expclass.pas

一个简单的表达式解析器 Delphi版

unit expclass;

interface
uses
  SysUtils;

type
  TExpression = class
  private
    CurPos: integer;
    MaxPos: integer;
    iExp: string;
    token: string;
    function IsDigit(ch: char):boolean;
    function IsLetter(ch: char):boolean;
    function CheckExp:boolean;      // 首先对表达式进行检查
    function gettoken:integer;
    function term:extended;
    function factor:extended;
  public
    oResok: boolean;
    
    constructor Create(AExpression: PChar);
    function Calc:extended;
  end;

const
  ID     = 1;   // var
  NUM    = 2;   // numbers
  PLUS   = 3;   // '+'
  MINUS  = 4;   // '-'
  TIMERS = 5;   // '*'
  OVER   = 6;   // '/'
  LPAREN = 7;   // '('
  RPAREN = 8;   // ')'
  POWER  = 9;   // '^'

  ERROR  = 255; // Errors

  OPS    = '+-*/()^';  // Oprators

implementation

function TExpression.IsLetter(ch: Char):boolean;
begin
  result := (ch in ['a'..'z']) or (ch in ['A'..'Z']);
end;

function TExpression.IsDigit(ch: Char):boolean;
begin
  result := (ch in ['0'..'9']) or (ch = '.');
end;

// Get Token and Token Type from curPos
function TExpression.gettoken:integer;
var
  Skip: boolean;
begin
  result := ERROR;
  Skip := False;          // 指示是否忽略指针移动
  token := '';
  while (CurPos <= MaxPos) and (iExp[CurPos] = ' ') do inc(CurPos);

  // 加入对负号的处理：如果前次token为 '('，或第一个符号就是'-'，则减号是负号
  if Pos(iExp[CurPos],OPS)>0 then begin
    case iExp[CurPos] of
      '+': result := PLUS;
      '-':
        begin
          if (CurPos > 1) and (iExp[CurPos-1] <> '(') then
            result := MINUS
          else begin
            token := '-';
            inc(CurPos);
            while (CurPos <= MaxPos) and IsDigit(iExp[CurPos]) do begin
              token := token + iExp[CurPos];
              inc(CurPos);
            end;

            Skip := True;
            result := NUM;
          end;
        end;
      '*': result := TIMERS;
      '/': result := OVER;
      '(': result := LPAREN;
      ')': result := RPAREN;
      '^': result := POWER;
    end;
    if not Skip then inc(CurPos);
  end else begin
    if IsLetter(iExp[CurPos]) then begin
      while (CurPos <= MaxPos) and (IsLetter(iExp[CurPos]) or IsDigit(iExp[CurPos])) do begin
        token := token + iExp[CurPos];
        inc(CurPos);
      end;
      result := ID;
    end else if IsDigit(iExp[CurPos]) then begin
      while (CurPos <= MaxPos) and IsDigit(iExp[CurPos]) do begin
        token := token + iExp[CurPos];
        inc(CurPos);
      end;
      result := NUM;
    end;
  end;

  if result = ERROR then oResok := false;
end;

function TExpression.term:extended;
var
  temp: extended;
begin
  result := factor();
  while (CurPos <= MaxPos) and ((iExp[CurPos] = '*') or (iExp[CurPos] = '/')) do begin
    case gettoken() of
      TIMERS:
        result := result * factor();
      OVER:
        begin
          temp := factor;
          if temp <> 0 then
            result := result / temp
          else begin
            oResok := false;
            result := 0;
          end;
        end;
    end;
  end;
end;

function TExpression.factor():extended;
begin
  result := 0.0;
  case gettoken() of
    NUM:
      result := StrtoFloatDef(token,0);
    LPAREN:
      begin
        result := calc;
        if gettoken() <> RPAREN then begin
          oResok := false;
          result := 0;
        end;
      end;
    ID:
      begin
      end;
  end;
end;

function TExpression.CheckExp: boolean;
begin
  result := true;
end;

constructor TExpression.Create(AExpression: PChar);
begin
  iExp := string(AExpression);
  CurPos := 1;
  MaxPos := Length(iExp);
  oResok := CheckExp;
end;

function TExpression.Calc:extended;
begin
  if oResok then begin
    result := term();
    while (CurPos <= MaxPos) and ((iExp[CurPos] = '+') or (iExp[CurPos] = '-')) do begin
      case gettoken() of
        PLUS:
          result := result + term();
        MINUS:
          result := result - term();
      end;
    end;
  end else
    result := 0;
end;

end.