# Shunting-yard algorithm

```ruby
var prec = Hash(
    '^' => 4,
    '*' => 3,
    '/' => 3,
    '+' => 2,
    '-' => 2,
    '(' => 1,
)

var assoc = Hash(
    '^' => 'right',
    '*' => 'left',
    '/' => 'left',
    '+' => 'left',
    '-' => 'left',
)

func shunting_yard(prog) {
    var inp = prog.words
    var ops = []
    var res = []
 
    func report (op) {
        printf("%25s    %-7s %10s %s\n",
            res.join(' '), ops.join(' '), op, inp.join(' '))
    }

    func shift  (t)  { report( "shift #{t}"); ops << t }
    func reduce (t)  { report("reduce #{t}"); res << t }
 
    while (inp) {
        given(var t = inp.shift) {
           when (/\d/) { reduce(t) }
           when ('(')  { shift(t) }
           when (')')  {
               while (ops) {
                 (var x = ops.pop) == '(' ? break : reduce(x)
               }
           }
           default {
                var newprec = prec{t}
                while (ops) {
                    var oldprec = prec{ops[-1]}
 
                    break if (newprec > oldprec)
                    break if ((newprec == oldprec) && (assoc{t} == 'right'))
 
                    reduce(ops.pop)
                }
                shift(t)
            }
        }
    }
    while (ops) { reduce(ops.pop) }
    return res
}
 
say shunting_yard('3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3').join(' ')
```

#### Output:

```
                                       reduce 3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3
                        3               shift + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3
                        3    +         reduce 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3
                      3 4    +          shift * 2 / ( 1 - 5 ) ^ 2 ^ 3
                      3 4    + *       reduce 2 / ( 1 - 5 ) ^ 2 ^ 3
                    3 4 2    +         reduce * ( 1 - 5 ) ^ 2 ^ 3
                  3 4 2 *    +          shift / ( 1 - 5 ) ^ 2 ^ 3
                  3 4 2 *    + /        shift ( 1 - 5 ) ^ 2 ^ 3
                  3 4 2 *    + / (     reduce 1 - 5 ) ^ 2 ^ 3
                3 4 2 * 1    + / (      shift - 5 ) ^ 2 ^ 3
                3 4 2 * 1    + / ( -   reduce 5 ) ^ 2 ^ 3
              3 4 2 * 1 5    + / (     reduce - ^ 2 ^ 3
            3 4 2 * 1 5 -    + /        shift ^ 2 ^ 3
            3 4 2 * 1 5 -    + / ^     reduce 2 ^ 3
          3 4 2 * 1 5 - 2    + / ^      shift ^ 3
          3 4 2 * 1 5 - 2    + / ^ ^   reduce 3
        3 4 2 * 1 5 - 2 3    + / ^     reduce ^
      3 4 2 * 1 5 - 2 3 ^    + /       reduce ^
    3 4 2 * 1 5 - 2 3 ^ ^    +         reduce /
  3 4 2 * 1 5 - 2 3 ^ ^ /              reduce +
3 4 2 * 1 5 - 2 3 ^ ^ / +
```


---

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