Issues evaluating postfix expressions in objective-c

Issues evaluating postfix expressions in objective-c

I want to evaluate mathematical expressions.I have an expression
(-55+44.5)/-6.9+(7.9999-1.234*-9.21)*31.9+(3.1-0000000009).I convert this
into postFix form. After conversion the expression is 55 - 44.5 + / 6.9 -
7.9999 1.234 * - 9.21 - 31.9 * + 3.1 0000000009 - + This is not a standard
postFix form.Hence I get wrong results.How can I solve this?
TIA