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 $6$

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

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 $3$

Found end of number:

append $10$ on temporary array

convert and assign to variable

o push number on stack

reset temp index to $0$

decrease input index by $1$

next state is $0$

Found space:

next state is $0$

Found end of input string:

pop result

return answer

Note: Watch for leftovers on stack

Found garbage

display error message

exit from program

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