Calculator

[T-bear]

Hi, I try to make a calculator where you just enter the whole operation (like 2/(3*(4+5)) ), press the "="-button, and get the answear. Has anyone made something like this before? How can i do this?
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Thanks!
 

[Leledumbo]

Easy way: use symbolic package
Hard way: learn compiler techniques

[Marc]

This sounds like homework. (at least it was when I took programming classes)
The global answer, start parsing and split your expression in (binary) tree nodes.
Then evaluate your tree (leftpart)(operator)(rightpart)

[marcov]

This sounds like homework. (at least it was when I took programming classes)
The global answer, start parsing and split your expression in (binary) tree nodes.
Then evaluate your tree (leftpart)(operator)(rightpart)

(not per se. IMHO for simple expression a stack based approach (one stack for operators, one for arguments) is easier than actuall building trees)

[zoiddani]

i think this might be helpful: http://en.wikipedia.org/wiki/Reverse_Polish_notation

[typo]

A simple (and understandable) example:

(It makes a single operation with 2 operands and 1 operator)

Code: [Select]

var
  S, Arg1, Arg2, Op :string;
  Index :byte;
  Res :integer;
begin
  S := Edit1.Text;
  Arg1 := '';
  Arg2 := '';
  Op := '';
  Index := 1;
  while S[Index] in ['0'..'9'] do
  begin
    Arg1 := Arg1 + S[Index];
    Inc(Index);
  end;

  while S[Index] = ' ' do
    Inc(Index);

  if S[Index] in ['+','-','*','/'] then
  begin
    op := S[Index];
    Inc(Index);
  end
  else
    begin
      ShowMessage('Operator expected at position ' + IntToStr(Index));
      Edit1.SetFocus;
      Edit1.SelStart := Index -1;
      Edit1.SelLength := 1;
      Exit;
    end;

  while S[Index] = ' ' do
    Inc(Index);

  while S[Index] in ['0'..'9'] do
  begin
    Arg2 := Arg2 + S[Index];
    Inc(Index);
  end;

  case op[1] of
    '+': begin
           Res := StrToInt(Arg1) + StrToInt(Arg2);
           ShowMessage(IntToStr(Res));
         end;
    '-':begin
           Res := StrToInt(Arg1) - StrToInt(Arg2);
           ShowMessage(IntToStr(Res));
         end;
    '*':begin
           Res := StrToInt(Arg1) * StrToInt(Arg2);
           ShowMessage(IntToStr(Res));
         end;
    '/':begin
           Res := StrToInt(Arg1) div StrToInt(Arg2);
           ShowMessage(IntToStr(Res));
         end;
  end;
end;   


[Marc]

This sounds like homework. (at least it was when I took programming classes)
The global answer, start parsing and split your expression in (binary) tree nodes.
Then evaluate your tree (leftpart)(operator)(rightpart)

(not per se. IMHO for simple expression a stack based approach (one stack for operators, one for arguments) is easier than actuall building trees)

I didn't want to think about it. I only recalled that when I made the assignment we came to a tree based solution

[idog]

Hi, I try to make a calculator where you just enter the whole operation (like 2/(3*(4+5)) ), press the "="-button, and get the answear. Has anyone made something like this before? How can i do this?

Using a stack is the "classic" solution for this problem. Whether you choose a stack or a tree, the basis is to be able to parse the string correctly - by identifying numbers and operators, while keeping track of the operator's priority (combine their algebraic priority with the parenthesis-induced priority).

[Leledumbo]

Either way chosen, the OP must break the string into tokens first and identify each correctly, then match them with the expected syntax.

[typo]

There are some libraries for it, like this one:

http://sourceforge.net/projects/tpsystools/

[typo]

The attached unit provides a class to calculate math expressions.

Usage:

Code: [Select]

  exp := TStExpression.Create(Application);
  exp.Expression := Edit1.Text;
  ShowMessage(FloatToStr(exp.AnalyzeExpression));