# Parsing/RPN calculator algorithm

Parsing/RPN calculator algorithm
You are encouraged to solve this task according to the task description, using any language you may know.

Create a stack-based evaluator for an expression in reverse Polish notation that also shows the changes in the stack as each individual token is processed as a table.

• Assume an input of a correct, space separated, string of tokens of an RPN expression
• Test with the RPN expression generated from the Parsing/Shunting-yard algorithm task `'3 4 2 * 1 5 - 2 3 ^ ^ / +'` then print and display the output here.
Note
• '^' means exponentiation in the expression above.

procedure RPN_Calculator is

``` package IIO is new Ada.Text_IO.Float_IO(Float);
```
```  package Float_Vec is new Ada.Containers.Vectors
(Index_Type => Positive, Element_Type => Float);
Stack: Float_Vec.Vector;
```
```  Input: String := Ada.Text_IO.Get_Line;
Cursor: Positive := Input'First;
New_Cursor: Positive;
```

begin

```  loop
while Cursor <= Input'Last and then Input(Cursor)=' ' loop
Cursor := Cursor + 1;
end loop;
```
```     exit when Cursor > Input'Last;
```
```     New_Cursor := Cursor;
while New_Cursor <= Input'Last and then Input(New_Cursor) /= ' ' loop
New_Cursor := New_Cursor + 1;
end loop;
```
```     -- try to read a number and push it to the stack
declare
Last: Positive;
Value: Float;
X, Y: Float;
begin
IIO.Get(From => Input(Cursor .. New_Cursor - 1),
Item => Value,
Last => Last);
Stack.Append(Value);
Cursor := New_Cursor;
```
```     exception -- if reading the number fails, try to read an operator token
when others =>
Y := Stack.Last_Element; Stack.Delete_Last; -- pick two elements
X := Stack.Last_Element; Stack.Delete_Last; -- from the stack
case Input(Cursor) is
when '+' => Stack.Append(X+Y);
when '-' => Stack.Append(X-Y);
when '*' => Stack.Append(X*Y);
when '/' => Stack.Append(X/Y);
when '^' => Stack.Append(X ** Integer(Float'Rounding(Y)));
when others => raise Program_Error with "unecpected token '"
& Input(Cursor) & "' at column" & Integer'Image(Cursor);
end case;
Cursor := New_Cursor;
end;
```
```     for I in Stack.First_Index .. Stack.Last_Index loop
IIO.Put(Stack.Element(I), Aft => 5, Exp => 0);
end loop;
end loop;
```
```  Ada.Text_IO.Put("Result = ");
IIO.Put(Item => Stack.Last_Element, Aft => 5, Exp => 0);
```

end RPN_Calculator;</lang>

Output:
```3 4 2 * 1 5 - 2 3 ^ ^ / +
3.00000
3.00000  4.00000
3.00000  4.00000  2.00000
3.00000  8.00000
3.00000  8.00000  1.00000
3.00000  8.00000  1.00000  5.00000
3.00000  8.00000 -4.00000
3.00000  8.00000 -4.00000  2.00000
3.00000  8.00000 -4.00000  2.00000  3.00000
3.00000  8.00000 -4.00000  8.00000
3.00000  8.00000 65536.00000
3.00000  0.00012
3.00012
Result =  3.00012```

## ALGOL 68

Works with: ALGOL 68G version Any - tested with release 2.8.win32

<lang algol68># RPN Expression evaluator - handles numbers and + - * / ^ #

1. the right-hand operand for ^ is converted to an integer #
1. expression terminator #

CHAR end of expression character = REPR 12;

1. evaluates the specified rpn expression #

PROC evaluate = ( STRING rpn expression )VOID: BEGIN

```   [ 256 ]REAL   stack;
INT           stack pos := 0;
```
```   # pops an element off the stack #
PROC pop = REAL:
BEGIN
stack pos -:= 1;
stack[ stack pos + 1 ]
END; # pop #
```
```   INT rpn pos := LWB rpn expression;
```
```   # evaluate tokens from the expression until we get the end of expression #
WHILE
```
```       # get the next token from the string #
```
```       STRING token type;
REAL   value;
```
```       # skip spaces #
WHILE rpn expression[ rpn pos ] = " "
DO
rpn pos +:= 1
OD;
```
```       # handle the token #
IF rpn expression[ rpn pos ] = end of expression character
THEN
# no more tokens #
FALSE
```
```       ELSE
# have a token #
```
```           IF  rpn expression[ rpn pos ] >= "0"
AND rpn expression[ rpn pos ] <= "9"
THEN
# have a number #
```
```               # find where the nmumber is in the expression #
INT  number start = rpn pos;
WHILE (   rpn expression[ rpn pos ] >= "0"
AND rpn expression[ rpn pos ] <= "9"
)
OR rpn expression[ rpn pos ] = "."
DO
rpn pos +:= 1
OD;
```
```               # read the number from the expression #
FILE number f;
associate( number f
, LOC STRING := rpn expression[ number start : rpn pos - 1 ]
);
get( number f, ( value ) );
close( number f );

token type := "number"
```
```           ELSE
# must be an operator #
CHAR op      = rpn expression[ rpn pos ];
rpn pos    +:= 1;
```
```               REAL arg1   := pop;
REAL arg2   := pop;
token type  := op;
```
```               value := IF   op = "+"
THEN
# add the top two stack elements #
arg1 + arg2
ELIF op = "-"
THEN
# subtract the top two stack elements #
arg2 - arg1
ELIF op = "*"
THEN
# multiply the top two stack elements #
arg2 * arg1
ELIF op = "/"
THEN
# divide the top two stack elements #
arg2 / arg1
ELIF op = "^"
THEN
# raise op2 to the power of op1 #
arg2 ^ ENTIER arg1
ELSE
# unknown operator #
print( ( "Unknown operator: """ + op + """", newline ) );
0
FI
```
```           FI;
```
```           TRUE
FI
DO
# push the new value on the stack and show the new stack #
```
```       stack[ stack pos +:= 1 ] := value;
```
```       print( ( ( token type + "            " )[ 1 : 8 ] ) );
FOR element FROM LWB stack TO stack pos
DO
print( ( " ", fixed( stack[ element ], 8, 4 ) ) )
OD;
print( ( newline ) )
```
```   OD;
```
```   print( ( "Result is: ", fixed( stack[ stack pos ], 12, 8 ), newline ) )
```

END; # evaluate #

main: (

```   # get the RPN expresson from the user #
```
```   STRING rpn expression;
```
```   print( ( "Enter expression: " ) );
read( ( rpn expression, newline ) );
```
```   # add a space to terminate the final token and an expression terminator #
rpn expression +:= " " + end of expression character;
```
```   # execute the expression #
evaluate( rpn expression )
```

)</lang>

Output:
```Enter expression: 3 4 2 * 1 5 - 2 3 ^ ^ / +
number    +3.0000
number    +3.0000  +4.0000
number    +3.0000  +4.0000  +2.0000
*         +3.0000  +8.0000
number    +3.0000  +8.0000  +1.0000
number    +3.0000  +8.0000  +1.0000  +5.0000
-         +3.0000  +8.0000  -4.0000
number    +3.0000  +8.0000  -4.0000  +2.0000
number    +3.0000  +8.0000  -4.0000  +2.0000  +3.0000
^         +3.0000  +8.0000  -4.0000  +8.0000
^         +3.0000  +8.0000 +65536.0
/         +3.0000  +0.0001
+         +3.0001
Result is:  +3.00012207
```

## ANTLR

### Java

<lang java> grammar rpnC ; // // rpn Calculator // // Nigel Galloway - April 7th., 2012 // @members { Stack<Double> s = new Stack<Double>(); } rpn : (WS* (num|op) (WS | WS* NEWLINE {System.out.println(s.pop());}))*; num : '-'? Digit+ ('.' Digit+)? {s.push(Double.parseDouble(\$num.text));}; Digit : '0'..'9'; op : '-' {double x = s.pop(); s.push(s.pop() - x);} | '/' {double x = s.pop(); s.push(s.pop() / x);} | '*' {s.push(s.pop() * s.pop());} | '^' {double x = s.pop(); s.push(Math.pow(s.pop(), x));} | '+' {s.push(s.pop() + s.pop());}; WS : (' ' | '\t'){skip()}; NEWLINE : '\r'? '\n'; </lang> Produces:

```>java Test
3 4 2 * 1 5 - 2 3 ^ ^ / +
^Z
3.0001220703125
```

## AutoHotkey

Works with: AutoHotkey_L

Output is in clipboard. <lang AHK>evalRPN("3 4 2 * 1 5 - 2 3 ^ ^ / +") evalRPN(s){ stack := [] out := "For RPN expression: '" s "'`r`n`r`nTOKEN`t`tACTION`t`t`tSTACK`r`n" Loop Parse, s If A_LoopField is number t .= A_LoopField else { If t stack.Insert(t) , out .= t "`tPush num onto top of stack`t" stackShow(stack) "`r`n" , t := "" If InStr("+-/*^", l := A_LoopField) { a := stack.Remove(), b := stack.Remove() stack.Insert( l = "+" ? b + a :l = "-" ? b - a :l = "*" ? b * a :l = "/" ? b / a :l = "^" ? b **a :0 ) out .= l "`tApply op " l " to top of stack`t" stackShow(stack) "`r`n" } } r := stack.Remove() out .= "`r`n The final output value is: '" r "'" clipboard := out return r } StackShow(stack){ for each, value in stack out .= A_Space value return subStr(out, 2) }</lang>

Output:
```For RPN expression: '3 4 2 * 1 5 - 2 3 ^ ^ / +'

TOKEN		ACTION			STACK
3	Push num onto top of stack	3
4	Push num onto top of stack	3 4
2	Push num onto top of stack	3 4 2
*	Apply op * to top of stack	3 8
1	Push num onto top of stack	3 8 1
5	Push num onto top of stack	3 8 1 5
-	Apply op - to top of stack	3 8 -4
2	Push num onto top of stack	3 8 -4 2
3	Push num onto top of stack	3 8 -4 2 3
^	Apply op ^ to top of stack	3 8 -4 8
^	Apply op ^ to top of stack	3 8 65536
/	Apply op / to top of stack	3 0.000122
+	Apply op + to top of stack	3.000122

The final output value is: '3.000122'```

## BBC BASIC

<lang bbcbasic> @% = &60B

```     RPN\$ = "3 4 2 * 1 5 - 2 3 ^ ^ / +"

DIM Stack(1000)
SP% = 0

FOR i% = 1 TO LEN(RPN\$)
Token\$ = MID\$(RPN\$,i%,1)
IF Token\$ <> " " THEN
PRINT Token\$ " :";
CASE Token\$ OF
WHEN "+": PROCpush(FNpop + FNpop)
WHEN "-": PROCpush(-FNpop + FNpop)
WHEN "*": PROCpush(FNpop * FNpop)
WHEN "/": n = FNpop : PROCpush(FNpop / n)
WHEN "^": n = FNpop : PROCpush(FNpop ^ n)
WHEN "0","1","2","3","4","5","6","7","8","9":
PROCpush(VALMID\$(RPN\$,i%))
WHILE ASCMID\$(RPN\$,i%)>=48 AND ASCMID\$(RPN\$,1)<=57
i% += 1
ENDWHILE
ENDCASE
FOR j% = SP%-1 TO 0 STEP -1 : PRINT Stack(j%); : NEXT
PRINT
ENDIF
NEXT i%
END

DEF PROCpush(n)
IF SP% > DIM(Stack(),1) ERROR 100, "Stack full"
Stack(SP%) = n
SP% += 1
ENDPROC

DEF FNpop
IF SP% = 0 ERROR 100, "Stack empty"
SP% -= 1
= Stack(SP%)</lang>
```
Output:
```3 :          3
4 :          4          3
2 :          2          4          3
* :          8          3
1 :          1          8          3
5 :          5          1          8          3
- :         -4          8          3
2 :          2         -4          8          3
3 :          3          2         -4          8          3
^ :          8         -4          8          3
^ :      65536          8          3
/ : 0.00012207          3
+ :    3.00012
```

## C

<lang c>#include <stdio.h>

1. include <stdlib.h>
2. include <string.h>
3. include <math.h>

void die(const char *msg) { fprintf(stderr, "%s", msg); abort(); }

1. define MAX_D 256

double stack[MAX_D]; int depth;

void push(double v) { if (depth >= MAX_D) die("stack overflow\n"); stack[depth++] = v; }

double pop() { if (!depth) die("stack underflow\n"); return stack[--depth]; }

double rpn(char *s) { double a, b; int i; char *e, *w = " \t\n\r\f";

for (s = strtok(s, w); s; s = strtok(0, w)) { a = strtod(s, &e); if (e > s) printf(" :"), push(a);

1. define binop(x) printf("%c:", *s), b = pop(), a = pop(), push(x)

else if (*s == '+') binop(a + b); else if (*s == '-') binop(a - b); else if (*s == '*') binop(a * b); else if (*s == '/') binop(a / b); else if (*s == '^') binop(pow(a, b));

1. undef binop

else { fprintf(stderr, "'%c': ", *s); die("unknown oeprator\n"); } for (i = depth; i-- || 0 * putchar('\n'); ) printf(" %g", stack[i]); }

if (depth != 1) die("stack leftover\n");

return pop(); }

int main(void) { char s[] = " 3 4 2 * 1 5 - 2 3 ^ ^ / + "; printf("%g\n", rpn(s)); return 0; }</lang>

It's also possible to parse RPN string backwards and recursively; good luck printing out your token stack as a table: there isn't one. <lang c>#include <stdio.h>

1. include <stdlib.h>
2. include <ctype.h>
3. include <string.h>
4. include <math.h>
1. define die(msg) fprintf(stderr, msg"\n"), abort();

double get(const char *s, const char *e, char **new_e) { const char *t; double a, b;

for (e--; e >= s && isspace(*e); e--); for (t = e; t > s && !isspace(t[-1]); t--);

if (t < s) die("underflow");

1. define get2(expr) b = get(s, t, (char **)&t), a = get(s, t, (char **)&t), a = expr

a = strtod(t, (char **)&e); if (e <= t) { if (t[0] == '+') get2(a + b); else if (t[0] == '-') get2(a - b); else if (t[0] == '*') get2(a * b); else if (t[0] == '/') get2(a / b); else if (t[0] == '^') get2(pow(a, b)); else { fprintf(stderr, "'%c': ", t[0]); die("unknown token"); } }

1. undef get2

*(const char **)new_e = t; return a; }

double rpn(const char *s) { const char *e = s + strlen(s); double v = get(s, e, (char**)&e);

while (e > s && isspace(e[-1])) e--; if (e == s) return v;

fprintf(stderr, "\"%.*s\": ", e - s, s); die("front garbage"); }

int main(void) { printf("%g\n", rpn("3 4 2 * 1 5 - 2 3 ^ ^ / +")); return 0; }</lang>

## C++

<lang cpp>#include <vector>

1. include <string>
2. include <sstream>
3. include <iostream>
4. include <cmath>
5. include <algorithm>
6. include <iterator>
7. include <cstdlib>

double rpn(const std::string &expr){

``` std::istringstream iss(expr);
std::vector<double> stack;
std::cout << "Input\tOperation\tStack after" << std::endl;
std::string token;
while (iss >> token) {
std::cout << token << "\t";
double tokenNum;
if (std::istringstream(token) >> tokenNum) {
std::cout << "Push\t\t";
stack.push_back(tokenNum);
} else {
std::cout << "Operate\t\t";
double secondOperand = stack.back();
stack.pop_back();
double firstOperand = stack.back();
stack.pop_back();
if (token == "*")
```

stack.push_back(firstOperand * secondOperand);

```     else if (token == "/")
```

stack.push_back(firstOperand / secondOperand);

```     else if (token == "-")
```

stack.push_back(firstOperand - secondOperand);

```     else if (token == "+")
```

stack.push_back(firstOperand + secondOperand);

```     else if (token == "^")
```

stack.push_back(std::pow(firstOperand, secondOperand));

```     else { //just in case
```

std::cerr << "Error" << std::endl; std::exit(1);

```     }
}
std::copy(stack.begin(), stack.end(), std::ostream_iterator<double>(std::cout, " "));
std::cout << std::endl;
}
return stack.back();
```

}

int main() {

``` std::string s = " 3 4 2 * 1 5 - 2 3 ^ ^ / + ";
std::cout << "Final answer: " << rpn(s) << std::endl;

return 0;
```

}</lang>

Output:
```Input	Operation	Stack after
3	Push		3
4	Push		3 4
2	Push		3 4 2
*	Operate		3 8
1	Push		3 8 1
5	Push		3 8 1 5
-	Operate		3 8 -4
2	Push		3 8 -4 2
3	Push		3 8 -4 2 3
^	Operate		3 8 -4 8
^	Operate		3 8 65536
/	Operate		3 0.00012207
+	Operate		3.00012
```

## C#

<lang csharp>using System; using System.Collections.Generic; using System.Linq; using System.Globalization; using System.Threading;

namespace RPNEvaluator {

```   class RPNEvaluator
{
static void Main(string[] args)
{
```
```           string rpn = "3 4 2 * 1 5 - 2 3 ^ ^ / +";
Console.WriteLine("{0}\n", rpn);
```
```           decimal result = CalculateRPN(rpn);
Console.WriteLine("\nResult is {0}", result);
}
```
```       static decimal CalculateRPN(string rpn)
{
string[] rpnTokens = rpn.Split(' ');
Stack<decimal> stack = new Stack<decimal>();
decimal number = decimal.Zero;
```
```           foreach (string token in rpnTokens)
{
if (decimal.TryParse(token, out number))
{
stack.Push(number);
}
else
{
switch (token)
{
case "^":
case "pow":
{
number = stack.Pop();
stack.Push((decimal)Math.Pow((double)stack.Pop(), (double)number));
break;
}
case "ln":
{
stack.Push((decimal)Math.Log((double)stack.Pop(), Math.E));
break;
}
case "sqrt":
{
stack.Push((decimal)Math.Sqrt((double)stack.Pop()));
break;
}
case "*":
{
stack.Push(stack.Pop() * stack.Pop());
break;
}
case "/":
{
number = stack.Pop();
stack.Push(stack.Pop() / number);
break;
}
case "+":
{
stack.Push(stack.Pop() + stack.Pop());
break;
}
case "-":
{
number = stack.Pop();
stack.Push(stack.Pop() - number);
break;
}
default:
Console.WriteLine("Error in CalculateRPN(string) Method!");
break;
}
}
PrintState(stack);
}
```
```           return stack.Pop();
}
```
```       static void PrintState(Stack<decimal> stack)
{
decimal[] arr = stack.ToArray();
```
```           for (int i = arr.Length - 1; i >= 0; i--)
{
Console.Write("{0,-8:F3}", arr[i]);
}

Console.WriteLine();
}
}
```

}</lang>

Output:
```3 4 2 * 1 5 - 2 3 ^ ^ / +

3.000
3.000   4.000
3.000   4.000   2.000
3.000   8.000
3.000   8.000   1.000
3.000   8.000   1.000   5.000
3.000   8.000   -4.000
3.000   8.000   -4.000  2.000
3.000   8.000   -4.000  2.000   3.000
3.000   8.000   -4.000  8.000
3.000   8.000   65536.000
3.000   0.000
3.000

Result is 3.0001220703125
```

## Clojure

This would be a lot simpler and generic if we were allowed to use something other than ^ for exponentiation. ^ isn't a legal clojure symbol. <lang clojure> (ns rosettacode.parsing-rpn-calculator-algorithm

``` (:require clojure.math.numeric-tower
clojure.string
clojure.pprint))
```

(def operators

``` "the only allowable operators for our calculator"
{"+" +
"-" -
"*" *
"/" /
"^" clojure.math.numeric-tower/expt})
```

(defn rpn

``` "takes a string and returns a lazy-seq of all the stacks"
[string]
(letfn [(rpn-reducer [stack item] ; this takes a stack and one item and makes a new stack
(if (contains? operators item)
(let [operand-1 (peek stack) ; if we used lists instead of vectors, we could use destructuring, but stacks would look backwards
stack-1 (pop stack)]   ;we're assuming that all the operators are binary
(conj (pop stack-1)
((operators item) (peek stack-1) operand-1)))
(conj stack (Long. item))))] ; if it wasn't an operator, we'll assume it's a long. Could choose bigint, or even read-line
(reductions rpn-reducer [] (clojure.string/split string #"\s+")))) ;reductions is like reduce only shows all the intermediate steps
```

(let [stacks (rpn "3 4 2 * 1 5 - 2 3 ^ ^ / +")] ;bind it so we can output the answer separately.

``` (println "stacks: ")
(clojure.pprint/pprint stacks)
(print "answer:" (->> stacks last first)))
```

</lang>

Output:

stacks: ([]

```[3]
[3 4]
[3 4 2]
[3 8]
[3 8 1]
[3 8 1 5]
[3 8 -4]
[3 8 -4 2]
[3 8 -4 2 3]
[3 8 -4 8]
[3 8 65536]
[3 1/8192]
[24577/8192])
```

## Common Lisp

<lang lisp>(setf (symbol-function '^) #'expt)  ; Make ^ an alias for EXPT

(defun print-stack (token stack)

```   (format T "~a: ~{~a ~}~%" token (reverse stack)))
```

(defun rpn (tokens &key stack verbose )

``` (cond
((and (not tokens) (not stack)) 0)
((not tokens) (car stack))
(T
(let* ((current (car tokens))
(next-stack (if (numberp current)
(cons current stack)
(let* ((arg2 (car stack))
(fun (car tokens)))
(cons (funcall fun arg1 arg2) (cddr stack))))))
(when verbose
(print-stack current next-stack))
(rpn (cdr tokens) :stack next-stack :verbose verbose)))))</lang>
```
Output:
```>(defparameter *tokens* '(3 4 2 * 1 5 - 2 3 ^ ^ / +))

*TOKENS*
> (rpn *tokens*)

24577/8192
> (rpn *tokens* :verbose T)
3: 3
4: 3 4
2: 3 4 2
*: 3 8
1: 3 8 1
5: 3 8 1 5
-: 3 8 -4
2: 3 8 -4 2
3: 3 8 -4 2 3
^: 3 8 -4 8
^: 3 8 65536
/: 3 1/8192
+: 24577/8192
24577/8192```

## Ela

<lang ela>open string console list format read

eval str = writen "Input\tOperation\tStack after" \$

```              eval' (split " " str) []
where eval' [] (s::_) = printfn "Result: {0}" s
eval' (x::xs) sta | "+"? = eval' xs <| op (+)
| "-"? = eval' xs <| op (-)
| "^"? = eval' xs <| op (**)
| "/"? = eval' xs <| op (/)
| "*"? = eval' xs <| op (*)
| else = eval' xs <| conv x
where c? = x == c
op (^) = out "Operate" st' \$ st'
where st' = (head ss ^ s) :: tail ss
conv x = out "Push" st' \$ st'
where st' = readStr x :: sta
(s,ss) | sta == [] = ((),[])
| else = (head sta,tail sta)
out op st' = printfn "{0}\t{1}\t\t{2}" x op st'
```

eval "3 4 2 * 1 5 - 2 3 ^ ^ / +"</lang>

Output:
```Input	Operation	Stack after
3	Push		[3]
4	Push		[4,3]
2	Push		[2,4,3]
*	Operate		[8,3]
1	Push		[1,8,3]
5	Push		[5,1,8,3]
-	Operate		[-4,8,3]
2	Push		[2,-4,8,3]
3	Push		[3,2,-4,8,3]
^	Operate		[8,-4,8,3]
^	Operate		[65536,8,3]
/	Operate		[0.0001220703125,3]
+	Operate		[3.0001220703125]
Result: 3.0001220703125```

## D

Translation of: Go

<lang d>import std.stdio, std.string, std.conv, std.typetuple;

void main() {

```   auto input = "3 4 2 * 1 5 - 2 3 ^ ^ / +";
writeln("For postfix expression: ", input);
writeln("\nToken            Action            Stack");
real[] stack;
foreach (tok; input.split()) {
auto action = "Apply op to top of stack";
switch (tok) {
foreach (o; TypeTuple!("+", "-", "*", "/", "^")) {
case o:
mixin("stack[\$ - 2]" ~
(o == "^" ? "^^" : o) ~ "=stack[\$ - 1];");
stack.length--;
break;
}
break;
default:
action = "Push num onto top of stack";
stack ~= to!real(tok);
}
writefln("%3s    %-26s  %s", tok, action, stack);
}
writeln("\nThe final value is ", stack[0]);
```

}</lang>

Output:
```For postfix expression: 3 4 2 * 1 5 - 2 3 ^ ^ / +

Token            Action            Stack
3    Push num onto top of stack  [3]
4    Push num onto top of stack  [3, 4]
2    Push num onto top of stack  [3, 4, 2]
*    Apply op to top of stack    [3, 8]
1    Push num onto top of stack  [3, 8, 1]
5    Push num onto top of stack  [3, 8, 1, 5]
-    Apply op to top of stack    [3, 8, -4]
2    Push num onto top of stack  [3, 8, -4, 2]
3    Push num onto top of stack  [3, 8, -4, 2, 3]
^    Apply op to top of stack    [3, 8, -4, 8]
^    Apply op to top of stack    [3, 8, 65536]
/    Apply op to top of stack    [3, 0.00012207]
+    Apply op to top of stack    [3.00012]

The final value is 3.00012```

## Erlang

<lang erlang>-module(rpn). -export([eval/1]).

parse(Expression) ->

```   parse(string:tokens(Expression," "),[]).
```

parse([],Expression) ->

```   lists:reverse(Expression);
```

parse(["+"|Xs],Expression) ->

```   parse(Xs,[fun erlang:'+'/2|Expression]);
```

parse(["-"|Xs],Expression) ->

```   parse(Xs,[fun erlang:'-'/2|Expression]);
```

parse(["*"|Xs],Expression) ->

```   parse(Xs,[fun erlang:'*'/2|Expression]);
```

parse(["/"|Xs],Expression) ->

```   parse(Xs,[fun erlang:'/'/2|Expression]);
```

parse(["^"|Xs],Expression) ->

```   parse(Xs,[fun math:pow/2|Expression]);
```

parse([X|Xs],Expression) ->

```   {N,_} = string:to_integer(X),
parse(Xs,[N|Expression]).
```

%% The expression should be entered as a string of numbers and %% operators separated by spaces. No error handling is included if %% another string format is used. eval(Expression) ->

```   eval(parse(Expression),[]).
```

eval([],[N]) ->

```   N;
```

eval([N|Exp],Stack) when is_number(N) ->

```   NewStack = [N|Stack],
print(NewStack),
eval(Exp,NewStack);
```

eval([F|Exp],[X,Y|Stack]) ->

```   NewStack = [F(Y,X)|Stack],
print(NewStack),
eval(Exp,NewStack).
```

print(Stack) ->

```   lists:map(fun (X) when is_integer(X) -> io:format("~12.12b ",[X]);
(X) when is_float(X) -> io:format("~12f ",[X]) end, Stack),
io:format("~n").</lang>
```
Output:
```145> rpn:eval("3 4 2 * 1 5 - 2 3 ^ ^ / +").
3
4            3
2            4            3
8            3
1            8            3
5            1            8            3
-4            8            3
2           -4            8            3
3            2           -4            8            3
8.000000           -4            8            3
65536.000000            8            3
0.000122            3
3.000122
3.0001220703125```

## FunL

<lang funl>def evaluate( expr ) =

``` stack = []
```
``` for token <- expr.split( \s+ )
case number( token )
Some( n ) ->
stack = n : stack
println( "push \$token: \${stack.reversed()}" )
None ->
case {'+': (+), '-': (-), '*': (*), '/': (/), '^': (^)}.>get( token )
Some( op ) ->
println( "perform \$token: \${stack.reversed()}" )
None -> error( "unrecognized operator '\$token'" )

```

res = evaluate( '3 4 2 * 1 5 - 2 3 ^ ^ / +' ) println( res + (if res is Integer then else " or \${float(res)}") )</lang>

Output:
```push 3: [3]
push 4: [3, 4]
push 2: [3, 4, 2]
perform *: [3, 8]
push 1: [3, 8, 1]
push 5: [3, 8, 1, 5]
perform -: [3, 8, -4]
push 2: [3, 8, -4, 2]
push 3: [3, 8, -4, 2, 3]
perform ^: [3, 8, -4, 8]
perform ^: [3, 8, 65536]
perform /: [3, 1/8192]
perform +: [24577/8192]
24577/8192 or 3.0001220703125
```

## Go

No error checking. <lang go>package main

import (

```   "fmt"
"math"
"strconv"
"strings"
```

)

var input = "3 4 2 * 1 5 - 2 3 ^ ^ / +"

func main() {

```   fmt.Printf("For postfix %q\n", input)
fmt.Println("\nToken            Action            Stack")
var stack []float64
for _, tok := range strings.Fields(input) {
action := "Apply op to top of stack"
switch tok {
case "+":
stack[len(stack)-2] += stack[len(stack)-1]
stack = stack[:len(stack)-1]
case "-":
stack[len(stack)-2] -= stack[len(stack)-1]
stack = stack[:len(stack)-1]
case "*":
stack[len(stack)-2] *= stack[len(stack)-1]
stack = stack[:len(stack)-1]
case "/":
stack[len(stack)-2] /= stack[len(stack)-1]
stack = stack[:len(stack)-1]
case "^":
stack[len(stack)-2] =
math.Pow(stack[len(stack)-2], stack[len(stack)-1])
stack = stack[:len(stack)-1]
default:
action = "Push num onto top of stack"
f, _ := strconv.ParseFloat(tok, 64)
stack = append(stack, f)
}
fmt.Printf("%3s    %-26s  %v\n", tok, action, stack)
}
fmt.Println("\nThe final value is", stack[0])
```

}</lang>

Output:
```For postfix "3 4 2 * 1 5 - 2 3 ^ ^ / +"

Token            Action            Stack
3    Push num onto top of stack  [3]
4    Push num onto top of stack  [3 4]
2    Push num onto top of stack  [3 4 2]
*    Apply op to top of stack    [3 8]
1    Push num onto top of stack  [3 8 1]
5    Push num onto top of stack  [3 8 1 5]
-    Apply op to top of stack    [3 8 -4]
2    Push num onto top of stack  [3 8 -4 2]
3    Push num onto top of stack  [3 8 -4 2 3]
^    Apply op to top of stack    [3 8 -4 8]
^    Apply op to top of stack    [3 8 65536]
/    Apply op to top of stack    [3 0.0001220703125]
+    Apply op to top of stack    [3.0001220703125]

The final value is 3.0001220703125
```

## Groovy

<lang groovy>def evaluateRPN(expression) {

```   def stack = [] as Stack
def binaryOp = { action -> return { action.call(stack.pop(), stack.pop()) } }
def actions = [
'+': binaryOp { a, b -> b + a },
'-': binaryOp { a, b -> b - a },
'*': binaryOp { a, b -> b * a },
'/': binaryOp { a, b -> b / a },
'^': binaryOp { a, b -> b ** a }
]
expression.split(' ').each { item ->
def action = actions[item] ?: { item as BigDecimal }
stack.push(action.call())
```
```       println "\$item: \$stack"
}
assert stack.size() == 1 : "Unbalanced Expression: \$expression (\$stack)"
stack.pop()
```

}</lang> Test <lang groovy>println evaluateRPN('3 4 2 * 1 5 - 2 3 ^ ^ / +')</lang>

Output:
```3: [3]
4: [3, 4]
2: [3, 4, 2]
*: [3, 8]
1: [3, 8, 1]
5: [3, 8, 1, 5]
-: [3, 8, -4]
2: [3, 8, -4, 2]
3: [3, 8, -4, 2, 3]
^: [3, 8, -4, 8]
^: [3, 8, 65536]
/: [3, 0.0001220703125]
+: [3.0001220703125]
3.0001220703125```

-- Show results main = mapM_ (\(x, y) -> putStrLn \$ x ++ " ==> " ++ show y) \$ reverse \$ zip b (a:c)

```       where (a, b, c) = solve "3 4 2 * 1 5 - 2 3 ^ ^ / +"
```

-- Solve and report RPN solve = foldl reduce ([], [], []) . words reduce (xs, ps, st) w =

```   if i == Nothing
then (read w:xs, ("Pushing " ++ w):ps, xs:st)
else (([(*),(+),(-),(/),(**)]!!o) a b:zs, ("Performing " ++ w):ps, xs:st)
where   i = elemIndex (head w) "*+-/^"
Just o = i
(b:a:zs) = xs
```

</lang>

Output:
```*Main> main
Pushing 3 ==> [3.0]
Pushing 4 ==> [4.0,3.0]
Pushing 2 ==> [2.0,4.0,3.0]
Performing * ==> [8.0,3.0]
Pushing 1 ==> [1.0,8.0,3.0]
Pushing 5 ==> [5.0,1.0,8.0,3.0]
Performing - ==> [-4.0,8.0,3.0]
Pushing 2 ==> [2.0,-4.0,8.0,3.0]
Pushing 3 ==> [3.0,2.0,-4.0,8.0,3.0]
Performing ^ ==> [8.0,-4.0,8.0,3.0]
Performing ^ ==> [65536.0,8.0,3.0]
Performing / ==> [1.220703125e-4,3.0]
Performing + ==> [3.0001220703125]
*Main>
```

## Icon and Unicon

<lang Icon>procedure main()

```  EvalRPN("3 4 2 * 1 5 - 2 3 ^ ^ / +")
```

end

procedure EvalRPN(expr) #: evaluate (and trace stack) an RPN string

```  stack := []
expr ? until pos(0) do {
tab(many(' '))                         # consume previous seperator
token := tab(upto(' ')|0)              # get token
if token := numeric(token) then {      # ... numeric
push(stack,token)
printf("pushed numeric   %i : %s\n",token,list2string(stack))
}
else {                                 # ... operator
every b|a := pop(stack)             # pop & reverse operands
case token of {
"+"|"-"|"*"|"^"   : push(stack,token(a,b))
"/"               : push(stack,token(real(a),b))
default           : runerr(205,token)
}
printf("applied operator %s : %s\n",token,list2string(stack))
}
}
```

end

procedure list2string(L) #: format list as a string

```  every (s := "[ ") ||:= !L || " "
return s || "]"
```

end</lang>

Output:
```pushed numeric   3 : [ 3 ]
pushed numeric   4 : [ 4 3 ]
pushed numeric   2 : [ 2 4 3 ]
applied operator * : [ 8 3 ]
pushed numeric   1 : [ 1 8 3 ]
pushed numeric   5 : [ 5 1 8 3 ]
applied operator - : [ -4 8 3 ]
pushed numeric   2 : [ 2 -4 8 3 ]
pushed numeric   3 : [ 3 2 -4 8 3 ]
applied operator ^ : [ 8 -4 8 3 ]
applied operator ^ : [ 65536 8 3 ]
applied operator / : [ 0.0001220703125 3 ]
applied operator + : [ 3.0001220703125 ]```

## J

Offered operations are all dyadic - having two arguments. So on each step we may either "shift" a number to the stack or "reduce" two topmost stack items to one.

The final verb is monad - it takes single argument, which contains both the input and accumulated stack. First, create initial state of the input: <lang J> a: , <;._1 ' ' , '3 4 2 * 1 5 - 2 3 ^ ^ / +' ┌┬─┬─┬─┬─┬─┬─┬─┬─┬─┬─┬─┬─┬─┐ ││3│4│2│*│1│5│-│2│3│^│^│/│+│ └┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┘</lang> As an example, let's add monadic operation _ which inverses the sign of the stack top element.

We're going to read tokens from input one by one. Each time we read a token, we're checking if it's a number - in this case we put the number to the stack - or an operation - in this case we apply the operation to the stack. The monad which returns 1 for operation and 0 otherwise is "isOp". Dyad, moving input token to the stack, is "doShift", and applying the operation to the stack is "doApply".

There are 6 operations - one monadic "_" and five dyadic "+", "-", "*", "/", "^". For operation, we need to translate input token into operation and apply it to the stack. The dyad which converts the input token to the operation is "dispatch". It uses two miscellaneous adverbs, one for monadic operations - "mo" - and another for dyadic - "dy".

The RPN driver is monad "consume", which handles one token. The output is the state of the program after the token was consumed - stack in the 0th box, and remaining input afterwards. As a side effect, "consume" is going to print the resulting stack, so running "consume" once for each token will produce intermediate states of the stack. <lang J> isOp=: '_+-*/^' e.~ {.@>@{.

```  mo=: 1 :'(}: , u@{:) @ ['
dy=: 1 :'(_2&}. , u/@(_2&{.)) @ ['
dispatch=: (-mo)`(+dy)`(-dy)`(*dy)`(%dy)`(^dy)@.('_+-*/^' i. {.@>@])
doShift=: (<@, ".@>@{.) , }.@]
doApply=: }.@] ,~ [ <@dispatch {.@]
consume=: [: ([ smoutput@>@{.) >@{. doShift`doApply@.(isOp@]) }.
consume ^: (<:@#) a: , <;._1 ' ' , '3 4 2 * 1 5 - 2 3 ^ ^ / +'
```

3 3 4 3 4 2 3 8 3 8 1 3 8 1 5 3 8 _4 3 8 _4 2 3 8 _4 2 3 3 8 _4 8 3 8 65536 3 0.00012207 3.00012 ┌───────┐ │3.00012│ └───────┘

```  consume ^: (<:@#) a: , <;._1 ' ' , '3 _ 4 +'
```

3 _3 _3 4 1 ┌─┐ │1│ └─┘</lang>

### Alternate Implementation

<lang J>rpn=: 3 :0

``` queue=. |.3 :'|.3 :y 0'::]each;: y
op=. 1 :'2 (u~/@:{.,}.)S:0 ,@]'
ops=. +op`(-op)`(*op)`(%op)`(^op)`(,&;)
choose=. ((;:'+-*/^')&i.@[)
,ops@.choose/queue
```

)</lang>

Example use:

<lang J> rpn '3 4 2 * 1 5 - 2 3 ^ ^ / +' 3.00012</lang>

To see intermediate result stacks, use this variant (the only difference is the definition of 'op'):

<lang J>rpnD=: 3 :0

``` queue=. |.3 :'|.3 :y 0'::]each;: y
op=. 1 :'2 (u~/@:{.,}.)S:0 ,@([smoutput)@]'
ops=. +op`(-op)`(*op)`(%op)`(^op)`(,&;)
choose=. ((;:'+-*/^')&i.@[)
,ops@.choose/queue
```

)</lang>

In other words:

<lang J> rpnD '3 4 2 * 1 5 - 2 3 ^ ^ / +' ┌─────┐ │2 4 3│ └─────┘ 5 1 8 3 3 2 _4 8 3 8 _4 8 3 65536 8 3 0.00012207 3 3.00012</lang>

Note that the seed stack is boxed while computed stacks are not. Note that top of stack here is on the left. Note also that adjacent constants are bundled in the parsing phase. Finally, note that the result of rpn (and of rpnD - lines previous to the last line in the rpnD example here are output and not a part of the result) is the final state of the stack - in the general case it may not contain exactly one value.

## Java

Works with: Java version 1.5+

Supports multi-digit numbers and negative numbers. <lang java5>import java.util.LinkedList;

public class RPN{ public static void evalRPN(String expr){ String cleanExpr = cleanExpr(expr); LinkedList<Double> stack = new LinkedList<Double>(); System.out.println("Input\tOperation\tStack after"); for(String token:cleanExpr.split("\\s")){ System.out.print(token+"\t"); Double tokenNum = null; try{ tokenNum = Double.parseDouble(token); }catch(NumberFormatException e){} if(tokenNum != null){ System.out.print("Push\t\t"); stack.push(Double.parseDouble(token+"")); }else if(token.equals("*")){ System.out.print("Operate\t\t"); double secondOperand = stack.pop(); double firstOperand = stack.pop(); stack.push(firstOperand * secondOperand); }else if(token.equals("/")){ System.out.print("Operate\t\t"); double secondOperand = stack.pop(); double firstOperand = stack.pop(); stack.push(firstOperand / secondOperand); }else if(token.equals("-")){ System.out.print("Operate\t\t"); double secondOperand = stack.pop(); double firstOperand = stack.pop(); stack.push(firstOperand - secondOperand); }else if(token.equals("+")){ System.out.print("Operate\t\t"); double secondOperand = stack.pop(); double firstOperand = stack.pop(); stack.push(firstOperand + secondOperand); }else if(token.equals("^")){ System.out.print("Operate\t\t"); double secondOperand = stack.pop(); double firstOperand = stack.pop(); stack.push(Math.pow(firstOperand, secondOperand)); }else{//just in case System.out.println("Error"); return; } System.out.println(stack); } System.out.println("Final answer: " + stack.pop()); }

private static String cleanExpr(String expr){ //remove all non-operators, non-whitespace, and non digit chars return expr.replaceAll("[^\\^\\*\\+\\-\\d/\\s]", ""); }

public static void main(String[] args){ evalRPN("3 4 2 * 1 5 - 2 3 ^ ^ / +"); } }</lang>

Output:
```Input	Operation	Stack after
3	Push		[3.0]
4	Push		[4.0, 3.0]
2	Push		[2.0, 4.0, 3.0]
*	Operate		[8.0, 3.0]
1	Push		[1.0, 8.0, 3.0]
5	Push		[5.0, 1.0, 8.0, 3.0]
-	Operate		[-4.0, 8.0, 3.0]
2	Push		[2.0, -4.0, 8.0, 3.0]
3	Push		[3.0, 2.0, -4.0, 8.0, 3.0]
^	Operate		[8.0, -4.0, 8.0, 3.0]
^	Operate		[65536.0, 8.0, 3.0]
/	Operate		[1.220703125E-4, 3.0]
+	Operate		[3.0001220703125]

## Julia

(This code takes advantage of the fact that all of the operands and functions in the specified RPN syntax are valid Julia expressions, so we can use the built-in `parse` and `eval` functions to turn them into numbers and the corresponding Julia functions.) <lang julia>function rpn(s)

```   stack = Any[]
for op in map(eval, map(parse, split(s)))
if isa(op, Function)
arg2 = pop!(stack)
arg1 = pop!(stack)
push!(stack, op(arg1, arg2))
else
push!(stack, op)
end
println("\$op: ", join(stack, ", "))
end
length(stack) != 1 && error("invalid RPN expression \$s")
return stack[1]
```

end rpn("3 4 2 * 1 5 - 2 3 ^ ^ / +")</lang>

Output:
```3: 3
4: 3, 4
2: 3, 4, 2
*: 3, 8
1: 3, 8, 1
5: 3, 8, 1, 5
-: 3, 8, -4
2: 3, 8, -4, 2
3: 3, 8, -4, 2, 3
^: 3, 8, -4, 8
^: 3, 8, 65536
/: 3, 0.0001220703125
+: 3.0001220703125```

(The return value is also `3.0001220703125`.)

## Liberty BASIC

<lang lb> global stack\$

expr\$ = "3 4 2 * 1 5 - 2 3 ^ ^ / +" print "Expression:" print expr\$ print

print "Input","Operation","Stack after"

stack\$="" token\$ = "#" i = 1 token\$ = word\$(expr\$, i) token2\$ = " "+token\$+" "

do

```   print "Token ";i;": ";token\$,
select case
'operation
case instr("+-*/^",token\$)<>0
print "operate",
op2\$=pop\$()
op1\$=pop\$()
if op1\$=""  then
print "Error: stack empty for ";i;"-th token: ";token\$
end
end if
```
```       op1=val(op1\$)
op2=val(op2\$)
```
```       select case token\$
case "+"
res = op1+op2
case "-"
res = op1-op2
case "*"
res = op1*op2
case "/"
res = op1/op2
case "^"
res = op1^op2
end select
```
```       call push str\$(res)
'default:number
case else
print "push",
call push token\$
end select
print "Stack: ";reverse\$(stack\$)
i = i+1
token\$ = word\$(expr\$, i)
token2\$ = " "+token\$+" "
```

loop until token\$ =""

res\$=pop\$() print print "Result:" ;res\$ extra\$=pop\$() if extra\$<>"" then

```   print "Error: extra things on a stack: ";extra\$
```

end if end

'--------------------------------------- function reverse\$(s\$)

```   reverse\$ = ""
token\$="#"
while token\$<>""
i=i+1
token\$=word\$(s\$,i,"|")
reverse\$ = token\$;" ";reverse\$
wend
```

end function '--------------------------------------- sub push s\$

```   stack\$=s\$+"|"+stack\$    'stack
```

end sub

function pop\$()

```   'it does return empty on empty stack
pop\$=word\$(stack\$,1,"|")
stack\$=mid\$(stack\$,instr(stack\$,"|")+1)
```

end function </lang>

Output:
```Expression:
3 4 2 * 1 5 - 2 3 ^ ^ / +

Input         Operation     Stack after
Token 1: 3    push          Stack:  3
Token 2: 4    push          Stack:  3 4
Token 3: 2    push          Stack:  3 4 2
Token 4: *    operate       Stack:  3 8
Token 5: 1    push          Stack:  3 8 1
Token 6: 5    push          Stack:  3 8 1 5
Token 7: -    operate       Stack:  3 8 -4
Token 8: 2    push          Stack:  3 8 -4 2
Token 9: 3    push          Stack:  3 8 -4 2 3
Token 10: ^   operate       Stack:  3 8 -4 8
Token 11: ^   operate       Stack:  3 8 65536
Token 12: /   operate       Stack:  3 0.12207031e-3
Token 13: +   operate       Stack:  3.00012207

Result:3.00012207
```

## NetRexx

Translation of: Java

<lang NetRexx>/* NetRexx */ options replace format comments java crossref symbols nobinary

numeric digits 20

rpnDefaultExpression = '3 4 2 * 1 5 - 2 3 ^ ^ / +' EODAD = '.*'

parse arg rpnString

if rpnString = '.' then rpnString = rpnDefaultExpression if rpnString = then do

``` say 'Enter numbers or operators [to stop enter' EODAD']:'
loop label rpnloop forever
if rpnval == EODAD then leave rpnloop
rpnString = rpnString rpnval
end rpnloop
end
```

rpnString = rpnString.space(1) say rpnString':' evaluateRPN(rpnString)

return

-- ----------------------------------------------------------------------------- method evaluateRPN(rpnString) public static returns Rexx

``` stack = LinkedList()
op = 0
L = 'L'
R = 'R'
rpnString = rpnString.strip('b')
say 'Input\tOperation\tStack after'
loop label rpn while rpnString.length > 0
parse rpnString token rest
rpnString = rest.strip('b')
say token || '\t\-'
select label tox case token
when '*' then do
say 'Operate\t\t\-'
op[R] = Rexx stack.pop()
op[L] = Rexx stack.pop()
stack.push(op[L] * op[R])
end
when '/' then do
say 'Operate\t\t\-'
op[R] = Rexx stack.pop()
op[L] = Rexx stack.pop()
stack.push(op[L] / op[R])
end
when '+' then do
say 'Operate\t\t\-'
op[R] = Rexx stack.pop()
op[L] = Rexx stack.pop()
stack.push(op[L] + op[R])
end
when '-' then do
say 'Operate\t\t\-'
op[R] = Rexx stack.pop()
op[L] = Rexx stack.pop()
stack.push(op[L] - op[R])
end
when '^' then do
say 'Operate\t\t\-'
op[R] = Rexx stack.pop()
op[L] = Rexx stack.pop()
-- If exponent is a whole number use Rexx built-in exponentiation operation, otherwise use Math.pow()
op[R] = op[R] + 0
if op[R].datatype('w') then stack.push(op[L] ** op[R])
else stack.push(Rexx Math.pow(op[L], op[R]))
end
otherwise do
if token.datatype('n') then do
say 'Push\t\t\-'
stack.push(token)
end
else do
say 'Error\t\t\-'
end
end
end tox
calc = Rexx
say stack.toString
end rpn
say
calc = stack.toString
return calc
```

</lang>

Output:
```Input	Operation	Stack after
3	Push		[3]
4	Push		[4, 3]
2	Push		[2, 4, 3]
*	Operate		[8, 3]
1	Push		[1, 8, 3]
5	Push		[5, 1, 8, 3]
-	Operate		[-4, 8, 3]
2	Push		[2, -4, 8, 3]
3	Push		[3, 2, -4, 8, 3]
^	Operate		[8, -4, 8, 3]
^	Operate		[65536, 8, 3]
/	Operate		[0.0001220703125, 3]
+	Operate		[3.0001220703125]

3 4 2 * 1 5 - 2 3 ^ ^ / +: [3.0001220703125]
```

## Nim

Translation of: Python

<lang nim>import math, rdstdin, strutils, tables

type Stack = seq[float]

proc lalign(s, x): string =

``` s & repeatChar(x - s.len, ' ')
```

proc opPow(s: var Stack) =

``` let b = s.pop
let a = s.pop
```

proc opMul(s: var Stack) =

``` let b = s.pop
let a = s.pop
```

proc opDiv(s: var Stack) =

``` let b = s.pop
let a = s.pop
```

``` let b = s.pop
let a = s.pop
```

proc opSub(s: var Stack) =

``` let b = s.pop
let a = s.pop
```

proc opNum(s: var Stack, num) = s.add num

let ops = toTable({"^": opPow,

```                  "*": opMul,
"/": opDiv,
"-": opSub})
```

proc getInput(inp = ""): seq[string] =

``` var inp = inp
if inp.len == 0:
result = inp.strip.split
```

proc rpnCalc(tokens): auto =

``` var s: Stack = @[]
result = @[@["TOKEN","ACTION","STACK"]]
for token in tokens:
var action = ""
action = "Apply op to top of stack"
ops[token](s)
else:
action = "Push num onto top of stack"
s.opNum token.parseFloat
result.add(@[token, action, s.map(proc (x: float): string = \$x).join(" ")])
```

let rpn = "3 4 2 * 1 5 - 2 3 ^ ^ / +" echo "For RPN expression: ", rpn let rp = rpnCalc rpn.getInput

var maxColWidths = newSeq[int](rp[0].len) for i in 0 .. rp[0].high:

``` for x in rp:
maxColWidths[i] = max(maxColWidths[i], x[i].len)
```

for x in rp:

``` for i, y in x:
stdout.write y.lalign(maxColWidths[i]), " "
echo ""</lang>
```
Output:
```For RPN expression: 3 4 2 * 1 5 - 2 3 ^ ^ / +
TOKEN ACTION                     STACK
3     Push num onto top of stack 3.0
4     Push num onto top of stack 3.0 4.0
2     Push num onto top of stack 3.0 4.0 2.0
*     Apply op to top of stack   3.0 8.0
1     Push num onto top of stack 3.0 8.0 1.0
5     Push num onto top of stack 3.0 8.0 1.0 5.0
-     Apply op to top of stack   3.0 8.0 -4.0
2     Push num onto top of stack 3.0 8.0 -4.0 2.0
3     Push num onto top of stack 3.0 8.0 -4.0 2.0 3.0
^     Apply op to top of stack   3.0 8.0 -4.0 8.0
^     Apply op to top of stack   3.0 8.0 65536.0
/     Apply op to top of stack   3.0 0.0001220703125
+     Apply op to top of stack   3.0001220703125```

## Objeck

<lang objeck> use IO; use Struct;

bundle Default {

``` class RpnCalc {
function : Main(args : String[]) ~ Nil {
Caculate("3 4 2 * 1 5 - 2 3 ^ ^ / +");
}

function : native : Caculate(rpn : String) ~ Nil {
rpn->PrintLine();

tokens := rpn->Split(" ");
stack := FloatVector->New();
each(i : tokens) {
token := tokens[i]->Trim();
if(token->Size() > 0) {
if(token->Get(0)->IsDigit()) {
}
else {
right := stack->Get(stack->Size() - 1); stack->RemoveBack();
left := stack->Get(stack->Size() - 1); stack->RemoveBack();
select(token->Get(0)) {
label '+': {
}
```
```             label '-': {
}
```
```             label '*': {
}
```
```             label '/': {
}
```
```             label '^': {
}
};
};
PrintStack(stack);
};
};
Console->Print("result: ")->PrintLine(stack->Get(0));
}
```
```   function : PrintStack(stack : FloatVector) ~ Nil {
"  ["->Print();
each(i : stack) {
stack->Get(i)->Print();
if(i + 1< stack->Size()) {
", "->Print();
};
};
']'->PrintLine();
}
}
```

} </lang>

Output:
```3 4 2 * 1 5 - 2 3 ^ ^ / +
[3]
[3, 4]
[3, 4, 2]
[3, 8]
[3, 8, 1]
[3, 8, 1, 5]
[3, 8, -4]
[3, 8, -4, 2]
[3, 8, -4, 2, 3]
[3, 8, -4, 8]
[3, 8, 65536]
[3, 0.00012207]
[3.00012]
result: 3.00012
```

## OCaml

<lang ocaml>(* binop : ('a -> 'a -> 'a) -> 'a list -> 'a list *) let binop op = function

``` | b::a::r -> (op a b)::r
| _ -> failwith "invalid expression"
```

(* interp : float list -> string -> string * float list *) let interp s = function

``` | "+" -> "add",    binop ( +. ) s
| "-" -> "subtr",  binop ( -. ) s
| "*" -> "mult",   binop ( *. ) s
| "/" -> "divide", binop ( /. ) s
| "^" -> "exp",    binop ( ** ) s
| str -> "push", (float_of_string str) :: s
```

(* interp_and_show : float list -> string -> float list *) let interp_and_show s inp =

``` let op,s' = interp s inp in
Printf.printf "%s\t%s\t" inp op;
List.(iter (Printf.printf "%F ") (rev s'));
print_newline ();
s'
```

(* rpn_eval : string -> float list *) let rpn_eval str =

``` Printf.printf "Token\tAction\tStack\n";
let ss = Str.(split (regexp_string " ") str) in
List.fold_left interp_and_show [] ss</lang>
```

Evaluation of the test expression:

```# rpn_eval "3 4 2 * 1 5 - 2 3 ^ ^ / +";;
Token	Action	Stack
3	push	3.
4	push	3. 4.
2	push	3. 4. 2.
*	mult	3. 8.
1	push	3. 8. 1.
5	push	3. 8. 1. 5.
-	subtr	3. 8. -4.
2	push	3. 8. -4. 2.
3	push	3. 8. -4. 2. 3.
^	exp	3. 8. -4. 8.
^	exp	3. 8. 65536.
/	divide	3. 0.0001220703125
- : float list = [3.0001220703125]
```

## ooRexx

<lang ooRexx>/* ooRexx *************************************************************

• 10.11.2012 Walter Pachl translated from PL/I via REXX
• /

fid='rpl.txt' ex=linein(fid) Say 'Input:' ex /* ex=' 3 4 2 * 1 5 - 2 3 ^ ^ / +' */ Numeric Digits 15 expr= st=.circularqueue~new(100) Say 'Stack contents:' do While ex<>

``` Parse Var ex ch +1 ex
expr=expr||ch;
if ch<>' ' then do
If pos(ch,'0123456789')>0 Then     /* a digit goes onto stack    */
st~push(ch)
Else Do                            /* an operator                */
op=st~pull                       /* get top element            */
select                           /* and modify the (now) top el*/
when ch='+' Then st~push(st~pull +  op)
when ch='-' Then st~push(st~pull -  op)
when ch='*' Then st~push(st~pull *  op)
when ch='/' Then st~push(st~pull /  op)
when ch='^' Then st~push(st~pull ** op)
end;
Say st~string(' ','L')           /* show stack in LIFO order   */
end
end
end
```

Say 'The reverse polish expression = 'expr Say 'The evaluated expression = 'st~pull</lang>

Output:
```Input: 3 4 2 * 1 5 - 2 3 ^ ^ / +
Stack contents:
3 8
3 8 -4
3 8 -4 8
3 8 65536
3 0.0001220703125
3.0001220703125
The reverse polish expression = 3 4 2 * 1 5 - 2 3 ^ ^ / +
The evaluated expression = 3.0001220703125
```

## Perl

<lang Perl>

1. RPN calculator
2. Nigel Galloway April 2nd., 2012

\$WSb = '(?:^|\s+)'; \$WSa = '(?:\s+|\$)'; \$num = '([+-/]?(?:\.\d+|\d+(?:\.\d*)?))'; \$op = '([-+*/^])'; sub myE {

``` my \$a = '('.\$1.')'.\$3.'('.\$2.')';
\$a =~ s/\^/**/;
return eval(\$a);
```

} while (<>) {

``` while (s/\$WSb\$num\s+\$num\s+\$op\$WSa/' '.myE().' '/e)  {}
print (\$_, "\n");
```

} </lang> Produces:

```>rpnC.pl
3 4 2 * 1 5 - 2 3 ^ ^ / +
3.0001220703125
```

## Perl 6

Works with: niecza version 2012-07-28

<lang perl6>my \$proggie = '3 4 2 * 1 5 - 2 3 ^ ^ / +';

class RPN is Array {

```   method binop(&infix:<op>) { self.push: self.pop Rop self.pop }
```
```   method run(\$p) {
for \$p.words {
say "\$_ ({self})";
when /\d/ { self.push: \$_ }
when '+'  { self.binop: &[+] }
when '-'  { self.binop: &[-] }
when '*'  { self.binop: &[*] }
when '/'  { self.binop: &[/] }
when '^'  { self.binop: &[**] }
default   { die "\$_ is bogus" }
}
say self;
}
```

}

RPN.new.run(\$proggie);</lang>

Output:
```3 ()
4 (3)
2 (3 4)
* (3 4 2)
1 (3 8)
5 (3 8 1)
- (3 8 1 5)
2 (3 8 -4)
3 (3 8 -4 2)
^ (3 8 -4 2 3)
^ (3 8 -4 8)
/ (3 8 65536)
+ (3 0.0001220703125)
3.0001220703125```

## PHP

<lang php> <?php function rpn(\$postFix){

```   \$stack = Array();
echo "Input\tOperation\tStack\tafter\n" ;
```

\$token = explode(" ", trim(\$postFix)); \$count = count(\$token);

```   for(\$i = 0 ; \$i<\$count;\$i++)
```

{

```       echo \$token[\$i] ." \t";
\$tokenNum = "";
```
```       if (is_numeric(\$token[\$i])) {
echo  "Push";
```

array_push(\$stack,\$token[\$i]);

```       }
else
{
echo "Operate";
\$secondOperand = end(\$stack);
```

array_pop(\$stack);

```           \$firstOperand = end(\$stack);
array_pop(\$stack);
```
```           if (\$token[\$i] == "*")
```

array_push(\$stack,\$firstOperand * \$secondOperand);

```           else if (\$token[\$i] == "/")
array_push(\$stack,\$firstOperand / \$secondOperand);
else if (\$token[\$i] == "-")
array_push(\$stack,\$firstOperand - \$secondOperand);
else if (\$token[\$i] == "+")
array_push(\$stack,\$firstOperand + \$secondOperand);
else if (\$token[\$i] == "^")
array_push(\$stack,pow(\$firstOperand,\$secondOperand));
else {
die("Error");
}
}
```

echo "\t\t" . implode(" ", \$stack) . "\n";

```   }
return end(\$stack);
```

}

echo "Compute Value: " . rpn("3 4 2 * 1 5 - 2 3 ^ ^ / + "); ?> </lang>

Output:
```Input	Operation	Stack	after
3 	Push		3
4 	Push		3 4
2 	Push		3 4 2
* 	Operate		3 8
1 	Push		3 8 1
5 	Push		3 8 1 5
- 	Operate		3 8 -4
2 	Push		3 8 -4 2
3 	Push		3 8 -4 2 3
^ 	Operate		3 8 -4 8
^ 	Operate		3 8 65536
/ 	Operate		3 0.0001220703125
+ 	Operate		3.0001220703125
Compute Value: 3.0001220703125
```

## PicoLisp

This is an integer-only calculator: <lang PicoLisp>(de rpnCalculator (Str)

```  (let (^ **  Stack)  # Define '^' from the built-in '**'
(prinl "Token  Stack")
(for Token (str Str "*+-/\^")
(if (num? Token)
(push 'Stack @)
(set (cdr Stack)
((intern Token) (cadr Stack) (pop 'Stack)) ) )
(prin Token)
(space 6)
(println Stack) )
(println (car Stack)) ) )</lang>
```

Test (note that the top-of-stack is in the left-most position): <lang PicoLisp>: (rpnCalculator "3 4 2 * 1 5 - 2 3 \^ \^ / +") Token Stack 3 (3) 4 (4 3) 2 (2 4 3)

• (8 3)

1 (1 8 3) 5 (5 1 8 3) - (-4 8 3) 2 (2 -4 8 3) 3 (3 2 -4 8 3) ^ (8 -4 8 3) ^ (65536 8 3) / (0 3) + (3) 3 -> 3</lang>

## PL/I

<lang PL/I>Calculator: procedure options (main); /* 14 Sept. 2012 */

```  declare expression character (100) varying initial ();
declare ch character (1);
declare (stack controlled, operand) float (18);
declare in file input;
```
```  open file (in) title ('/CALCULAT.DAT,type(text),recsize(100)');
on endfile (in) go to done;
```
```  put ('Stack contents:');
```

main_loop:

```  do forever;
get file (in) edit (ch) (a(1));
expression = expression || ch;
if ch = ' ' then iterate;
select (ch);
when ('0', '1', '2', '3', '4', '5', '6', '7', '8', '9')
do; allocate stack; stack = ch; iterate main_loop; end;
when ('+') do; operand = stack; free stack; stack = stack +  operand; end;
when ('-') do; operand = stack; free stack; stack = stack -  operand; end;
when ('*') do; operand = stack; free stack; stack = stack *  operand; end;
when ('/') do; operand = stack; free stack; stack = stack /  operand; end;
when ('^') do; operand = stack; free stack; stack = stack ** operand; end;
end;
call show_stack;
end;
```

done:

```  put skip list ('The reverse polish expression = ' || expression);
put skip list ('The evaluated expression = ' || stack);
```

end Calculator;</lang>

```Stack contents:
3.0000000000      8.0000000000
3.0000000000      8.0000000000     -4.0000000000
3.0000000000      8.0000000000     -4.0000000000      8.0000000000
3.0000000000      8.0000000000  65536.0000000000
3.0000000000      0.0001220703
3.0001220703
The reverse polish expression = 3 4 2 * 1 5 - 2 3 ^ ^ / +
The evaluated expression =  3.00012207031250000E+0000
```

The procedure to display the stack:

```/* As the stack is push-down pop-up, need to pop it to see what's inside. */
show_stack: procedure;
declare ts float (18) controlled;

do while (allocation(stack) > 0);
allocate ts; ts = stack; free stack;
end;
put skip;
do while (allocation(ts) > 0);
allocate stack; stack = ts; free ts; put edit (stack) (f(18,10));
end;
end show_stack;```

## Prolog

Works with SWI-Prolog. <lang Prolog>rpn(L) :- writeln('Token Action Stack'), parse(L, [],[X] ,[]), format('~nThe final output value is ~w~n', [X]).

% skip spaces parse([X|L], St) --> {char_type(X, white)}, parse(L, St).

% detect operators parse([Op|L], [Y, X | St]) --> { is_op(Op, X, Y, V), writef(' %s', Op), with_output_to(atom(Str2), writef('Apply %s on top of stack', Op)), writef(' %35l', [Str2]), writef('%w\n', St)}, parse(L, [V | St]).

% detect number parse([N|L], St) --> {char_type(N, digit)}, parse_number(L, [N], St).

% string is finished parse([], St) --> St.

% compute numbers parse_number([N|L], NC, St) --> {char_type(N, digit)}, parse_number(L, [N|NC], St).

parse_number(S, NC, St) --> { reverse(NC, RNC), number_chars(V, RNC), writef('%5r', [V]), with_output_to(atom(Str2), writef('Push num %w on top of stack', [V])), writef(' %35l', [Str2]), writef('%w\n', St)}, parse(S, [V|St]).

% defining operations is_op(42, X, Y, V) :- V is X*Y. is_op(43, X, Y, V) :- V is X+Y. is_op(45, X, Y, V) :- V is X-Y. is_op(47, X, Y, V) :- V is X/Y. is_op(94, X, Y, V) :- V is X**Y.</lang>

Output:
```5 ?- rpn("3 4 2 * 1 5 - 2 3 ^ ^ / +").
Token  Action                             Stack
3  'Push num 3 on top of stack'       [3]
4  'Push num 4 on top of stack'       [4,3]
2  'Push num 2 on top of stack'       [2,4,3]
*  'Apply * on top of stack'          [8,3]
1  'Push num 1 on top of stack'       [1,8,3]
5  'Push num 5 on top of stack'       [5,1,8,3]
-  'Apply - on top of stack'          [-4,8,3]
2  'Push num 2 on top of stack'       [2,-4,8,3]
3  'Push num 3 on top of stack'       [3,2,-4,8,3]
^  'Apply ^ on top of stack'          [8,-4,8,3]
^  'Apply ^ on top of stack'          [65536,8,3]
/  'Apply / on top of stack'          [0.0001220703125,3]
+  'Apply + on top of stack'          [3.0001220703125]

The final output value is 3.0001220703125
true .```

## Python

<lang python>def op_pow(stack):

```   b = stack.pop(); a = stack.pop()
stack.append( a ** b )
```

def op_mul(stack):

```   b = stack.pop(); a = stack.pop()
stack.append( a * b )
```

def op_div(stack):

```   b = stack.pop(); a = stack.pop()
stack.append( a / b )
```

```   b = stack.pop(); a = stack.pop()
stack.append( a + b )
```

def op_sub(stack):

```   b = stack.pop(); a = stack.pop()
stack.append( a - b )
```

def op_num(stack, num):

```   stack.append( num )

```

ops = {

```'^': op_pow,
'*': op_mul,
'/': op_div,
'-': op_sub,
}
```

def get_input(inp = None):

```   'Inputs an expression and returns list of tokens'

if inp is None:
inp = input('expression: ')
tokens = inp.strip().split()
```

def rpn_calc(tokens):

```   stack = []
table = ['TOKEN,ACTION,STACK'.split(',')]
for token in tokens:
if token in ops:
action = 'Apply op to top of stack'
ops[token](stack)
table.append( (token, action, ' '.join(str(s) for s in stack)) )
else:
action = 'Push num onto top of stack'
op_num(stack, eval(token))
table.append( (token, action, ' '.join(str(s) for s in stack)) )
return table
```

if __name__ == '__main__':

```   rpn = '3 4 2 * 1 5 - 2 3 ^ ^ / +'
print( 'For RPN expression: %r\n' % rpn )
rp = rpn_calc(get_input(rpn))
maxcolwidths = [max(len(y) for y in x) for x in zip(*rp)]
row = rp[0]
print( ' '.join('{cell:^{width}}'.format(width=width, cell=cell) for (width, cell) in zip(maxcolwidths, row)))
for row in rp[1:]:
print( ' '.join('{cell:<{width}}'.format(width=width, cell=cell) for (width, cell) in zip(maxcolwidths, row)))
```
```   print('\n The final output value is: %r' % rp[-1][2])</lang>
```
Output:
```For RPN expression: '3 4 2 * 1 5 - 2 3 ^ ^ / +'

TOKEN           ACTION                 STACK
3     Push num onto top of stack 3
4     Push num onto top of stack 3 4
2     Push num onto top of stack 3 4 2
*     Apply op to top of stack   3 8
1     Push num onto top of stack 3 8 1
5     Push num onto top of stack 3 8 1 5
-     Apply op to top of stack   3 8 -4
2     Push num onto top of stack 3 8 -4 2
3     Push num onto top of stack 3 8 -4 2 3
^     Apply op to top of stack   3 8 -4 8
^     Apply op to top of stack   3 8 65536
/     Apply op to top of stack   3 0.0001220703125
+     Apply op to top of stack   3.0001220703125

The final output value is: '3.0001220703125'```

## Racket

<lang racket>

1. lang racket

(define (calculate-RPN expr)

``` (for/fold ([stack '()]) ([token expr])
(printf "~a\t -> ~a~N" token stack)
(match* (token stack)
[((? number? n) s) (cons n s)]
[('+ (list x y s ___)) (cons (+ x y) s)]
[('- (list x y s ___)) (cons (- y x) s)]
[('* (list x y s ___)) (cons (* x y) s)]
[('/ (list x y s ___)) (cons (/ y x) s)]
[('^ (list x y s ___)) (cons (expt y x) s)]
[(x s) (error "calculate-RPN: Cannot calculate the expression:"
(reverse (cons x s)))])))
```

</lang> Test case

```-> (calculate-RPN '(3.0 4 2 * 1 5 - 2 3 ^ ^ / +))
3.0	 -> ()
4	 -> (3.0)
2	 -> (4 3.0)
*	 -> (2 4 3.0)
1	 -> (8 3.0)
5	 -> (1 8 3.0)
-	 -> (5 1 8 3.0)
2	 -> (-4 8 3.0)
3	 -> (2 -4 8 3.0)
^	 -> (3 2 -4 8 3.0)
^	 -> (8 -4 8 3.0)
/	 -> (65536 8 3.0)
+	 -> (1/8192 3.0)
3.0001220703125
```

Reading from a string: <lang racket> (calculate-RPN (in-port read (open-input-string "3.0 4 2 * 1 5 - 2 3 ^ ^ / +"))) </lang>

## REXX

### version 1

<lang rexx>/* REXX ***************************************************************

• 09.11.2012 Walter Pachl translates from PL/I
• /

fid='rpl.txt' ex=linein(fid) Say 'Input:' ex /* ex=' 3 4 2 * 1 5 - 2 3 ^ ^ / +' */ Numeric Digits 15 expr= st.=0 Say 'Stack contents:' do While ex<>

``` Parse Var ex ch +1 ex
expr=expr||ch;
if ch<>' ' then do
select
When pos(ch,'0123456789')>0 Then Do
Call stack ch
Iterate
End
when ch='+' Then do; operand=getstack(); st.sti = st.sti +  operand; end;
when ch='-' Then do; operand=getstack(); st.sti = st.sti -  operand; end;
when ch='*' Then do; operand=getstack(); st.sti = st.sti *  operand; end;
when ch='/' Then do; operand=getstack(); st.sti = st.sti /  operand; end;
when ch='^' Then do; operand=getstack(); st.sti = st.sti ** operand; end;
end;
call show_stack
end
end
```

Say 'The reverse polish expression = 'expr Say 'The evaluated expression = 'st.1 Exit stack: Procedure Expose st. /* put the argument on top of the stack */

``` z=st.0+1
st.z=arg(1)
st.0=z
Return
```

getstack: Procedure Expose st. sti /* remove and return the stack's top element */

``` z=st.0
stk=st.z
st.0=st.0-1
sti=st.0
Return stk
```

show_stack: procedure Expose st. /* show the stack's contents */

``` ol=
do i=1 To st.0
ol=ol format(st.i,5,10)
End
Say ol
Return</lang>
```
Output:
```Input: 3 4 2 * 1 5 - 2 3 ^ ^ / +
Stack contents:
3.0000000000     8.0000000000
3.0000000000     8.0000000000    -4.0000000000
3.0000000000     8.0000000000    -4.0000000000     8.0000000000
3.0000000000     8.0000000000 65536.0000000000
3.0000000000     0.0001220703
3.0001220703
The reverse polish expression = 3 4 2 * 1 5 - 2 3 ^ ^ / +
The evaluated expression = 3.0001220703125
```

### version 2

This REXX version handles tokens (not characters). <lang REXX>/*REXX program evaluates a Reverse Polish notation (RPN) expression.*/ parse arg x; if x= then x = '3 4 2 * 1 5 - 2 3 ^ ^ / +'; ox=x showSteps=1 /*set to 0 (zero) if working steps not wanted.*/ x=space(x); tokens=words(x)

``` do i=1  for tokens;  @.i=word(x,i);  end /*i*/   /*assign input tokens*/
```

L=max(20,length(x)) /*use 20 for the min show width. */ numeric digits L /*ensure enough digits for answer*/ say center('operand',L,'─') center('stack',L*2,'─'); e='***error!***' op='- + / * ^'; add2s='add to───►stack'; z=; stack=

``` do #=1  for tokens;   ?=@.#;   ??=?  /*process each token from @. list*/
w=words(stack)                              /*stack count (# entries).*/
if datatype(?,'N') then do; stack=stack ?; call show add2s; iterate; end
if ?=='^'          then ??="**"      /*REXXify  ^ ──► **  (make legal)*/
interpret 'y=' word(stack,w-1) ?? word(stack,w)             /*compute.*/
if datatype(y,'W') then y=y/1        /*normalize the number with  ÷   */
_=subword(stack,1,w-2);   stack=_ y  /*rebuild the stack with answer. */
call show ?
end   /*#*/
```

z=space(z stack) /*append any residual entries. */ say; say ' RPN input:' ox; say ' answer──►' z /*show input & ans.*/ parse source upper . y . /*invoked via C.L. or REXX pgm?*/ if y=='COMMAND' | \datatype(z,'W') then exit /*stick a fork in it, done.*/

```                                  else return z   /*RESULT ──► invoker.*/
```

/*──────────────────────────────────SHOW subroutine─────────────────────*/ show: if showSteps then say center(arg(1),L) left(space(stack),L); return</lang>

Output:
when using the default input
```─────────operand───────── ──────────────────────stack───────────────────────
*             3 8
add to───►stack      3 8 1 5
-             3 8 -4
add to───►stack      3 8 -4 2
add to───►stack      3 8 -4 2 3
^             3 8 -4 8
^             3 8 65536
/             3 0.0001220703125
+             3.0001220703125

RPN input: 3 4 2 * 1 5 - 2 3 ^ ^ / +
```

### version 3 (error checking)

This REXX version is the same as above, but also checks for various errors and allows more operators:

• checks for illegal operator
• checks for illegal number
• checks for illegal bit (logical) values
• checks for malformed RPN expression
• checks for division by zero
• allows alternative exponentiation symbol   **
• allows logical operations   &   &&   |
• allows alternative division symbol   ÷
• allows integer division   %
• allows remainder division   //
• allows concatenation   ||

<lang REXX>/*REXX program evaluates a Reverse Polish notation (RPN) expression.*/ parse arg x; if x= then x = '3 4 2 * 1 5 - 2 3 ^ ^ / +'; ox=x showSteps=1 /*set to 0 (zero) if working steps not wanted.*/ x=space(x); tokens=words(x) /*elide extra blanks;count tokens*/

``` do i=1  for tokens;  @.i=word(x,i);  end /*i*/   /*assign input tokens*/
```

L=max(20,length(x)) /*use 20 for the min show width. */ numeric digits L /*ensure enough digits for answer*/ say center('operand',L,'─') center('stack',L*2,'─'); e='***error!***' add2s='add to───►stack'; z=; stack= dop='/ // % ÷'; bop='& | &&' /*division ops; binary operands*/ aop='- + * ^ **' dop bop; lop=aop '||' /*arithmetic ops; legal operands*/

``` do #=1  for tokens;   ?=@.#;  ??=?   /*process each token from @. list*/
w=words(stack);  b=word(stack,max(1,w))     /*stack count; last entry.*/
a=word(stack,max(1,w-1))   /*stack's "first" operand.*/
division  =wordpos(?,dop)\==0               /*flag:  doing a division.*/
arith     =wordpos(?,aop)\==0               /*flag:  doing arithmetic.*/
bitOp     =wordpos(?,bop)\==0               /*flag:  doing binary math*/
if datatype(?,'N') then do; stack=stack ?; call show add2s; iterate; end
if wordpos(?,lop)==0 then do; z=e 'illegal operator:' ?;      leave; end
if w<2               then do; z=e 'illegal RPN expression.';  leave; end
if ?=='^'            then ??="**"    /*REXXify  ^ ──► **  (make legal)*/
if ?=='÷'            then ??="/"     /*REXXify  ÷ ──► /   (make legal)*/
if division  &  b=0  then do; z=e 'division by zero: '    b;  leave; end
if bitOp & \isBit(a) then do; z=e "token isn't logical: " a;  leave; end
if bitOp & \isBit(b) then do; z=e "token isn't logical: " b;  leave; end
interpret 'y=' a ?? b                /*compute with two stack operands*/
if datatype(y,'W')   then y=y/1      /*normalize number with  ÷  by 1.*/
_=subword(stack,1,w-2);   stack=_ y  /*rebuild the stack with answer. */
call show ?
end   /*#*/
```

if word(z,1)==e then stack= /*handle special case of errors. */ z=space(z stack) /*append any residual entries. */ say; say ' RPN input:' ox; say ' answer──►' z /*show input & ans.*/ parse source upper . how . /*invoked via C.L. or REXX pgm?*/ if how=='COMMAND' | ,

```  \datatype(z,'W') then exit          /*stick a fork in it, we're done.*/
```

return z /*return Z ──► invoker (RESULT).*/ /*──────────────────────────────────subroutines─────────────────────────*/ isBit: return arg(1)==0 | arg(1)==1 /*returns 1 if arg1 is bin bit.*/ show: if showSteps then say center(arg(1),L) left(space(stack),L); return</lang> output is identical to version 2.

## Ruby

See Parsing/RPN/Ruby <lang ruby>rpn = RPNExpression("3 4 2 * 1 5 - 2 3 ^ ^ / +") value = rpn.eval</lang>

Output:
```for RPN expression: 3 4 2 * 1 5 - 2 3 ^ ^ / +
Term	Action	Stack
3	PUSH	[3]
4	PUSH	[3, 4]
2	PUSH	[3, 4, 2]
*	MUL	[3, 8]
1	PUSH	[3, 8, 1]
5	PUSH	[3, 8, 1, 5]
-	SUB	[3, 8, -4]
2	PUSH	[3, 8, -4, 2]
3	PUSH	[3, 8, -4, 2, 3]
^	EXP	[3, 8, -4, 8]
^	EXP	[3, 8, 65536]
/	DIV	[3, 0.0001220703125]
Value = 3.0001220703125```

## Run BASIC

<lang runbasic>prn\$ = "3 4 2 * 1 5 - 2 3 ^ ^ / + "

j = 0 while word\$(prn\$,i + 1," ") <> "" i = i + 1

``` n\$ = word\$(prn\$,i," ")
if n\$ < "0" or n\$ > "9" then
num1   = val(word\$(stack\$,s," "))
num2   = val(word\$(stack\$,s-1," "))
n      = op(n\$,num2,num1)
s      = s - 1
stack\$ = stk\$(stack\$,s -1,str\$(n))
print "Push Opr ";n\$;" to stack:  ";stack\$
else
s = s + 1
stack\$ = stack\$ + n\$ + " "
print "Push Num ";n\$;" to stack:  ";stack\$
```

end if wend

function stk\$(stack\$,s,a\$) for i = 1 to s

``` stk\$ = stk\$ + word\$(stack\$,i," ") + " "
```

next i stk\$ = stk\$ + a\$ + " " end function

FUNCTION op(op\$,a,b) if op\$ = "*" then op = a * b if op\$ = "/" then op = a / b if op\$ = "^" then op = a ^ b if op\$ = "+" then op = a + b if op\$ = "-" then op = a - b end function</lang>

```Push Num 3 to stack:  3
Push Num 4 to stack:  3 4
Push Num 2 to stack:  3 4 2
Push Opr * to stack:  3 8
Push Num 1 to stack:  3 8 1
Push Num 5 to stack:  3 8 1 5
Push Opr - to stack:  3 8 -4
Push Num 2 to stack:  3 8 -4 2
Push Num 3 to stack:  3 8 -4 2 3
Push Opr ^ to stack:  3 8 -4 8
Push Opr ^ to stack:  3 8 65536
Push Opr / to stack:  3 1.22070312e-4
Push Opr + to stack:  3.00012207```

## Swift

Translation of: Go

<lang Swift>let opa = [

```   "^": (prec: 4, rAssoc: true),
"*": (prec: 3, rAssoc: false),
"/": (prec: 3, rAssoc: false),
"+": (prec: 2, rAssoc: false),
"-": (prec: 2, rAssoc: false),
```

]

func rpn(tokens: [String]) -> [String] {

```   var rpn : [String] = []
var stack : [String] = [] // holds operators and left parenthesis
```
```   for tok in tokens {
switch tok {
case "(":
stack += [tok] // push "(" to stack
case ")":
while !stack.isEmpty {
let op = stack.removeLast() // pop item from stack
if op == "(" {
} else {
rpn += [op] // add operator to result
}
}
default:
if let o1 = opa[tok] { // token is an operator?
for op in stack.reverse() {
if let o2 = opa[op] {
if !(o1.prec > o2.prec || (o1.prec == o2.prec && o1.rAssoc)) {
// top item is an operator that needs to come off
rpn += [stack.removeLast()] // pop and add it to the result
continue
}
}
break
}
```
```               stack += [tok] // push operator (the new one) to stack
} else { // token is not an operator
rpn += [tok] // add operand to result
}
}
}
```
```   return rpn + stack.reverse()
```

}

func parseInfix(e: String) -> String {

```   return join(" ", rpn(split(e, { \$0 == " " })))
```

}

var input : String

input = "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3" "infix: \(input)" "postfix: \(parseInfix(input))"</lang>

Output:
`"postfix: 3 4 2 * 1 5 - 2 3 ^ ^ / +"`

## Tcl

<lang tcl># Helper proc pop stk {

```   upvar 1 \$stk s
set val [lindex \$s end]
set s [lreplace \$s end end]
return \$val
```

}

proc evaluate rpn {

```   set stack {}
foreach token \$rpn {
```

set act "apply" switch \$token { "^" { # Non-commutative operation set a [pop stack] lappend stack [expr {[pop stack] ** \$a}] } "/" { # Non-commutative, special float handling set a [pop stack] set b [expr {[pop stack] / double(\$a)}] if {\$b == round(\$b)} {set b [expr {round(\$b)}]} lappend stack \$b } "*" { # Commutative operation lappend stack [expr {[pop stack] * [pop stack]}] } "-" { # Non-commutative operation set a [pop stack] lappend stack [expr {[pop stack] - \$a}] } "+" { # Commutative operation lappend stack [expr {[pop stack] + [pop stack]}] } default { set act "push" lappend stack \$token } } puts "\$token\t\$act\t\$stack"

```   }
return [lindex \$stack end]
```

}

puts [evaluate {3 4 2 * 1 5 - 2 3 ^ ^ / +}]</lang>

Output:
```3	push	3
4	push	3 4
2	push	3 4 2
*	apply	3 8
1	push	3 8 1
5	push	3 8 1 5
-	apply	3 8 -4
2	push	3 8 -4 2
3	push	3 8 -4 2 3
^	apply	3 8 -4 8
^	apply	3 8 65536
/	apply	3 0.0001220703125
+	apply	3.0001220703125
3.0001220703125
```

## VBA

Translation of: Liberty BASIC

<lang VBA>Global stack\$

Function RPN(expr\$) Debug.Print "Expression:" Debug.Print expr\$ Debug.Print "Input", "Operation", "Stack after"

stack\$ = "" token\$ = "#" i = 1 token\$ = Split(expr\$)(i - 1) 'split is base 0 token2\$ = " " + token\$ + " "

Do

```   Debug.Print "Token "; i; ": "; token\$,
'operation
If InStr("+-*/^", token\$) <> 0 Then
Debug.Print "operate",
op2\$ = pop\$()
op1\$ = pop\$()
If op1\$ = "" Then
Debug.Print "Error: stack empty for "; i; "-th token: "; token\$
End
End If

op1 = Val(op1\$)
op2 = Val(op2\$)

Select Case token\$
Case "+"
res = CDbl(op1) + CDbl(op2)
Case "-"
res = CDbl(op1) - CDbl(op2)
Case "*"
res = CDbl(op1) * CDbl(op2)
Case "/"
res = CDbl(op1) / CDbl(op2)
Case "^"
res = CDbl(op1) ^ CDbl(op2)
End Select

Call push2(str\$(res))
'default:number
Else
Debug.Print "push",
Call push2(token\$)
End If
Debug.Print "Stack: "; reverse\$(stack\$)
i = i + 1
If i > Len(Join(Split(expr, " "), "")) Then
token\$ = ""
Else
token\$ = Split(expr\$)(i - 1) 'base 0
token2\$ = " " + token\$ + " "
End If
```

Loop Until token\$ = ""

Debug.Print Debug.Print "Result:"; pop\$() 'extra\$ = pop\$() If stack <> "" Then

```   Debug.Print "Error: extra things on a stack: "; stack\$
```

End If End End Function

'--------------------------------------- Function reverse\$(s\$)

```   reverse\$ = ""
token\$ = "#"
While token\$ <> ""
i = i + 1
token\$ = Split(s\$, "|")(i - 1) 'split is base 0
reverse\$ = token\$ & " " & reverse\$
Wend
```

End Function '--------------------------------------- Sub push2(s\$)

```   stack\$ = s\$ + "|" + stack\$ 'stack
```

End Sub

Function pop\$()

```   'it does return empty on empty stack
pop\$ = Split(stack\$, "|")(0)
stack\$ = Mid\$(stack\$, InStr(stack\$, "|") + 1)
```

End Function</lang>

Output:
```?RPN("3 4 2 * 1 5 - 2 3 ^ ^ / +")
Expression:
3 4 2 * 1 5 - 2 3 ^ ^ / +
Input         Operation     Stack after
Token  1 : 3  push          Stack:  3
Token  2 : 4  push          Stack:  3 4
Token  3 : 2  push          Stack:  3 4 2
Token  4 : *  operate       Stack:  3  8
Token  5 : 1  push          Stack:  3  8 1
Token  6 : 5  push          Stack:  3  8 1 5
Token  7 : -  operate       Stack:  3  8 -4
Token  8 : 2  push          Stack:  3  8 -4 2
Token  9 : 3  push          Stack:  3  8 -4 2 3
Token  10 : ^ operate       Stack:  3  8 -4  8
Token  11 : ^ operate       Stack:  3  8  65536
Token  12 : / operate       Stack:  3  .0001220703125
Token  13 : + operate       Stack:   3.0001220703125

Result: 3.0001220703125```

## zkl

<lang zkl>var ops=D("^",True, "*",'*, "/",'/, "+",'+, "-",'-);

fcn parseRPN(e){

```  println("\npostfix: ", e);
stack:=L();
foreach tok in (e.split()){
op:=ops.find(tok);
if(op){
```

y := stack.pop(); x := stack.pop(); if(True==op) x=x.pow(y); else x=op(x,y); stack.append(x);

```     }
else stack.append(tok.toFloat());
println(tok," --> ",stack);
}
println("result: ", stack[0])
```

}</lang> <lang zkl>tests:=T("3 4 2 * 1 5 - 2 3 ^ ^ / +"); foreach t in (tests) { parseRPN(t) }</lang>

Output:
```postfix: 3 4 2 * 1 5 - 2 3 ^ ^ / +
3 --> L(3)
4 --> L(3,4)
2 --> L(3,4,2)
* --> L(3,8)
1 --> L(3,8,1)
5 --> L(3,8,1,5)
- --> L(3,8,-4)
2 --> L(3,8,-4,2)
3 --> L(3,8,-4,2,3)
^ --> L(3,8,-4,8)
^ --> L(3,8,65536)
/ --> L(3,0.00012207)
+ --> L(3.00012)
result: 3.00012
```