BASIC language interpreter, in about 3 days. I think MyBasic has a lot of shortcuts and it is enough for my requirements. From time to time I got code from CodeProject and so now I would like to share my code with you.

Details

This interpreter uses Zoly Farkas's CStringTokenizer as it's tokenizer but it does not work for 16-bit character sets, for example Chinese, and the interpreter needs to locate tokens in any place of the source code. Therefore I made some changes to CStringTokenizer as you can see below :

void Locate(int nLocation);
int GetLocation(){ return m_iPChar;}

void LoadCode(CString &strCode);

private:
inline int GetChar()
{ 
  if(m_iChar == m_sString.GetLength())
    return -1;
  else
    return (unsigned char)m_sString[m_iChar++];
}

CStringTokenizer::CStringTokenizer()
{
  m_bEolIsSignificant = FALSE;
  m_bSlSlComments = TRUE;
  m_bSlStComments = TRUE;
  m_bPascalComments = TRUE;

  Locate(0);

  // Reset syntax
  ResetSyntax();
  // Set the word Chars
  WordChars('a','z');
  WordChars('A','Z');
  WordChars(128,255);
  QuoteChar('"');
  QuoteChar('\'');
  WhiteSpaceChars(0,' ');
  ParseNumbers();
}

void CStringTokenizer::LoadCode(CString &strCode)
{
  m_sString = strCode;
  m_sString += -1;
}

void CStringTokenizer::Locate(int nLocation)
{
  m_bPushedBack = FALSE;
  m_iLineNo = 1;  // the first line
  m_iChar = nLocation;
  m_iPChar= nLocation;
  m_peekc = ' ';
}

In the main loop the interpreter gets the keywords one by one and calls the function accordingly, that's all.

do{
nToken = m_BasicLex.NextToken();
switch(nToken)
{
case TT_INTEGER://LABLE
exec_lable();
break;
case TT_WORD://ASSIGNMENT
m_BasicLex.PushBack();
exec_assignment();
break;
case TT_KW_PRINT://PRINT
exec_print();
break;
case TT_KW_INPUT:
m_BasicLex.PushBack();
exec_default();
break;
case TT_KW_IF://IF
exec_if();
break;
case TT_KW_ENDIF://ENDIF
exec_endif();
break;
case TT_KW_FOR://FOR
exec_for();
break;
case TT_KW_NEXT://NEXT
exec_next();
break;
case TT_KW_GOSUB:
exec_gosub();//gosub
break;
case TT_KW_RETURN://return
exec_return();
break;
case TT_KW_GOTO://GOTO
exec_goto();
break;
case TT_KW_END://END 
m_BasicLex.PushBack();
exec_end();
case TT_EOF:
goto ex;
default://
Error(0,"not except begin of line !");
};
}while(nToken!=TT_EOF);
ex:;

Custom-BASIC keywords

The keywords Custom-BASIC language interpreter supports.

PRINT
IF
THEN
ENDIF
FOR
NEXT
TO
GOTO
GOSUB
RETURN
END

Sample BASIC program

PRINT "Hello world!\r\nHello to all of you!"

FOR X=1 To 2 
    FOR Y=1 To 2 
      PRINT "X=",X,"Y=",Y 
    NEXT 
NEXT


A = 12.5*2+(100+100)-2^2+4/6 
PRINT A

IF A >= 0 THEN 
GOTO 100 
ENDIF

PRINT "..."

100 PRINT "GOTO 100" 
200 PRINT "GOTO 200..."

GOSUB 300

END

300 PRINT "GOSUB 300" 
RETURN

How to use the code

Include header file in your project and use like this :

#include "Basic.h"
public:
  CBasic m_Basic;
//
// Load source code and parse it .
//

CString strCode(pBuffer);
m_Basic.Init();
m_Basic.Parse(strCode);

Conclusion

All comments and feedback and criticism are welcome. I hope someone can benefit from my interpreter.