How can I fix my while loop?

Hi!

I am trying to create a program that reads a cell array and computes the answer ex:

t = {'mult','add','2','3','stop','add','4','5','stop','stop'} (So the equation is (2+3)*(4+5))

ans= 45.0000

here is my current code:

function [pos prod] = doMult ( expr, pos )
    prod=1;
    lm=length(expr)
   expo=1;
   sub=1;
   quot=1;
   sum=1;
   while ~strcmp(expr{pos}, 'stop')
       pos=pos+1
           if strcmp(expr{pos}, 'pow')
               [pos expo]=doPow(expr, pos)
           elseif strcmp(expr{pos},'mult')
               [pos prod]=doMult(expr, pos)
           elseif strcmp(expr{pos}, 'div')
               [pos quot]=doDiv(expr, pos) 
           elseif strcmp(expr{pos},'add')
               [pos sum]=doAdd(expr, pos)
           elseif strcmp(expr{pos},'sub')
               [pos diff]=doSub(expr, pos)
         
             
          else
            
              ExprNUM=str2double(expr) %double type 
              prod=prod* ExprNUM(1,pos)*sum*diff*quot*expo
              pos=pos
          end
   end
end

and the other finctions for doAdd and such would contain some variation of this . the output gives me an answer of 5, which is not correct. After going through what it calculates here:

lm =
    10
pos =
     2
lm =
    10
pos =
     3
ExprNUM =
   NaN   NaN     2     3   NaN   NaN     4     5   NaN   NaN
sum =
     2
pos =
     3
pos =
     4
ExprNUM =
   NaN   NaN     2     3   NaN   NaN     4     5   NaN   NaN
sum =
     5
pos =
     4
pos =
     5
ExprNUM =
   NaN   NaN     2     3   NaN   NaN     4     5   NaN   NaN
sum =
   NaN
pos =
     5
pos =
     5
sum =
   NaN
answer =
     5
ans =
     5

and it looks like it is not recognizing the stop and the function following it. How can I fix this?

Thanks!

[Geoff Hayes]

Ashley - maybe you need to start with a simpler example and/or post more of your code. Is doMult the calling function or do you have some other function that you call first?

Note that if I try something simple like

g = {'mult'    '2'    '3'    'stop' }
doMult(g,1)

then I get an error

Undefined function or variable "diff".

Error in doMult (line 25)
              prod=prod* ExprNUM(1,pos)*sum*diff*quot*expo

which makes sense because neither sum nor diff have been defined.

NOTE that sum and diff are built-in MATLAB functions so you should not be creating variables with the same names as these functions.