News:

Building a 3D Ray Tracer  By stevmjon

Main Menu

Mock up lexical scanner routine

Started by kevin, September 13, 2016, 09:54:10 PM

Previous topic - Next topic

kevin

  Here's a mock up lexical scanner routine that would sit at the bottom of the compiler/parser.  


PlayBASIC Code: [Select]
   ac   = 1
constant Token_UNKNOWN =ac(1)
constant Token_INTEGER =ac(1)
constant Token_TEXT =ac(1)
constant Token_OPERATOR_ADD =ac(1)
constant Token_OPERATOR_SUB =ac(1)
constant Token_OPERATOR_MULT =ac(1)
constant Token_OPERATOR_DIV =ac(1)
constant Token_ENDOFFILE =ac(1)


Type tParser
SourcePointer ; Current Position (character) within the source
CurrentToken ; a numeric value of this 'token' so we can just compare values and not strings
CurrentTokenText$ ; The 'string' of the this token.
EndTYPE


; Declare the global Parser structure
Dim Parser as tParser
Parser = new tParser


; declare our source code
global SourceCode$ = " stuff 10 + 20 - 30 "


print "SOURCE CODE:"+SourceCode$
print ""
print ""


For lp=0 to 100

; get the next token
ThisTOKEN=GetNextToken()

#print "TOKEN:"+STR$(ThisTOKEN)

select ThisTOKEN

case Token_INTEGER
print " Integer= "+ Parser.CurrentTokenText$

case Token_TEXT
print " Text= "+ Parser.CurrentTokenText$

case Token_OPERATOR_ADD,Token_OPERATOR_SUB, Token_OPERATOR_DIV, Token_OPERATOR_MULT
print "Operator= "+ Parser.CurrentTokenText$

endselect


if ThisTOKEN = Token_ENDOFFILE
exitfor
endif



next

sync
waitkey




//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------
//------------------------------>> GetNextToken <<-----------------------------
//-----------------------------------------------------------------------------
//-----------------------------------------------------------------------------

Function GetNextToken()

; Set Token to
Pos=Parser.SourcePointer


// check if
if (pos+1)>=Len(SourceCode$)
CurrentToken = Token_ENDOFFILE

print "DONE"
goto done
endif


ThisCHR = mid(SourceCode$,1+pos)

// is this a white space chr ?
if THISCHR = 32 or ThisCHR=9

// Skip WHITE SPACE

For lp = pos to Len(SourceCode$)
ThisCHR = mid(SourceCode$ , 1+lp)
if thisCHR<>32 and ThisCHR<>9
pos =lp
exitfor
endif
next
ThisCHR = mid(SourceCode$,1+pos)
Parser.SourcePointer=pos

endif



// ----------------------------------------------
// check if the next 'hard' character is a number
// ----------------------------------------------
if ThisCHR=>asc("0") and ThisCHR<=asc("9")
// ----------------------------------------------

Number$=""

CurrentTOKEN = Token_INTEGER

For lp = pos to Len(SourceCode$)-1

ThisCHR = mid(SourceCode$,1+lp)
if ThisCHR=>asc("0") and ThisCHR<=asc("9")
Number$+=Chr$(thisCHR)
else

Parser.SourcePointer = lp
Parser.CurrentTokenText$= Number$

;print Number$
exitfor
endif
next
goto DONE
endif





// ----------------------------------------------
// check if the next 'hard' character is a word
// ----------------------------------------------
if (ThisCHR=>asc("a") and ThisCHR<=asc("z") ) or (ThisCHR=>asc("A") and ThisCHR<=asc("Z") )
// ----------------------------------------------

ThisTEXT$=""
Login required to view complete source code




 Related links:

      * Simple Formula Evaluation / Compilers