RPN Calculator

Reverse Polish Notation (RPN) is mathmatical convention for handling expressions. RPN is most commonly found in Hewlett-Packard calculators. RPN, like Forth and Postscript, is stack oriented. Operators only handle data stored on the stack. The infix (or normal) expression 3 + 5 would be expressed as 3 5 +. Note the operator goes at the end of the expression.

What really happens is that the 3 is pushed on the stack, then the 5 is pushed, and finally, when the + is encountered, it takes (pops) two numbers off the stack, adds them and puts the results back on the stack.

For the expression (1 + 2) * 3, you would enter 1 2 + 3 *. You don’t need ()’s to control what operation takes place first. RPN executes each operator as it comes across it.

Below is a Structured SuperBasic program that implements a simple RPN calculator. The left part of the screen is used for entry and results. The right part of the screen is used to show the current stack. With RPN, you can put a lot of numbers on the stack, then enter the operators for them.

Reverse Polish Notation Calculator

T = 0 : F = 0

CLS #1 : CLS #2
num = 0
sx = 0
DIM stack(30)
stackp = 1
num_entered = F

REPeat loop

    in$=INKEY$(-1)
    IF (in$ INSTR "0123456789") THEN
       num = (10*num) + in$
       PRINT #2,in$;
       num_entered = T
    END IF
    
    IF in$ = "+" THEN
       IF num_entered = T THEN
            push(num)
            num_entered = F
       END IF
       plus : num = 0
    END IF
    IF in$ = "-" THEN
        IF num_entered = T THEN
            push(num)
            num_entered = F
        END IF
        minus : num = 0
    END IF
    IF in$ = "*" THEN
        IF num_entered = T THEN
            push(num)
            num_entered = F
        END IF
        times : num = 0
    END IF
    IF in$ = "/" THEN
        IF num_entered = T THEN
            push(num)
            num_entered = F
        END IF
        divide : num = 0
    END IF
    IF in$ = "^" THEN
        IF num_entered = T THEN
            push(num)
            num_entered = F
        END IF
        power : num = 0
    END IF

    IF in$ = CHR$(10) THEN push(num) : num = 0 : PRINT #2

END REPeat loop

DEFine PROCedure push(x)
    sx = sx + 1
    AT sx,0 : PRINT x
    stackp = stackp + 1
    stack(stackp) = x
END DEFine push

DEFine FuNction pop
  LOCal temp
    IF stackp = 0 THEN
        PRINT #2,"Stack Error"
        STOP
    ELSE
        temp = stack(stackp)
        AT sx,0 : PRINT "            "
        sx = sx - 1
        IF sx = -1 THEN sx = 0
        stackp = stackp - 1
        RETURN temp
    END IF
END DEFine pop 
          
DEFine PROCedure plus
  LOCal temp1, temp2
    PRINT #2 : PRINT #2,"   +"
    temp1 = pop
    temp2 = pop
    temp1 = temp1 + temp2
    push(temp1)
    PRINT #2,"   ="
    PRINT #2,temp1
END DEFine plus

DEFine PROCedure minus
  LOCal temp1, temp2
    PRINT #2 : PRINT #2,"   -"
    temp1 = pop
    temp2 = pop
    temp1 = temp1 - temp2
    push(temp1)
    PRINT #2,"   ="
    PRINT #2,temp1
END DEFine minus

DEFine PROCedure times
  LOCal temp1, temp2
    PRINT #2 : PRINT #2,"   *"
    temp1 = pop
    temp2 = pop
    temp1 = temp1 * temp2
    push(temp1)
    PRINT #2,"   ="
    PRINT #2,temp1
END DEFine times

DEFine PROCedure divide
  LOCal temp1, temp2
    PRINT #2 : PRINT #2,"   /"
     temp1 = pop
    temp2 = pop
    IF temp2 = 0 THEN
       PRINT #2,"Divide by Zero Error"
    ELSE
        temp1 = temp1 / temp2
        push(temp1)
        PRINT #2,"   ="
        PRINT #2,temp1
    END IF
END DEFine divide

DEFine PROCedure power
  LOCal temp1, temp2
    PRINT #2 : PRINT #2,"   ^"
    temp1 = pop
    temp2 = pop
    IF temp2 = 0 THEN
        PRINT #2,"Negative Power Error"
    ELSE
       temp1 = temp1 ^ temp2
       push(temp1)
       PRINT #2,"   ="
       PRINT #2,temp1
    END IF
END DEFine power