Parsing/Shunting-yard algorithm
Given the operator characteristics and input from the Shunting-yard algorithm page and tables Use the algorithm to show the changes in the operator stack and RPN output as each individual token is processed.
- Assume an input of a correct, space separated, string of tokens representing an infix expression
- Generate a space separated output string representing the RPN
- Test with the input string
'3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3'
then print and display the output here. - Operator precedence is given in this table:
You are encouraged to solve this task according to the task description, using any language you may know.
operator precedence associativity ^ 4 Right * 3 Left / 3 Left + 2 Left - 2 Left
- Extra credit
- Add extra text explaining the actions and an optional comment for the action on receipt of each token.
- Note
- the handling of functions and arguments is not required.
- See also
- Parsing/RPN calculator algorithm for a method of calculating a final value from this output RPN expression.
- Parsing/RPN to infix conversion.
AutoHotkey
<lang AHK>SetBatchLines -1
- NoEnv
expr := "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3"
output := "Testing string '" expr "'`r`n`r`nToken`tOutput Queue"
. Space(StrLen(expr)-StrLen("Output Queue")+2) "OP Stack"
- define a stack with semantic .push() and .pop() funcs
stack := {push: func("ObjInsert"), pop: func("ObjRemove"), peek: func("Peek")}
Loop Parse, expr, %A_Space% {
token := A_LoopField if token is number Q .= token A_Space if isOp(token){ o1 := token while isOp(o2 := stack.peek()) and ((isLeft(o1) and Precedence(o1) <= Precedence(o2)) or (isRight(o1) and Precedence(o1) < Precedence(o2))) Q .= stack.pop() A_Space stack.push(o1) } If ( token = "(" ) stack.push(token) If ( token = ")" ) { While ((t := stack.pop()) != "(") && (t != "") Q .= t A_Space if (t = "") throw Exception("Unmatched parenthesis. " . "Character number " A_Index) } output .= "`r`n" token Space(7) Q Space(StrLen(expr)+2-StrLen(Q)) . Disp(stack)
} output .= "`r`n(empty stack to output)" While (t := stack.pop()) != ""
if InStr("()", t) throw Exception("Unmatched parenthesis.") else Q .= t A_Space, output .= "`r`n" Space(8) Q . Space(StrLen(expr)+2-StrLen(Q)) Disp(stack)
output .= "`r`n`r`nFinal string: '" Q "'" clipboard := output
isOp(t){
return (!!InStr("+-*/^", t) && t)
} Peek(this){
r := this.Remove(), this.Insert(r) return r
} IsLeft(o){
return !!InStr("*/+-", o)
} IsRight(o){
return o = "^"
} Precedence(o){
return (InStr("+-/*^", o)+3)//2
} Disp(obj){
for each, val in obj o := val . o return o
} Space(n){
return n>0 ? A_Space Space(n-1) : ""
}</lang>
- Output
Testing string '3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3' Token Output Queue OP Stack 3 3 + 3 + 4 3 4 + * 3 4 *+ 2 3 4 2 *+ / 3 4 2 * /+ ( 3 4 2 * (/+ 1 3 4 2 * 1 (/+ - 3 4 2 * 1 -(/+ 5 3 4 2 * 1 5 -(/+ ) 3 4 2 * 1 5 - /+ ^ 3 4 2 * 1 5 - ^/+ 2 3 4 2 * 1 5 - 2 ^/+ ^ 3 4 2 * 1 5 - 2 ^^/+ 3 3 4 2 * 1 5 - 2 3 ^^/+ (empty stack to output) 3 4 2 * 1 5 - 2 3 ^ ^/+ 3 4 2 * 1 5 - 2 3 ^ ^ /+ 3 4 2 * 1 5 - 2 3 ^ ^ / + 3 4 2 * 1 5 - 2 3 ^ ^ / + Final string: '3 4 2 * 1 5 - 2 3 ^ ^ / + '
C
Requires a functioning ANSI terminal and Enter key. <lang c>#include <sys/types.h>
- include <regex.h>
- include <stdio.h>
typedef struct { const char *s; int len, prec, assoc; } str_tok_t;
typedef struct { const char * str; int assoc, prec; regex_t re; } pat_t;
enum assoc { A_NONE, A_L, A_R }; pat_t pat_eos = {"", A_NONE, 0};
pat_t pat_ops[] = { {"^\\)", A_NONE, -1}, {"^\\*\\*", A_R, 3}, {"^\\^", A_R, 3}, {"^\\*", A_L, 2}, {"^/", A_L, 2}, {"^\\+", A_L, 1}, {"^-", A_L, 1}, {0} };
pat_t pat_arg[] = { {"^[-+]?[0-9]*\\.?[0-9]+([eE][-+]?[0-9]+)?"}, {"^[a-zA-Z_][a-zA-Z_0-9]*"}, {"^\\(", A_L, -1}, {0} };
str_tok_t stack[256]; /* assume these are big enough */ str_tok_t queue[256]; int l_queue, l_stack;
- define qpush(x) queue[l_queue++] = x
- define spush(x) stack[l_stack++] = x
- define spop() stack[--l_stack]
void display(const char *s) { int i; printf("\033[1;1H\033[JText | %s", s); printf("\nStack| "); for (i = 0; i < l_stack; i++) printf("%.*s ", stack[i].len, stack[i].s); // uses C99 format strings printf("\nQueue| "); for (i = 0; i < l_queue; i++) printf("%.*s ", queue[i].len, queue[i].s); puts("\n\n<press enter>"); getchar(); }
int prec_booster;
- define fail(s1, s2) {fprintf(stderr, "[Error %s] %s\n", s1, s2); return 0;}
int init(void) { int i; pat_t *p;
for (i = 0, p = pat_ops; p[i].str; i++) if (regcomp(&(p[i].re), p[i].str, REG_NEWLINE|REG_EXTENDED)) fail("comp", p[i].str);
for (i = 0, p = pat_arg; p[i].str; i++) if (regcomp(&(p[i].re), p[i].str, REG_NEWLINE|REG_EXTENDED)) fail("comp", p[i].str);
return 1; }
pat_t* match(const char *s, pat_t *p, str_tok_t * t, const char **e) { int i; regmatch_t m;
while (*s == ' ') s++; *e = s;
if (!*s) return &pat_eos;
for (i = 0; p[i].str; i++) { if (regexec(&(p[i].re), s, 1, &m, REG_NOTEOL)) continue; t->s = s; *e = s + (t->len = m.rm_eo - m.rm_so); return p + i; } return 0; }
int parse(const char *s) { pat_t *p; str_tok_t *t, tok;
prec_booster = l_queue = 0; display(s); while (*s) { p = match(s, pat_arg, &tok, &s); if (!p || p == &pat_eos) fail("parse arg", s);
/* Odd logic here. Don't actually stack the parens: don't need to. */ if (p->prec == -1) { prec_booster += 100; continue; } qpush(tok); display(s);
re_op: p = match(s, pat_ops, &tok, &s); if (!p) fail("parse op", s);
tok.assoc = p->assoc; tok.prec = p->prec;
if (p->prec > 0) tok.prec = p->prec + prec_booster; else if (p->prec == -1) { if (prec_booster < 100) fail("unmatched )", s); tok.prec = prec_booster; }
while (l_stack) { t = stack + l_stack - 1; if (!(t->prec == tok.prec && t->assoc == A_L) && t->prec <= tok.prec) break; qpush(spop()); display(s); }
if (p->prec == -1) { prec_booster -= 100; goto re_op; }
if (!p->prec) { display(s); if (prec_booster) fail("unmatched (", s); return 1; }
spush(tok); display(s); }
return 1; }
int main() { int i; const char *tests[] = { "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3", /* RC mandated: OK */ "123", /* OK */ "3+4 * 2 / ( 1 - 5 ) ^ 2 ^ 3.14", /* OK */ "(((((((1+2+3**(4 + 5))))))", /* bad parens */ "a^(b + c/d * .1e5)!", /* unknown op */ "(1**2)**3", /* OK */ 0 };
if (!init()) return 1; for (i = 0; tests[i]; i++) { printf("Testing string `%s' <enter>\n", tests[i]); getchar();
printf("string `%s': %s\n\n", tests[i], parse(tests[i]) ? "Ok" : "Error"); }
return 0; }</lang>
- Output
Note: This cannot give a flavour of the true interactive output where tokens are shuffled around every time the enter key is pressed.
Testing string `3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3' <enter> Text | 3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3 Stack| Queue| <press enter> Text | + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3 Stack| Queue| 3 <press enter> Text | 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3 Stack| + Queue| 3 <press enter> Text | * 2 / ( 1 - 5 ) ^ 2 ^ 3 Stack| + Queue| 3 4 <press enter> Text | 2 / ( 1 - 5 ) ^ 2 ^ 3 Stack| + * Queue| 3 4 <press enter> Text | / ( 1 - 5 ) ^ 2 ^ 3 Stack| + * Queue| 3 4 2 <press enter> Text | ( 1 - 5 ) ^ 2 ^ 3 Stack| + Queue| 3 4 2 * <press enter> Text | ( 1 - 5 ) ^ 2 ^ 3 Stack| + / Queue| 3 4 2 * <press enter> Text | - 5 ) ^ 2 ^ 3 Stack| + / Queue| 3 4 2 * 1 <press enter> Text | 5 ) ^ 2 ^ 3 Stack| + / - Queue| 3 4 2 * 1 <press enter> Text | ) ^ 2 ^ 3 Stack| + / - Queue| 3 4 2 * 1 5 <press enter> Text | ^ 2 ^ 3 Stack| + / Queue| 3 4 2 * 1 5 - <press enter> Text | 2 ^ 3 Stack| + / ^ Queue| 3 4 2 * 1 5 - <press enter> Text | ^ 3 Stack| + / ^ Queue| 3 4 2 * 1 5 - 2 <press enter> Text | 3 Stack| + / ^ ^ Queue| 3 4 2 * 1 5 - 2 <press enter> Text | Stack| + / ^ ^ Queue| 3 4 2 * 1 5 - 2 3 <press enter> Text | Stack| + / ^ Queue| 3 4 2 * 1 5 - 2 3 ^ <press enter> Text | Stack| + / Queue| 3 4 2 * 1 5 - 2 3 ^ ^ <press enter> Text | Stack| + Queue| 3 4 2 * 1 5 - 2 3 ^ ^ / <press enter> Text | Stack| Queue| 3 4 2 * 1 5 - 2 3 ^ ^ / + <press enter> Text | Stack| Queue| 3 4 2 * 1 5 - 2 3 ^ ^ / + <press enter> string `3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3': Ok Testing string `123' <enter> ^C
Go
No error checking. No extra credit output, but there are some comments in the code. <lang go>package main
import (
"fmt" "strings"
)
var input = "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3"
var opa = map[string]struct {
prec int rAssoc bool
}{
"^": {4, true}, "*": {3, false}, "/": {3, false}, "+": {2, false}, "-": {2, false},
}
func main() {
fmt.Println("infix: ", input) fmt.Println("postfix:", parseInfix(input))
}
func parseInfix(e string) (rpn string) {
var stack []string // holds operators and left parenthesis for _, tok := range strings.Fields(e) { switch tok { case "(": stack = append(stack, tok) // push "(" to stack case ")": var op string for { // pop item ("(" or operator) from stack op, stack = stack[len(stack)-1], stack[:len(stack)-1] if op == "(" { break // discard "(" } rpn += " " + op // add operator to result } default: if o1, isOp := opa[tok]; isOp { // token is an operator for len(stack) > 0 { // consider top item on stack op := stack[len(stack)-1] if o2, isOp := opa[op]; !isOp || o1.prec > o2.prec || o1.prec == o2.prec && o1.rAssoc { break } // top item is an operator that needs to come off stack = stack[:len(stack)-1] // pop it rpn += " " + op // add it to result } // push operator (the new one) to stack stack = append(stack, tok) } else { // token is an operand if rpn > "" { rpn += " " } rpn += tok // add operand to result } } } // drain stack to result for len(stack) > 0 { rpn += " " + stack[len(stack)-1] stack = stack[:len(stack)-1] } return
}</lang> Output:
infix: 3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3 postfix: 3 4 2 * 1 5 - 2 3 ^ ^ / +
Icon and Unicon
<lang Icon>procedure main()
infix := "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3" printf("Infix = %i\n",infix) printf("RPN = %i\n",Infix2RPN(infix))
end
link printf
record op_info(pr,as) # p=precedence, a=associativity (left=null)
procedure Infix2RPN(expr) #: Infix to RPN parser - shunting yard static oi initial {
oi := table() # precedence & associativity every oi[!"+-"] := op_info(2) # 2L every oi[!"*/"] := op_info(3) # 3L oi["^"] := op_info(4,1) # 4R } ostack := [] # operator stack rpn := "" # rpn pat := sprintf("%%5s : %%-%ds : %%s\n",*expr) # fmt printf(pat,"Token","Output","Op Stack") # header
expr ? until pos(0) do { # while tokens tab(many(' ')) # consume any seperator token := tab(upto(' ')|0) # get token printf(pat,token,rpn,list2string(ostack)) # report if token := numeric(token) then # ... numeric rpn ||:= token || " " else if member(oi,token) then { # ... operator while member(oi,op2 := ostack[1]) & ( /oi[token].as & oi[token].pr <= oi[op2].pr ) | ( \oi[token].as & oi[token].pr < oi[op2].pr ) do rpn ||:= pop(ostack) || " " push(ostack,token) } else # ... parenthesis if token == "(" then push(ostack,token) else if token == ")" then { until ostack[1] == "(" do rpn ||:= pop(ostack) || " " | stop("Unbalanced parenthesis") pop(ostack) # discard "(" } }
while token := pop(ostack) do # ... input exhausted if token == ("("|")") then stop("Unbalanced parenthesis") else { rpn ||:= token || " " printf(pat,"",rpn,list2string(ostack)) } return rpn
end
procedure list2string(L) #: format list as a string
every (s := "[ ") ||:= !L || " " return s || "]"
end</lang>
printf.icn provides formatting
Output:
Infix = "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3" Token : Output : Op Stack 3 : : [ ] + : 3 : [ ] 4 : 3 : [ + ] * : 3 4 : [ + ] 2 : 3 4 : [ * + ] / : 3 4 2 : [ * + ] ( : 3 4 2 * : [ / + ] 1 : 3 4 2 * : [ ( / + ] - : 3 4 2 * 1 : [ ( / + ] 5 : 3 4 2 * 1 : [ - ( / + ] ) : 3 4 2 * 1 5 : [ - ( / + ] ^ : 3 4 2 * 1 5 - : [ / + ] 2 : 3 4 2 * 1 5 - : [ ^ / + ] ^ : 3 4 2 * 1 5 - 2 : [ ^ / + ] 3 : 3 4 2 * 1 5 - 2 : [ ^ ^ / + ] : 3 4 2 * 1 5 - 2 3 ^ : [ ^ / + ] : 3 4 2 * 1 5 - 2 3 ^ ^ : [ / + ] : 3 4 2 * 1 5 - 2 3 ^ ^ / : [ + ] : 3 4 2 * 1 5 - 2 3 ^ ^ / + : [ ] RPN = "3 4 2 * 1 5 - 2 3 ^ ^ / + "
PicoLisp
Note: "^" is a meta-character and must be escaped in strings <lang PicoLisp>(de operator (Op)
(member Op '("\^" "*" "/" "+" "-")) )
(de leftAssoc (Op)
(member Op '("*" "/" "+" "-")) )
(de precedence (Op)
(case Op ("\^" 4) (("*" "/") 3) (("+" "-") 2) ) )
(de shuntingYard (Str)
(make (let (Fmt (-7 -30 -4) Stack) (tab Fmt "Token" "Output" "Stack") (for Token (str Str "_") (cond ((num? Token) (link @)) ((= "(" Token) (push 'Stack Token)) ((= ")" Token) (until (= "(" (car Stack)) (unless Stack (quit "Unbalanced Stack") ) (link (pop 'Stack)) ) (pop 'Stack) ) (T (while (and (operator (car Stack)) ((if (leftAssoc (car Stack)) <= <) (precedence Token) (precedence (car Stack)) ) ) (link (pop 'Stack)) ) (push 'Stack Token) ) ) (tab Fmt Token (glue " " (made)) Stack) ) (while Stack (when (= "(" (car Stack)) (quit "Unbalanced Stack") ) (link (pop 'Stack)) (tab Fmt NIL (glue " " (made)) Stack) ) ) ) )</lang>
Output: <lang PicoLisp>: (shuntingYard "3 + 4 * 2 / (1 - 5) \^ 2 \^ 3") Token Output Stack 3 3 + 3 + 4 3 4 +
- 3 4 *+
2 3 4 2 *+ / 3 4 2 * /+ ( 3 4 2 * (/+ 1 3 4 2 * 1 (/+ - 3 4 2 * 1 -(/+ 5 3 4 2 * 1 5 -(/+ ) 3 4 2 * 1 5 - /+ ^ 3 4 2 * 1 5 - ^/+ 2 3 4 2 * 1 5 - 2 ^/+ ^ 3 4 2 * 1 5 - 2 ^^/+ 3 3 4 2 * 1 5 - 2 3 ^^/+
3 4 2 * 1 5 - 2 3 ^ ^/+ 3 4 2 * 1 5 - 2 3 ^ ^ /+ 3 4 2 * 1 5 - 2 3 ^ ^ / + 3 4 2 * 1 5 - 2 3 ^ ^ / +
-> (3 4 2 "*" 1 5 "-" 2 3 "\^" "\^" "/" "+")</lang>
PL/I
<lang PL/I> cvt: procedure options (main); /* 15 January 2012. */
declare (in, stack, out) character (100) varying; declare (ch, chs) character (1); declare display bit (1) static initial ('0'b);
in = '3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3';
in = '(' || in || ' ) '; /* Initialize with parentheses */
put skip edit ('INPUT', 'STACK', 'OUTPUT') (a, col(37), a, col(47), a);
stack = ' '; out = ; /* Initialize */ do while (length (in) > 0); ch = substr(in, 1, 1); select (ch); when (' ') ;
when ('+', '-', '*', '/', '^') do; /* Copy any equal or higher-priority operators from the stack */ /* to the output string */ chs = substr(stack, 1, 1); do while ((spriority(chs) >= priority(ch)) & ( chs ^= ')' ) ); if display then put skip list ('unstacking: ' || chs); out = out || ' ' || chs; stack = substr(stack, 2); chs = substr(stack, 1, 1); end; /* Now copy the input to the TOS. */ if display then put skip list ('copying ' || ch || ' to TOS'); stack = ch || stack; end; when ( '(' ) do; stack = '(' || stack; if display then put skip list ('stacking the (' ); end; when ( ')' ) do; /* copy all operators from the stack to the output, */ /* until a '(' is encountered. */ chs = substr(stack, 1, 1); do while (chs ^= '(' ); if display then put skip list ('copying stack ' || chs || ' to output'); put skip edit (stack, out) (col(37), a, col(47), a); out = out || ' ' || chs; stack = substr(stack, 2); chs = substr(stack, 1, 1); end; /* Now delete the '(' from the input and */ /* the ')' from the top of the stack. */ if display then put skip edit ('Deleting ( from TOS') (col(30), a); stack = substr(stack, 2); /* The '(' on the input is removed at the end of the loop. */ end; otherwise /* it's an operand. */ do; out = out || ' '; do while (ch ^= ' '); if display then put skip list ('copying ' || ch || ' to output'); out = out || ch; in = substr(in, 2); ch = substr(in, 1, 1); end; end; end; in = substr(in, 2); /* Remove one character from the input */ put skip edit (in, stack, out) (a, col(37), a, col(47), a); end;
priority: procedure (ch) returns (character(1));
declare ch character (1);
return ( translate(ch, '1122335', '()+-*/^' ) );
end priority;
spriority: procedure (ch) returns (character(1));
declare ch character (1);
return ( translate(ch, '1122334', '()+-*/^' ) );
end spriority;
end cvt; </lang> Output to come.
Python
Parenthesis are added to the operator table then special-cased in the code. This solution includes the extra credit. <lang python>from collections import namedtuple from pprint import pprint as pp
OpInfo = namedtuple('OpInfo', 'prec assoc') L, R = 'Left Right'.split()
ops = {
'^': OpInfo(prec=4, assoc=R), '*': OpInfo(prec=3, assoc=L), '/': OpInfo(prec=3, assoc=L), '+': OpInfo(prec=2, assoc=L), '-': OpInfo(prec=2, assoc=L), '(': OpInfo(prec=9, assoc=L), ')': OpInfo(prec=0, assoc=L), }
NUM, LPAREN, RPAREN = 'NUMBER ( )'.split()
def get_input(inp = None):
'Inputs an expression and returns list of (TOKENTYPE, tokenvalue)' if inp is None: inp = input('expression: ') tokens = inp.strip().split() tokenvals = [] for token in tokens: if token in ops: tokenvals.append((token, ops[token])) #elif token in (LPAREN, RPAREN): # tokenvals.append((token, token)) else: tokenvals.append((NUM, token)) return tokenvals
def shunting(tokenvals):
outq, stack = [], [] table = ['TOKEN,ACTION,RPN OUTPUT,OP STACK,NOTES'.split(',')] for token, val in tokenvals: note = action = if token is NUM: action = 'Add number to output' outq.append(val) table.append( (val, action, ' '.join(outq), ' '.join(s[0] for s in stack), note) ) elif token in ops: t1, (p1, a1) = token, val v = t1 note = 'Pop ops from stack to output' while stack: t2, (p2, a2) = stack[-1] if (a1 == L and p1 <= p2) or (a1 == R and p1 < p2): if t1 != RPAREN: if t2 != LPAREN: stack.pop() action = '(Pop op)' outq.append(t2) else: break else: if t2 != LPAREN: stack.pop() action = '(Pop op)' outq.append(t2) else: stack.pop() action = '(Pop & discard "(")' table.append( (v, action, ' '.join(outq), ' '.join(s[0] for s in stack), note) ) break table.append( (v, action, ' '.join(outq), ' '.join(s[0] for s in stack), note) ) v = note = else: note = break note = note = if t1 != RPAREN: stack.append((token, val)) action = 'Push op token to stack' else: action = 'Discard ")"' table.append( (v, action, ' '.join(outq), ' '.join(s[0] for s in stack), note) ) note = 'Drain stack to output' while stack: v = t2, (p2, a2) = stack[-1] action = '(Pop op)' stack.pop() outq.append(t2) table.append( (v, action, ' '.join(outq), ' '.join(s[0] for s in stack), note) ) v = note = return table
if __name__ == '__main__':
infix = '3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3' print( 'For infix expression: %r\n' % infix ) rp = shunting(get_input(infix)) maxcolwidths = [len(max(x, key=len)) 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 RPN is: %r' % rp[-1][2])</lang>
- Sample output
For infix expression: '3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3' TOKEN ACTION RPN OUTPUT OP STACK NOTES 3 Add number to output 3 + Push op token to stack 3 + 4 Add number to output 3 4 + * Push op token to stack 3 4 + * 2 Add number to output 3 4 2 + * / (Pop op) 3 4 2 * + Pop ops from stack to output Push op token to stack 3 4 2 * + / ( Push op token to stack 3 4 2 * + / ( 1 Add number to output 3 4 2 * 1 + / ( - Push op token to stack 3 4 2 * 1 + / ( - 5 Add number to output 3 4 2 * 1 5 + / ( - ) (Pop op) 3 4 2 * 1 5 - + / ( Pop ops from stack to output (Pop & discard "(") 3 4 2 * 1 5 - + / Discard ")" 3 4 2 * 1 5 - + / ^ Push op token to stack 3 4 2 * 1 5 - + / ^ 2 Add number to output 3 4 2 * 1 5 - 2 + / ^ ^ Push op token to stack 3 4 2 * 1 5 - 2 + / ^ ^ 3 Add number to output 3 4 2 * 1 5 - 2 3 + / ^ ^ (Pop op) 3 4 2 * 1 5 - 2 3 ^ + / ^ Drain stack to output (Pop op) 3 4 2 * 1 5 - 2 3 ^ ^ + / (Pop op) 3 4 2 * 1 5 - 2 3 ^ ^ / + (Pop op) 3 4 2 * 1 5 - 2 3 ^ ^ / + The final output RPN is: '3 4 2 * 1 5 - 2 3 ^ ^ / +'
Ruby
See Parsing/RPN/Ruby
<lang ruby>rpn = RPNExpression.from_infix("3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3")</lang> outputs
for Infix expression: 3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3 Term Action Output Stack 3 PUSH V ["3"] [] + PUSH OP ["3"] ["+"] 4 PUSH V ["3", "4"] ["+"] * PUSH OP ["3", "4"] ["+", "*"] 2 PUSH V ["3", "4", "2"] ["+", "*"] / MUL ["3", "4", "2", "*"] ["+"] * has higher precedence than / / PUSH OP ["3", "4", "2", "*"] ["+", "/"] ( OPEN_P ["3", "4", "2", "*"] ["+", "/", "("] 1 PUSH V ["3", "4", "2", "*", "1"] ["+", "/", "("] - PUSH OP ["3", "4", "2", "*", "1"] ["+", "/", "(", "-"] 5 PUSH V ["3", "4", "2", "*", "1", "5"] ["+", "/", "(", "-"] ) SUB ["3", "4", "2", "*", "1", "5", "-"] ["+", "/", "("] unwinding parenthesis ) CLOSE_P ["3", "4", "2", "*", "1", "5", "-"] ["+", "/"] ^ PUSH OP ["3", "4", "2", "*", "1", "5", "-"] ["+", "/", "^"] 2 PUSH V ["3", "4", "2", "*", "1", "5", "-", "2"] ["+", "/", "^"] ^ PUSH OP ["3", "4", "2", "*", "1", "5", "-", "2"] ["+", "/", "^", "^"] 3 PUSH V ["3", "4", "2", "*", "1", "5", "-", "2", "3"] ["+", "/", "^", "^"] RPN = 3 4 2 * 1 5 - 2 3 ^ ^ / +
Tcl
<lang tcl>package require Tcl 8.5
- Helpers
proc tokenize {str} {
regexp -all -inline {[\d.]+|[-*^+/()]} $str
} proc precedence op {
dict get {^ 4 * 3 / 3 + 2 - 2} $op
} proc associativity op {
if {$op eq "^"} {return "right"} else {return "left"}
}
proc shunting {expression} {
set stack {} foreach token [tokenize $expression] {
if {[string is double $token]} { puts "add to output: $token" lappend output $token } elseif {$token eq "("} { puts "push parenthesis" lappend stack $token } elseif {$token eq ")"} { puts "popping to parenthesis" while {[lindex $stack end] ne "("} { lappend output [lindex $stack end] set stack [lreplace $stack end end] puts "...popped [lindex $output end] to output" } set stack [lreplace $stack end end] puts "...found parenthesis" } else { puts "adding operator: $token" set p [precedence $token] set a [associativity $token] while {[llength $stack]} { set o2 [lindex $stack end] if { $o2 ne "(" && (($a eq "left" && $p <= [precedence $o2]) || ($a eq "right" && $p < [precedence $o2])) } then { puts "...popped operator $o2 to output" lappend output $o2 set stack [lreplace $stack end end] } else { break } } lappend stack $token } puts "\t\tOutput:\t$output\n\t\tStack:\t$stack"
} puts "transferring tokens from stack to output" lappend output {*}[lreverse $stack]
}
puts [shunting "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3"]</lang> Output:
add to output: 3 Output: 3 Stack: adding operator: + Output: 3 Stack: + add to output: 4 Output: 3 4 Stack: + adding operator: * Output: 3 4 Stack: + * add to output: 2 Output: 3 4 2 Stack: + * adding operator: / ...popped operator * to output Output: 3 4 2 * Stack: + / push parenthesis Output: 3 4 2 * Stack: + / ( add to output: 1 Output: 3 4 2 * 1 Stack: + / ( adding operator: - Output: 3 4 2 * 1 Stack: + / ( - add to output: 5 Output: 3 4 2 * 1 5 Stack: + / ( - popping to parenthesis ...popped - to output ...found parenthesis Output: 3 4 2 * 1 5 - Stack: + / adding operator: ^ Output: 3 4 2 * 1 5 - Stack: + / ^ add to output: 2 Output: 3 4 2 * 1 5 - 2 Stack: + / ^ adding operator: ^ Output: 3 4 2 * 1 5 - 2 Stack: + / ^ ^ add to output: 3 Output: 3 4 2 * 1 5 - 2 3 Stack: + / ^ ^ transferring tokens from stack to output 3 4 2 * 1 5 - 2 3 ^ ^ / +