Contributor: SWAG SUPPORT TEAM ³I'm writing a Program that draws equations. It's fairly easy if you put ³the equation in a pascal Variable like Y := (X+10) * 2, but I would like ³the user to enter the equation, but I don't see any possible way to do ³it. ...One way of doing it is by using an "expression trees". Suppose you have the equation Y := 20 ö 2 + 3. In this equation, you can represent the expression 20 ö 2 + 3 by using "full" binary trees as such: figure 1 a ÚÄ¿ ³+³ <----- root of your expression ÀÄÙ b / \ ÚÄ¿ ÚÄ¿ e ³ö³ ³3³ ÀÄÙ ÀÄÙ / \ c ÚÄÄ¿ ÚÄ¿ d ³20³ ³2³ ÀÄÄÙ ÀÄÙ (Note: a "leaf" is a node With no left or right children - ie: a value ) ...The above expression are called infix arithmetic expressions; the operators are written in between the things on which they operate. In our example, the nodes are visited in the order c, d, b, e, a, and their Labels in this order are 20, 2, ö, 3, +. Function Evaluate(p: node): Integer; { return value of the expression represented by the tree With root p } { p - points to the root of the expression tree } Var T1, T2: Integer; op: Char; begin if (p^.left = nil) and (p^.right = nil) then { node is a "leaf" } Evaluate := (p^.Value) { simple Case } else begin T1 := Evaluate(p^.Left); T2 := Evaluate(p^.Right); op := p^.Label; { apply operation } Case op of '+': Evaluate := (T1 + T2); '-': Evaluate := (T1 - T2); 'ö': Evaluate := (T1 div T2); '*': Evaluate := (T1 * T2); end; end end; ...Thus, using figure 1, we have: ÚÄÄ ÚÄÄ ³ ³ Evaluate(c) = 20 ³ Evaluate(b) ³ Evaluate(d) = 2 ³ ³ ApplyOp('ö',20,2) = 10 Evaluate(a)³ ÀÄÄ ³ Evaluate(e) = 3 ³ ³ ApplyOp('+',10,3) = 13 ÀÄ