$0.$ Starting State:

$$ Increment input string's index value

$$ If is operator, next state is $1$

$$ If is digit, next state is $2$

$$ If is space, next state is $4$

$$ If is end of string, next state is $5$

$$ If anything else, next state is $6$

$1.$ Found operator:

$$ Pop $2$nd number off stack

$$ Pop $1$st number off stack

$$ Perform operation

$$ Push result on stack

$$ Next state is $0$

Note: Watch popping empty stack and division by zero

$2.$ Found digit:

$$ Copy digit to temporary string

$$ increment both the temporary and input string index values

$$ If character in input string is a digit or decimal point

$$ next state is $2$

$$ otherwise

$$ next state $3$

$3.$ Found end of number:

$$ append $\backslash 0$ on temporary array

$$ convert and assign to variable

$$ push number on stack

$$ reset temp index to $0$

$$ decrease input index by $1$

$$ next state is $0$

$4.$ Found space:

$$ next state is $0$

$5.$ Found end of input string:

$$ pop result

$$ return answer

Note: Watch for leftovers on stack

$6.$ Found garbage

$$ display error message

$$ exit from program